MAN ALONE
RICHARD J.KOSCIEJEW
Our closest living relative are three surviving species of great apes: the gorilla, the common chimpanzee, And the pygmy chimpanzee (also known as bonobos). Their confinement to Africa, along with abundant fossil evidence, strongly suggests that they also played the earliest stages of human evolution out in Africa, human history, as something separate from the history of animals, occurring about seven million years ago (estimates range from five to nine million years ago). Around that time, a population of African apes broke off into several populations, of which one preceded to evolve into modern gorillas, a second into the two modern chimps, and the third into humans. The gorilla line apparently split slightly before the split between the chimp and the human lines.
The primate, is the order of mammals that includes humans, apes, which are the closest living relatives to humans, monkeys, and some less familiar mammals, such as tarsiers, lorises, and lemurs. Humans and other primates share a common evolutionary descent. Consequently, primates have always fascinated scientists because their physical features, social organization, behavioural patterns, and fossil remains provide clues about our earliest human ancestors.
Primates evolved from tree-dwelling ancestors. Although some species, such as humans, have since taken to the ground, all primates’ share features that are related to their tree-climbing ancestry. These include arms and legs that can move more freely than those of most other mammals, flexible fingers and toes, forward-facing eyes that can judge distances accurately - a vital aid when moving about high above the ground - and large brains.
Primates live in a wide range of habitats but are restricted by their need for warmth. Most primates live in tropical jungles or dry forests, but some live in dry grasslands, and others have settled in cold, mountainous regions of China and Japan. The world's most northerly primate, the Japanese macaque, has learned to bathe in hot springs to survive through the winter snows. In parts of the tropics, monkeys can be seen within a few miles of busy city centres, but despite this adaptability, most of the world’s primates retain a close dependence on trees. Apart from humans, baboons are the only primates that have fully made the transition to life out in the open, and even they instinctively climb to safety if danger threatens.
Some primates, especially the smaller species, are active only at night, or nocturnal, while others are diurnal, active during the day. Most primate species - particularly monkeys - are highly sociable animals, sometimes living in troops of more than 100 members. Smaller primates, especially nocturnal ones, tend to be solitary and secretive.
Primates range in size from quite small to quite large. The world's largest species, the lowland gorilla at 200 kg. (400 lb.) is more than 6,000 times the weight of the smallest primate, the pygmy mouse lemur from Madagascar. Measuring only 20 cm. (8 in.) from nose to tail, and weighing about 30 g. (1 oz.), this tiny animal was first identified about two centuries ago, but was later assumed to be extinct until its rediscovery in 1993.
There are about 235 species of primates. Scientists use more than one way to classify primates, and one system divides the order into two overall groups, or suborders: the prosimians and the anthropoids.
The prosimians, or "primitive primates," make up the smaller of these two groups, with about 60 species, and include lemurs, Pontos, galagos, lorises, and, in some classification systems, tarsiers. Lemurs are only found on the islands of Madagascar and Comoros, where they have flourished in isolation for millions of years. Pontos and galagos are found in Africa, while lorises and tarsiers are found in southeast Asia. Typical prosimians are small to medium-sized mammals with long whiskers, pointed muzzles, and well-developed senses of smell and hearing. Most prosimians are nocturnal, although in Madagascar some larger lemurs are active by day.
In the past, tree shrews were often classified as primates, but their place in mammal classification has been the subject of much debate. Today, based on reproductive patterns and on new fossil evidence, most zoologists classify them in an order of their own, the Scandentia.
The remainder of the world's primates makes up the anthropoid, or “humanlike” suborder, which contains about 175 species. This group consists of humans, apes, and monkeys. Most anthropoids, apart from baboons, have flat faces and a poor sense of smell. With a few exceptions, anthropoids are usually active during the day, and they find their food mainly by sight.
Apes are found only in Africa and Asia. They have no tails, and their arms are longer than their legs. Monkeys from Central and South America, known as New World monkeys, have broad noses and nostrils that open sideways. They are called platyrrhine, which means broad-nosed. Monkeys from Africa and Asia, known as Old World monkeys, have narrow noses and nostrils that face downward - a characteristic also seen in apes and humans. Old World Monkeys are called catarrhine, which means downward-nosed.
During evolution, primates have kept several physical features that most other mammals have lost. One of these is the clavicle, or collarbone. In primates, the clavicle forms an important part of the shoulder joint. It helps to stabilize the shoulder, permitting a primate to support its weight by hanging from its arms alone - something that few other mammals can do. Some primates, particularly gibbons and the siamang, use this ability to move through the trees from one branch to another by swinging from arm to arm. This type of locomotion is called the brachiation.
During evolution, many mammals have gradually lost limb bones as they have adapted to different ways of life: horses, for example, have lost all but a single toe on each foot. Nearly all primates, by contrast, have retained a full set of five fingers and toes, and usually these digits have become increasingly flexible as time has gone through. In the aye-aye, a prosimian from Madagascar, the third finger on each hand is long and thin with a special claw at the end. Aye-ayes use these bony fingers to extract insect grubs from bark.
Evolution has affected the thumbs and big toes of primates. In most mammals, these digits bend in the same plane as the other fingers and toes. However, in many primates, the thumbs or big toes are opposable, meaning that they are set apart in a way that permits them to meet the other digits at the tips to form a circle. This enables primates to grip branches, and equally importantly, pick up and handle small objects. Instead of having claws, most primates have flat nails that cover soft, sensitive fingertips—another adaptation that helps primates to manipulate objects with great dexterity.
Tails are absent in humans and apes, but in most monkeys and prosimians, the tail plays a special role in maintaining balance during movement through the treetops. Many New World monkeys have prehensile tails, which can be wrapped around branches, gripping them like an extra hand or foot.
Primate skulls show several distinctive features. One of these is the position of the eyes, which in most species is on the front of the skull looking forward, rather than on the side of the skull looking to the side as in many other mammals. The two forward-facing eyes have overlapping fields of view, which give primates stereoscopic vision. Stereoscopic vision permits accurate perception of distance, which is helpful for handling food or swinging from branch to branch high above the ground. Another distinctive feature of primate skulls, in anthropoids particularly, is the large domed cranium that protects the brain. The inside surface of this dome clearly shows the outline of an unusually large brain—one of the most remarkable characteristics of this group. The shapes of anthropoid brains are different from other mammals; the portion of the brain devoted to vision is especially large, while the portion devoted to smell is comparatively small.
The primate order includes a handful of species that live entirely on meat (carnivores) and a few that are strict vegetarians (herbivores), but it is composed chiefly of animals that have varied diets (omnivores). The carnivorous primates are the four species of tarsiers, which live in Southeast Asia. Using their long back legs, these pocket-sized nocturnal hunters leap on their prey, pinning it down with their hands and then killing it with their needle-sharp teeth. Tarsiers primarily eat insects but will also eat lizards, bats, and snakes.
Other prosimians, such as galagos and mouse lemurs, also hunt for insects, but they supplement their diet with different kinds of food, including lizards, bird eggs, fruit, and plant sap. This opportunistic approach to feeding is seen in most of monkeys and in chimpanzees. Several species of monkeys, and chimpanzees, but not the other apes, have been known to attack and eat other monkeys. Baboons, the most adept hunters on the ground, often eat meat and sometimes manage to kill small antelope.
Most apes and monkeys eat a range of plant-based foods, but a few specialize in eating leaves. South American howler monkeys and African colobus monkeys eat the leaves of many different trees, but the proboscis monkey on the island of Borneo is more selective, surviving largely on the leaves of mangroves. These leaf-eating monkeys have modified digestive systems, similar to cows, which enable them to break down food that few other monkeys can digest. Other apes and monkeys eat mostly fruit, while some marmosets and lemurs depend on tree gum and sap.
Compared with many other mammals, primates have few young, and their offspring take a long time to develop. The gestational period, the time between conception and birth, is remarkably long compared with other mammals of similar size. A tarsier, for example, gives birth to a single young after a gestational period of nearly six months. By contrast, a similarly sized rodent will often give birth to six or more young after a gestational period lasting just three weeks. Most primates usually give birth to a single baby, although some species, such as dwarf lemurs, usually have twins or triplets.
Once the young are born, the period of parental feeding and protection can be even more drawn out. In small prosimians the young are often weaned after about five weeks, but in apes they are often fed on their mother's milk for three or four years, and they may continue to rely on her protection for six or more years. This long childhood - which reaches its extreme in humans - is a crucial feature of a primate's life because it enables complex patterns of behaviour to be passed on by learning.
Some primates have fixed breeding seasons, but many can breed anytime of the year. In many species, females signal that they are in estrus - receptive and ready to mate - by releasing special scents. In other species, females develop conspicuous swelling around their genitals to signal their readiness for mating. Such swelling is especially noticeable in chimpanzees. While most copulation occurs when the females are receptive, in some species, such as humans and pygmy chimpanzees, copulation frequently occurs even if the female is not in estrus.
Primates display a wide range of mating behaviours. Solitary primates, such as aye-ayes and orangutans, have simple reproductive behaviour. Within the territory that each male control, his imperative territorial rights are in assess of several females live, each with their own territory. The male mates with any females within his territory that are receptive. Other species, such as gibbons, form small family groups consisting of a monogamous pair and their young. Gorillas form harems, in which one adult male life with several adult females and their young. Among social primates, breeding can be complicated by the presence of many adults. Males may cooperate in defending their troop's territory, but they often fight each other for the chance to mate. In some species, only the dominant male mates with the females in the group. Chimpanzee females mate promiscuously with several adult males, although they usually pair up with one high-ranking male during the final few days of estrus, spending all of their time together and mating together exclusively.
Primates have the most highly developed brains in the animal kingdom, rivalled only by those of dolphins, whales, and possibly elephants. Anthropoid primates in particular are intelligent and inquisitive animals that are quick to learn new patterns of behaviour. This resourcefulness enables them to exploit a wide range of foods and may help them to escape attacks by predators.
Many zoologists believe that primates' large brains initially evolved in response to their tree-dwelling habits and their way of feeding. Anthropoid primates, which have the largest brains, live in a visual world, relying on sight to move about and to find and manipulate food. Unlike smell or hearing, vision generates a large amount of complex sensory information that has to be processed and stored. In primate brains, these operations are carried out by part of the brain called the cerebral cortex, which evolved into such a large structure that the rest of the brain is hidden beneath it. Some unrelated mammals, such as squirrels, also live in trees, but they have less-developed eyesight and much smaller brains.
Increased brainpower has had important effects on the way primates live. It has helped them to move about and find food and enabled them to develop special skills. One of the most remarkable of these is toolmaking, seen in chimpanzees and, to a far greater extent, in humans. Toolmaking, as opposed to simple tool use, involves a preconceived image of what the finished tool should look like - something that is only possible with an advanced brain.
The intelligence of primates is also evident in their social behaviour. For species that live in groups, daily life involves countless interactions with relatives, allies, and rivals. Mutual cleaning and grooming of the fur, which removes parasites, helps to reinforce relationships, while threats - sometimes followed by combat - maintain the hierarchy of dominance that permeates typical primate troops.
Primates use a variety of methods to communicate. In solitary prosimians, when animals are not within sight of each other, communication is often accomplished by using scents. Such animals use urine, faeces, or special scent glands to mark territory or to show a readiness to mate. In social anthropoids, visual and vocal signals are much more important. Most monkeys and apes speak with a complex array of facial expressions, some of which are similar to the facial expressions used by humans.
Primates also talk with a repertoire of sounds. These range from the soft clicks and grunts of the colobus to the songs of the gibbon and the roaring of the howler monkey, which can sometimes be heard more than 3 km. (2 mi.) away. Far-carrying calls are used in courtship, both to keep group members from getting separated and to mark and maintain feeding territories. Some primate calls convey more precise messages, often denoting specific kinds of danger. In the wild, researchers have observed that chimpanzees use as many as 34 different calls, and evidence suggests that they can pass on information-such as the location of food-using this form of communication.
Comparatively, little is known about the origins of primates compared with many other groups of mammals, because primates have left relatively few fossil remains. The chief reason for the scarcity of fossils is that forests, the primary home for most early primates, do not create good conditions for fossilization. Instead of being buried by sediment, the bodies of early primates were more likely to have been eaten by scavengers and their bones dispersed.
The earliest fossils of primates discovered dates from the end of the Cretaceous Period, about 65 million years ago. These early fossils include specimens of a species called Notharctus, which resembles today's lemurs and had a long pointed snout. The ancestors of another prosimian group, the tarsiers, are known from fossils that date from the early Eocene Epoch, about 50 million years ago. In 1996 researchers in China recovered fossil bones of a primitive primate no bigger than a human thumb. The animal, named Eosimias, lived 45 million years ago. Many scientists believe that Eosimias is an example of a transitional animal in the evolution of prosimians to anthropoids.
The origin of anthropoids has been difficult to pin down. A single anthropoid fossil has been found that may come from the Eocene Epoch, but conclusive fossil evidence of anthropoids does not appear until the Oligocene Epoch, which began 38 million years ago. These early anthropoids belonged to a lineage that led to the catarrhine primates - the Old World monkeys, apes, and humans. The platyrrhine primates, which include all New World monkeys, are presumed to have diverged from the Old World monkeys during the Eocene Epoch. They evolved in isolation on what was then the island continent of South America. Genetic analysis shows that New World monkeys clearly have the same ancestry with the catarrhines, which means that they must have reached the island continent from the Old World. Exactly how they did this is unclear. One possibility is that they floated across from Africa on logs or rafts of vegetation, journeying across an Atlantic Ocean that was much narrower than it is today.
Of all primate groups, the apes and the direct ancestors of humans have been the most intensively studied. One key question concerns when the two groups diverged. Based on the comparisons of genes and the structure of body parts, scientists think that the line leading to the orangutan diverged from the one leading to humans about 12 million years ago. The ancestral line leading to chimpanzees did not diverge until more recently, probably between five and seven million years ago. This evidence strongly suggests that chimpanzees are our closest living relatives.
The word primate means "the first.” When it was originally coined more than two centuries ago, it conveyed the widely held idea that primates were superior to all other mammals. This notion has since been discarded, but nonhuman primates still generate great interest because of their humanlike characteristics.
In scientific research, much of this interest has focussed on primate behaviour and its correspondence with human behaviour. Attempts have been made to train chimpanzees and orangutans to mimic human speech, but differences in anatomy make it very difficult for apes to produce recognizable words. A more revealing series of experiments has involved training chimpanzees, and later gorillas, to understand words and to respond using American Sign Language. In the late 1960s, a chimp named Washoe learned more than 130 signs. In the 1970s and 1980s, a gorilla named Koko learned to use more than 500 signs and to recognize an additional 500 signs. One outcome of these long-running experiments was that the chimps or gorillas occasionally produced new combinations of signs, suggesting that the animals were not simply repeating tricks that they had learned. More recently, chimps have been trained to talk with humans by using coloured shapes or computer keyboards. They too have shown an ability to associate abstract symbols with objects and ideas - the underlying basis of language.
Apes and monkeys also play an important role in the field of medical research. Because their body systems work very much like our own, new vaccines and new forms of surgery are sometimes tried on apes and monkeys before they are approved for use on humans. Species that are most often used in this way include chimpanzees, baboons, and rhesus monkeys. This kind of animal experimentation has undoubtedly contributed to human welfare, but the medical use of primates is an increasingly controversial area, particularly when it involves animals captured in the wild.
According to figures published by the World Conservation Union (IUCN), more than 110 species of primates - nearly half the world's total—are currently under threat of extinction. This makes the primates among the most vulnerable animals on earth.
The species most under threats are those affected by deforestation. This has been particularly severe in Madagascar, the only home of the lemurs, and it is also taking place at a rapid rate in Southeast Asia, threatening gibbons and orangutans. The almost total destruction of Brazil's Atlantic rainforest has proved catastrophic for several species, including the lion tamarins, which are found only in this habitat. Primates are also threatened by collection for the pet trade and by hunting. Illegal hunting is the chief threat facing the mountain gorilla, a rare African subspecies that lives in the politically volatile border region straddling Uganda, Rwanda, and the Democratic Republic of the Congo.
In the face of these threats, urgent action is currently underway to protect many of these endangered species. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) currently forbids the export of many primates, although not all countries have chosen to follow this law. More direct methods of species preservation include habitat protection and captive breeding programs. Sometimes - for example, the lion tamarin—these programs have met with considerable success. However, without the preservation of extensive and suitable natural habitats, many primate species are destined for extinction.
Humans as primates, have themselves of a physical and genetic similarities showing that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa - chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas - have the same ancestor that lived sometime between eight million and six million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between six million and two million years ago come entirely from Africa.
Humans and great apes of Africa have the same ancestor that lived between eight million and five million years ago. Most scientists distinguish among 12 to 19 different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species - probably most of them - left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.
The tree of Human Evolution Fossil evidence shows that the first humans evolved from ape ancestors at least six million years ago. Many species of humans followed, but only some left descendants on the branch leading to The Homo sapiens. In this slide show, white skulls represent species that lived during the period shown; gray skulls represent extinct human species.
Early humans first migrated out of Africa into Asia probably between two million and 1.7 million years ago. They entered Europe much later, generally within the past one million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.
The scientific study of human evolution is called Paleoanthropology. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behaviour. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, Paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, and the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world developed. Yet many people have come to reconcile such beliefs with the scientific evidence.
Modern and Early Humans have undergone major anatomical changes during evolution. This illustration depicts Australopithecus afarensis, the earliest of the three species; A Homo erectus, an intermediate species; The Homo sapiens, a modern human, and Homo’s ergaster. The modern humans are much taller than A. afarensis and have flatter faces and much larger brains. Modern humans have a larger brain than H. erectus and almost flat face beneath the front of the braincase.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offsprings that are, offsprings themselves capable of reproducing. Scientists classify each species with a unique, but two-part scientific names. In this system, modern humans are classified as Homo sapiens.
The mechanism for evolutionary change resides in genes—the basic units of heredity. Genes affect how the body and behaviour of an organism develop during its life. The information contained in genes can change - a process known as mutation. The way particular genes are expressed - how they affect the body or behaviour of an organism - can also change. Over time, genetic change can alter a species’s overall way of life, such as what it eats, how it grows, and where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population - a group of organisms of the same species that share a particular local habitat. Many factors can favour new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure), physiology (bodily functions, such as digestion), and behaviour. Over long periods, evolution dramatically transformed humans and their ways of life.
Geneticists estimate that the human line began to diverge from that of the African apes between eight million and five million years ago (paleontologists have dated the earliest human fossils to at least six million years ago). This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000. Years ago. belong to the scientific order named Primates, a group of more than 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities plus some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, and the significance of each step in human evolution.
All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) sight; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.
Australopithecines, were the earliest humanlike primates. Known from fossil remains found in Africa, australopithecines, or australopiths, represent the group from which the ancestors of modern humans emerged. As generally used, the term australopithecines covers all early human fossils dated from about 7 million to 2.5 million years ago, and some of those dated from 2.5 million to 1.4 million years ago. The group became extinct after that time.
At some time before about 2.5 million years ago, a currently unknown australopithecine species gave rise to the ancestor of our own genus Homo. The earliest members of Homo resembled late australopiths, and coexisted with them for more than a million years, but the Homo species had larger brains.
Australopiths evolved from apes and had a combination of apelike and humanlike traits. Their faces protruded like those of apes, and they probably had a similar amount of body hair. However, several traits distinguished australopithecines from the apes. First, australopiths are believed to have walked upright on two legs most of the time—a practice known as bipedalism—instead of using all four limbs for locomotion. In addition, australopiths had smaller canine teeth than those of apes. Studies of australopith hand bones suggest that at least one species could have made the earliest stone tools around 2.5 million years ago. However, some scientists believe only members of the larger-brained genus Homo would have had the mental capacity and hand manipulation skills needed to make these tools.
Scientists have identified many australopithecine species. Some of these are classified in the genus Australopithecus (meaning “southern ape”). Other australopithecines belong to different genera, including Sahelanthropus, Orrorin, Ardipithecus, Kenyanthropus, and, by some classifications, Paranthropus.
The first australopithecine specimen was identified by South African anatomist Raymond Dart from a fossilized skull discovered in 1924 at a quarry in Taung, South Africa. Dart named the fossil Australopithecus africanus, meaning “southern ape from Africa.” At the time of this discovery, most anthropologists believed that humans had evolved in Asia rather than Africa. The scientific community mostly rejected Dart’s claim to have found an ancestor of modern humans in southern Africa. Dart’s work was followed up and confirmed by the Scottish-born paleontologist Robert Broom, who discovered many more australopithecine fossils in South Africa. By about 1950, the scientific community finally accepted these australopithecine fossils as proof that humans had evolved in Africa, discovered Australopithecus (or Paranthropus) boisei specimens at Tanzania’s Olduvai Gorge in 1959. East Africa subsequently became the center of research into the evolutionary origins of human beings. In the late 1960s, Kenyan paleoanthropologist Richard Leakey began prospecting for fossils around Lake Turkana in northern Kenya. Fossil fragments of more than a thousand early humans have since been recovered from that region.
Research teams led by American palaeanthropologist Donald Johanson and his French colleague Maurice Taieb made equally spectacular finds in Ethiopia during the 1970s. These finds include the famous partial skeleton named Lucy and the remains of more than 13 Australopithecus afarensis individuals, known as the First Family. Since that time, paleoanthropologists have continued to discover australopithecine fossils in southern, eastern, and central Africa.
One of the most important recent fossil finds was announced in 2006. A team led by Ethiopian scientist Zeresenay Alemseged unearthed the partial skeleton of a three-year-old Australopithecus afarensis female at Dikika in the Afar region of Ethiopia. Nicknamed “Selam,” the Dikika child dates from around 3.3 million years ago and is one of the most complete specimens of an australopith ever found. The well-preserved bones provide previously undocumented details of the skull and skeleton. Some features, such as the shape of the shoulder blades, the long, curved fingers, and the semi-circular ear canals involved in balance, are more ape-like, suggesting an adaptation for climbing trees. The leg bones and feet, however, indicate an ability to walk upright even at an early age. The shape of the brain was preserved and its size indicates australopiths grew to adulthood more slowly than chimpanzees, a characteristic found in later hominids, including modern humans. The hyoid bone that supports the tongue was found as well. The bone is crucial to speech in modern humans but its shape in the Dikika child is like that found in modern great apes, and not humans
Many scientists believe that robust australopiths represent a distinct evolutionary group of early humans because these species share features associated with heavy chewing. According to this view, Australopithecus aethiopicus diverged from other australopiths and later gave rise to A. boisei and A. robustus. Paleoanthropologists who strongly support this view think that the robusts should be classified in the genus Paranthropus, the original name given to the southern species. Thus, these three species are sometimes referred to as P. aethiopicus, P. boisei, and P. robustus.
Other paleoanthropologists believe that the eastern robust species, A. aethiopicus and A. boisei, may have evolved from an early australopith of the same region, perhaps A. afarensis. According to this view, A. africanus gave rise only to the southern species, A. robustus. Scientists refer to such a case—in which two or more independent species evolve similar characteristics in different places or at different times — as parallel evolution. If parallel evolution occurred in australopiths, the robust species would make up two separate branches of the human family tree.
The last robust australopiths died out about 1.4 million years ago. At about this time, climate patterns around the world entered a period of fluctuation, and these changes may have reduced the food supply on which robusts depended. Interaction with larger-brained members of the genus Homo, such as Homo erectus, may also have contributed to the decline of late australopiths, although no compelling evidence exists of such direct contact. Competition with several other species of plant-eating monkeys and pigs, which thrived in Africa at the time, may have been an even more important factor. But the reasons why the robust australopiths became extinct after flourishing for such a long time are not yet known for sure.
Australopithecines, very early human ancestors, spent some of their time in trees. Australopithecines had long, curved fingers that helped them grasp branches for climbing. In this artist’s rendering, members of a group of the species Australopithecus africanus forage for fruits and leaves in the treetops, where they are safe from such potential predators as lions. Although australopithecines were good tree-climbers, they also walked fully upright and spent much time on the ground.
Scientists have several ideas about why australopiths first split off from the apes, initiating the course of human evolution. Virtually all hypotheses suggest that environmental change was an important factor, specifically in influencing the evolution of bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis.
The global climate cooled and became drier between 8 million and 5 million years ago, near the end of the Miocene Epoch. According to the savanna hypothesis, this climate change broke up and reduced the area of African forests. As the forests shrunk, an ape population in eastern Africa became separated from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to its drier environment, which contained larger areas of grassy savanna.
The expansion of dry terrain favoured the evolution of terrestrial living, and made it more difficult to survive by living in trees. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators-activities that also may have required the ability to communicate well. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans.
Critics of the savanna hypothesis argue against it on several grounds, but particularly for two reasons. First, discoveries by a French scientific team of australopith fossils in Chad, in Central Africa, suggests that the environments of East Africa may not have been fully separated from those farther west. Second, recent research suggests that open savannas were not prominent in Africa until sometime after 2 million years ago.
Criticism of the savanna hypothesis has spawned alternative ideas about early human evolution. The woodland-mosaic hypothesis proposes that the early australopiths evolved in patchily wooded areas — a mosaic of woodland and grassland — that offered opportunities for feeding both on the ground and in the trees, and that ground feeding favored bipedalism.
The variability hypothesis suggests that early australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Scientists have found that this range of habitats existed at the time when the early australopiths evolved. So the development of new anatomical characteristics — particularly bipedalism — combined with an ability to climb trees, may have given early humans the versatility to live in a variety of habitats.
Scientists also have many ideas about which benefits of bipedalism may have influenced its evolution. Ideas about the benefits of regular bipedalism include that it freed the hands, making it easier to carry food and tools; allowed early humans to see over tall grass to watch for predators; reduced exposure of the body to hot sun and increased exposure to cooling winds; improved the ability to hunt or use weapons, which became easier with an upright posture; and made extensive feeding from bushes and low branches easier than it would have been for a quadruped. Scientists do not overwhelmingly support any one of these ideas. Recent studies of chimpanzees suggest, though, that the ability to feed more easily might have particular relevance. Chimps move on two legs most often when they feed from the ground on the leaves and fruits of bushes and low branches. Chimps cannot, however, walk in this way over long distances.
Bipedalism in early humans would have enabled them to travel efficiently over long distances, giving them an advantage over quadrupedal apes in moving across barren open terrain between groves of trees. In addition, the earliest humans continued to have the advantage from their ape ancestry of being able to escape into the trees to avoid predators. The benefits of both bipedalism and agility in the trees may explain the unique anatomy of australopiths. Their long, powerful arms and curved fingers probably made them good climbers, while their pelvis and lower limb structure was reshaped for upright walking.
People belong to the genus Homo, which first evolved at least 2.3 million to 2.5 million years ago. The earliest members of this genus differed from the australopiths in at least one important respect—they had larger brains than did their predecessors.
The evolution of the modern human genus can be divided roughly into three periods: early, middle, and late. Species of early Homo resembled gracile australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of middle Homo began perhaps between 2 million and 1.8 million years ago, overlapping with the end of early Homo. Species of middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from middle to late Homo probably occurred sometime around 200,000 years ago. Species of late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution.
The origin of the genus Homo has long intrigued paleoanthropologists and prompted much debate. One of several known species of australopiths, or one not yet discovered, could have given rise to the first species of Homo. Scientists also do not know exactly what factors favoured the evolution of a larger and more complex brain—the defining physical trait of modern humans.
Louis Leakey originally argued that the origin of Homo related directly to the development of toolmaking—specifically, the making of stone tools. Toolmaking requires certain mental skills and fine hand manipulation that may exist only in members of our own genus. Indeed, the name Homo habilis (meaning “handy man”) refers directly to the making and use of tools.
However, several species of australopiths lived at the same time as early Homo, making it unclear which species produced the earliest stone tools. Recent studies of australopith hand bones have suggested that at least one of the robust species, Australopithecus robustus, could have made tools. In addition, during the 1960s and 1970s researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that australopiths and the apes that preceded them probably also made some kinds of tools.
According to some scientists, however, early Homo probably did make the first stone tools. The ability to cut and pound foods would have been most useful to these smaller-toothed humans, whereas the robust australopiths could chew even very tough foods. Furthermore, early humans continued to make stone tools similar to the oldest known kinds for a time long after the gracile australopiths died out.
Some scientists think that a period of environmental cooling and drying in Africa set the stage for the evolution of Homo. According to this idea, many types of animals suited to the challenges of a drier environment originated during the period between about 2.8 million and 2.4 million years ago, including the first species of Homo. A toolmaking human might have had an advantage in obtaining alternative food sources as vegetation became sparse in increasingly dry environments. The new foods might have included underground roots and tubers, as well as meat obtained through scavenging or hunting. However, some scientists disagree with this idea, arguing that the period during which Homo evolved fluctuated between drier and wetter conditions, rather than just becoming dry. In this case, the making and use of stone tools and an expansion of the diet in early Homo—as well as an increase in brain size — may all have been adaptations to unpredictable and fluctuating environments. In either case, more scientific documentation is necessary to strongly support or refute the idea that early Homo arose as part of a larger trend of rapid species extinction and the evolution of many new species during a period of environmental change.
Early species of the genus Homo may have been the first human ancestors to eat meat on a regular basis. In the lower foreground of this artist’s rendering, a mother and child share meat from an animal carcass. Rather than hunting prey themselves, these early humans often may have scavenged the kills of predatory animals, using simple stone tools to cut up carcasses.
Paleoanthropologists generally recognize two species of early Homo — Homo habilis and H. rudolfensis (although other species may also have existed). The record is unclear because most of the early fossils that scientists have identified as species of Homo—rather than robust australopiths who lived at the same time—occur as isolated fragments. In many places, only teeth, jawbones, and pieces of skull — without any other skeletal remains—indicate that new species of smaller-toothed humans had evolved as early as 2.5 million years ago. Scientists cannot always tell whether these fossils belong to late-surviving gracile australopiths or early representatives of Homo. The two groups resemble each other because Homo likely descended directly from a species of gracile australopith.
Between 1960 and 1963, at Olduvai Gorge, Tanzania, a team led by Louis and Mary Leakey discovered the remains of an early human that seemed distinctly different from the australopiths. In 1964 Louis Leakey, South African palaeanthropologist Philip Tobias, and British primate researcher John Napier concluded that these remains represented a new species, which they named Homo habilis. The scientists placed the species in the genus Homo because its brain was estimated to be significantly larger than that of any known australopith. Other scientists questioned whether the amount of brain enlargement was sufficient for inclusion of the species in Homo, and even whether H. habilis was different from Australopithecus africanus, as the teeth of the two species look similar. However, scientists now widely accept both the genus and species names designated by the Olduvai team. According to recent estimates, H. habilis had a brain volume that ranged from 590 to 690 cu cm (36 to 42 cu in), well above the range for australopithecines.
H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species somewhat resemble those of australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust australopiths.
A fragmented skeleton of a female from Olduvai shows that she stood only about 1 m (3.3 ft) tall, and the ratio of the length of her arms to her legs was greater than that in the australopith Lucy. At least in the case of this individual, therefore, H. habilis had very apelike body proportions. However, H. habilis had more modern-looking feet and hands capable of producing tools. Some of the earliest stone tools from Olduvai have been found with H. habilis fossils, suggesting that this species made and used the tools at this site.
Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have indicated that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with some other fossils — exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size actually exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa.
In 1972 in East Turkana, Kenya, a research team led by Kenyan palaeanthropologist Richard Leakey discovered this 1.8-million-year-old skull. British-Kenyan zoologist Meave Leakey (Richard’s wife) reconstructed the skull, shown here, from over 150 fragments of bone. Because the size and several anatomical features of the skull differed from those of other early humans known at the time, scientists eventually classified it as belonging to a new species, Homo rudolfensis.
This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana). The best-known fossils of H. rudolfensis come from the area surrounding this lake and date from about 1.9 million years ago. Paleoanthropologists have not determined the entire time range during which H. rudolfensis may have lived.
This species had a larger face and body than did H. habilis. The cranial capacity of H. rudolfensis averaged about 750 cu cm (46 cu in). Scientists need more evidence to know whether the brain of H. rudolfensis in relation to its body size was larger than that proportion in H. habilis. A larger brain-to-body-size ratio can indicate increased mental abilities. H. rudolfensis also had fairly large teeth, approaching the size of those in robust australopiths. The discovery of even a partial fossil skeleton would reveal whether this larger form of early Homo had apelike or more modern body proportions. Scientists have found several modern-looking thighbones that date from between 2 million and 1.8 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m (5 ft) and 52 kg (114 lb).
Cro-Magnons, were the prehistoric people who lived in Europe from about 40,000 to 10,000 years ago. The Cro-Magnons were anatomically modern people, and are scientifically classified as Homo sapiens just as are people today. They were the first modern humans to inhabit Europe, living there at a time when glaciers covered much of the continent and the climate was often bitterly cold.
Cro-Magnons are named after the Cro-Magnon rock shelter in Les Eyzies, southwestern France, where their skeletal remains were first discovered in 1868. Like other modern humans, Cro-Magnons had a high forehead, small brow ridges, and a well-defined chin. These physical features set them apart from the Neanderthal, Homo neanderthalensis, early humans who lived in Europe from about 200,000 to 28,000 years ago.
Cro-Magnon people first appeared in Europe about 40,000 years ago, probably from the east and with an ultimate origin in Africa. By just under 30,000 years ago, the Cro-Magnons had entirely displaced the Neanderthal, who had previously been the only human occupants of Europe.
Like the Neanderthal, the Cro-Magnons were hunters and gatherers who lived off the bounty of nature. Neanderthal and earlier humans had skilfully made tools out of stone. The Cro-Magnons not only made an unprecedented variety of stone tools, they also made tools and weapons out of softer materials such as bone, ivory, and antler.
In cultural and technological sophistication, the Cro-Magnons far surpassed their Neanderthal predecessors, leaving behind a dazzling output of symbolic and decorative artifacts. By about 35,000 years ago, Cro-Magnons were making sculptures of the finest quality. They played music on bone flutes of surprisingly complex sound capability, and they unquestionably sang and danced. They made notations on bone and antler plaques. They buried their dead sometimes sumptuously, with elaborate grave goods. They decorated the walls of caves with some of the most impressive artwork ever made. Delicate, eyed bone needles dated to under 30,000 years ago suggest they made fitted clothing. They even baked ceramic figurines in simple but highly effective kilns. The Cro-Magnons, in a word, were us, with language and all the complexities of symbolic thought.
The origins of the Cro-Magnons’ complex cognitive abilities—which they exhibited virtually from the first moment of their occupation of Europe—are unknown. The most ancient indications of complex symbolic behaviors come from sites in Africa close to 100,000 years old, and the archaeological record in between is very thin. Without question, the Cro-Magnons provide us with the most dramatic evidence we have of the arrival of full-fledged modern human sensibility.
Neanderthal or Neanderthals, prehistoric humans who lived in Europe, the Middle East, and western Asia from about 200,000 to 28,000 years ago. Scientifically, they are usually classified as a separate species, Homo neanderthalensis. Although closely related to modern humans (Homo sapiens), Neanderthal were physically distinct. Short and stocky in build, they had large, protruding faces, prominent brows, and low, sloping foreheads. Their brains, however, were fully as big as modern humans. The typical lifespan of Neanderthal was much shorter than that of people today, with few individuals living beyond 40 years.
Scientists believe that Neanderthal regularly used fire. It would have provided them with heat, light, a way to cook food, and protection from carnivores. In this diorama from the Field Museum in Chicago, Illinois, a Neanderthal man prepares to start a fire.
Neanderthal have often been caricatured as clumsy, dim-witted brutes who walked with a slouch. This misconception emerged from faulty conclusions by the anthropologists who first studied Neanderthal fossils. In fact, Neanderthal walked completely upright without bent knees. Moreover, in recent years scientists have come to appreciate that Neanderthal were remarkable in their achievements and sophistication. They used fire, made complex stone tools and weapons, wore clothing, and buried their dead. They successfully adapted to harsh, cold climates of the late Ice Age and survived as a species for more than 150,000 years—longer than modern humans have existed.
Neanderthal were apparently the sole humans in Europe when the first members of Homo sapiens arrived there, probably from the Middle East, about 40,000 years ago. Just over 10,000 years after this event, Neanderthal became extinct. Some scientists theorize that competition or conflict with modern humans played a role in the extinction of Neanderthal, but this is a subject of debate. The exact reason for their disappearance remains a mystery.
The term Neanderthal comes from the discovery in 1856 of human fossils in the Little Feldhofer Cave of the Neander Valley, near Düsseldorf in western Germany (tal means “valley” in German). These bones were the first to be recognized as an early type of human. Since then, archaeologists have discovered more fossils of Neanderthal than of any other early human species. Because of this abundance of evidence, Neanderthal are among the best understood of all our fossil relatives.
Neanderthal were built on exactly the same basic body plan as modern humans are, but their skulls and skeletons reveal some significant differences. Their large brains were housed in long skulls (as measured front-to-back) with low foreheads and bulging rears, in contrast to the short skulls and high foreheads of modern humans. The brains of Neanderthal were, on average, as large as those of modern humans, and all were within the Homo sapiens size range. In front, the face was quite forwardly positioned compared to the flatter face of modern humans. Neanderthal had prominent brow ridges with a bony arch over each eye, and the cheekbones retreated sharply from a large nasal cavity (indicating a large nose). They had long and powerful jaws but no chin.
The skull of Homo neanderthalensis, left, differs considerably from that of anatomically modern humans, or Homo sapiens, right. Neanderthal had large, protruding faces, low, sloping foreheads, and heavy brow ridges. In contrast, modern humans have flatter faces, high foreheads, and less prominent brow ridges. Neanderthal also had more pronounced and powerful jaws than do modern humans.
`Neanderthal skeletons show numerous differences from those of modern humans, notably in the pelvis and in the limb joints, which were large and robust. Most Neanderthal were relatively short, with males standing about 1 m 60 cm (5 ft 3 in) tall, but some topped 1 m 83 cm (6 ft). Their short limbs and stocky bodies tended to minimize heat loss from the head and extremities and suggest an adaptation to extreme cold. The limb bones of Neanderthal were rather thick-walled in comparison to our own, and joint surfaces were large. Just as we do, Neanderthal differed a bit from place to place in stature and features.
In their heyday some 75,000 years ago, Neanderthal groups occupied a vast region encompassing Europe and southwestern Asia, from the Atlantic coast to western Asia and the eastern Mediterranean. They adjusted with considerable success to the climatic extremes of the late Ice Age, when bitterly cold glacial periods alternated with warmer periods known as interglacials. During glacial periods, much of Europe was covered with thick ice sheets and the European plains were treeless steppes and tundra. But even in milder interglacial periods, Neanderthal had to survive cold winters and long periods of food scarcity.
Most European Neanderthal groups flourished in the sheltered valleys of northern Spain, southwestern France, and elsewhere in southern Europe, where caves and rock shelters often provided good winter homes. In southwestern Asia, other Neanderthal groups adapted to arid, chilly conditions, where constant mobility was the key to survival. Some of these populations were more lightly built than their European relatives.
Most Neanderthal fossils have been discovered in rock shelters or cave mouths, but this does not mean Neanderthal did not camp in the open. Caves and rock shelters are simply more likely to preserve evidence of occupation than sites in the open. There are indications that Neanderthal rigged up artificial shelters where required.
Like other human species before them, Neanderthal were hunters and gatherers, living off the resources provided by nature. They almost certainly lived and hunted in small, nomadic groups that roamed over large territories. By all indications, Neanderthal were expert hunters who relied on exceptional stalking skills to get close to animals of all sizes. Animal bones found at Neanderthal sites suggest that they hunted most of the animals in their environment, including wild cattle, deer, horses, and reindeer. Many Neanderthal skeletons display signs of healed broken limbs and other traumatic injuries resulting from hunting accidents or other mishaps.
Analyses of Neanderthal bone chemistry suggest that Neanderthal lived mostly on meat, but they did not depend on hunting alone. Scavenging of dead carcasses, rather than active hunting, might account for a proportion of the animal bones found a Neanderthal living sites. Seeds and other plant remains found at Neanderthal sites demonstrate that wild plant foods were an important part of their diet.
Evidence suggests that Neanderthal might at least occasionally have practised cannibalism, a behavior documented among the earliest humans in Europe 780,000 years ago. Neanderthal bones from a cave in southeastern France show cut marks indicating they were scraped of flesh with stone knives.
The Neanderthal made stone tools quite skilfully and relied on them for their survival. Triangular spear points may have been shafted (attached to a wooden handle or shaft) to make hunting weapons. Scrapers, hand axes, and backed knives (sharp flakes with one side dulled to fit comfortably in the hand) would have been highly effective for butchering animals and scraping hides for clothing or shelter. Sharp-edged chopper stones were probably used for cracking open animal bones to get at marrow. The cutting surfaces of Neanderthal tools also show wear consistent with woodworking. Wood does not preserve well, and only a handful of wooden artifacts have been recovered from Neanderthal sites. However, a pre-Neanderthal find in Germany of finely-shaped throwing spears suggests that Neanderthal would have made these too and thus have been quite sophisticated ambush-hunters.
Neanderthal made stone tools by striking flakes from rock “cores.” The cores were carefully selected and prepared so that only a single blow was normally required to detach a flake. A number of relatively standardized flakes were sometimes produced from a single core. These sharp flakes served as “blanks” that were further worked and shaped into the desired tools. Suitable stone was sometimes rare, and often tools were sharpened and resharpened to make new tools, yielding a whole variety of shapes and sizes. Unlike the Cro-Magnons, their modern human successors, Neanderthal rarely used bone or antler as materials for tool making.
Neanderthal used this same basic toolmaking technology, termed Mousterian by archeologists, for most of their existence. But about 35,000 years ago—shortly after the arrival of modern humans in Europe—they acquired a more advanced toolmaking technology, called Châtelperronian where it occurred in France, characterized by long, thin stone “blades” and greater use of antler and bone. Similar technology is also associated with modern humans of this era, leading some experts to suggest that Neanderthal somehow learned from modern humans how to make these tools. Alternatively, Neanderthal may have invented this toolmaking technology independently.
Most experts believe that the Neanderthal must have had clothing of some sort in order to survive the climate in Europe, which was at times severely cold. However, little is known about what type of clothing they wore. They could have easily made simple skin cloaks by scraping animal hides with stone tools, and they did make bone awls that would have served to pierce hides for binding. Neanderthal never developed perforated bone needles, which would have allowed them to fashion tailored, layered clothing.
Neanderthals also controlled fire. At some Neanderthal sites, thick piles of ash and burned rocks attest to years of campfires burning. No evidence exists that reveals how Neanderthal used fire, but it would have provided them with heat, light, and a way to cook food.
Neanderthal were the first humans known to have buried their dead. Numerous burial pits have been discovered in the floors of caves and rock shelters, sometimes accompanied by stone tools or a few animal bones. At one Neanderthal grave, in Shanidar Cave in Iraq, large amounts of pollen were discovered, perhaps suggesting a burial with flowers. A Neanderthal child skeleton from Teshik-Tash in the western foothills of the Himalayas lay in a pit surrounded by six pairs of mountain goat horns. At many other burial sites, Neanderthal skeletons have their knees and arms drawn close to the chest in a fetal position, possibly but not necessarily indicating a ritual burial position.
To some authorities, these burials and grave items represent evidence that Neanderthal practiced religious rituals, believed in the afterlife, and had the ability to think symbolically. Other experts challenge such interpretations as overly enthusiastic, and offer more mundane explanations. For example, stone tools and animal bones were common objects in Neanderthal living sites and could have been buried in graves unintentionally, as part of the filling process. Neanderthal may have buried their dead simply to avoid attracting unwelcome scavengers to their settlements, not because burials held symbolic importance. The flower pollen found in the Shanidar grave could have been deposited by burrowing rodents.
The possibility that Neanderthal used language, a hallmark of symbolic thought, has intrigued researchers for decades. Some scholars believe the Neanderthal had fully articulate speech. Some support for this claim comes from a Neanderthal skeleton discovered in Kebara, Israel. The skeleton still possesses its hyoid bone, a bone situated at the base of the tongue that affects the movement of the larynx, where speech originates. The Kebara Neanderthal hyoid is identical to that of modern humans, suggesting these people were physically capable of articulate speech. While some studies of the base of the Neanderthal skull suggest the larynx may have been positioned too high in the throat to produce articulate speech, the bending of the cranial base in earlier fossils suggests that the ability to produce the sounds of speech may have been present in human precursors well before Neanderthal times.
Objects with possible symbolic connotations have been discovered at a few Neanderthal sites, including pierced animal teeth that may have been used as pendants, incised bone fragments, and a polished plaque made from a mammoth tooth. Bone and tooth ornaments, including an elegant bone pendant, were found with Neanderthal remains at Arcy-sur-Cure in central France. But the extreme rarity of these objects contrasts sharply with the remarkable abundance of symbolic and decorative artifacts—such as cave paintings, figurines, carvings, and beads—produced by the Cro-Magnons, the Neanderthal’ successors in Europe. Thus, it seems likely that Neanderthal did not have symbolic thought or language as we know it today, though their intuitive intelligence was probably highly developed.
The earliest evidence for the human occupation of Europe comes from Gran Dolina cavern in the Atapuerca hills of northern Spain, where in the mid-1990s archaeologists unearthed human fossils dated to 780,000 years ago. These fossils are sometimes classified as a separate species, Homo mauritanicus (or Homo antecessor). Between 500,000 and 200,000 years ago, humans in Europe show some of the features of Neanderthal, but not all of them, as well as some features of modern humans. Some of these specimens have been classified as Homo heidelbergensis. Whether either of these species represents the direct ancestor of the Neanderthal or modern humans is not known, though Neanderthal origins must lie with ancient human groups in Europe.
The Cro-Magnons, a group of early Homo sapiens, entered Europe about 40,000 years ago, a time when Neanderthal were the region’s only human inhabitants. Neanderthal and modern humans thus coexisted in Europe for more than 10,000 years. Did Neanderthal interbreed with modern humans? Why did the Neanderthal die out around 28,000 years ago while modern humans thrived?
Scientists disagree about whether Neanderthal were a distinct species from modern humans. Largely because Neanderthal were big-brained, some palaeanthropologists continue to regard them as a version of ourselves. They classify Neanderthal as a subspecies of Homo sapiens—Homo sapiens neanderthalensis—and anatomically modern humans as Homo sapiens sapiens. According to this school of thought, Neanderthal and Cro-Magnons interbred, and anatomically distinctive Neanderthal features were simply “swamped” genetically by waves of Cro-Magnons intruding into the Neanderthal’ homeland. If true, some Neanderthal genes probably survive today in modern humans of European descent.
Supporters of interbreeding between Neanderthal and modern humans turn to fossil evidence; some late Neanderthal fossils are said to look more “modern” than earlier ones, and some early moderns are said to have some “Neanderthal-like” features. The claim that these fossils represent evidence of interbreeding is controversial and remains unproven. At a few sites there is evidence of a short-lived culture that combined Neanderthal and Cro-Magnon elements, but this was probably achieved without biological intermixing.
The more we learn about Neanderthal, the clearer it becomes that they deserve recognition as a species in their own right, Homo neanderthalensis. In a dramatic series of studies, scientists have extracted fragments of deoxyribonucleic acid (DNA, the basic unit of heredity) from several Neanderthal specimens and compared them to DNA from living humans. The studies show that Neanderthal DNA is genetically distant from modern human DNA, falling well outside the range of variation seen among humans today. This large difference in genetic structure indicates that Neanderthal could not have been ancestral to modern humans. The researchers calculated that the last common ancestor of modern humans and Neanderthal probably lived 500,000 to 600,000 years ago. Taken together, these genetic studies offer powerful evidence that Homo neanderthalensis was a fully individuated species.
If Neanderthal and modern humans in Europe were indeed different species, as the evidence overwhelmingly suggests, they could not have interbred. No biologically significant exchange of genes would have been possible. The existence of two distinct species also suggests that they competed for the same territory. How this competition played itself out is unknown, but the general pattern in Europe seems to have been one of abrupt replacement of Neanderthal by moderns at site after site. The end result was the extinction of the Neanderthal. Whether this extinction occurred because of direct conflict or indirect economic competition is not known, but a combination of these factors seems likely
By around 6 million years ago in Africa, an apelike species had evolved with two important traits that distinguished it from apes: (1) small canine, or eye, teeth (teeth next to the four incisors, or front teeth) and (2) bipedalism—that is, walking on two legs as the primary form of locomotion. Scientists refer to these earliest human species as australopithecines, or australopiths for short. The earliest australopith species known today belong to three genera: Sahelanthropus, Orrorin, and Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as “southern ape,” in reference to South Africa, where the first known australopith fossils were found.
The Great Rift Valley, a region in eastern Africa in which past movements in Earth’s crust have exposed ancient deposits of fossils, has become famous for its australopith finds. Countries in which scientists have found australopith fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, australopiths ranged widely over the African continent.
`Fossils from several different early australopith species that lived between 4 million and 2 million years ago clearly show a variety of adaptations that mark the transition from ape to human. The very early period of this transition, prior to 4 million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features.
Fossils reveal much about the physical build and activities of early australopiths, but not everything about outward physical features such as the colour and texture of skin and hair, or about certain behaviours, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes—particularly the African apes—to better understand how early australopiths might have looked and behaved, and how the transition from ape to human might have occurred.
For example, australopiths probably resembled the great apes in characteristics such as the shape of the face and the amount of hair on the body. Australopiths also had brains roughly equal in size to those of the great apes, so they probably had apelike mental abilities. Their social life probably resembled that of chimpanzees.
Most of the distinctly human physical qualities in australopiths related to their bipedal stance. Before australopiths, no mammal had ever evolved an anatomy for habitual upright walking. Australopiths also had small canine teeth, as compared with long canines found in almost all other catarrhine primates.
Other characteristics of australopiths reflected their ape ancestry. They had a low cranium behind a projecting face, and a brain size of 390 to 550 cu cm (24 to 34 cu in)—in the range of an ape’s brain. The body weight of australopiths, as estimated from their bones, ranged from 27 to 49 kg (60 to 108 lb), and they stood 1.1 to 1.5 m (3.5 to 5 ft) tall. Their weight and height compare closely to those of chimpanzees (chimp height measured standing). Some australopith species had a large degree of sexual dimorphism—males were much larger than females—a trait also found in gorillas, orangutans, and some other primates.
Australopiths also had curved fingers and long thumbs with a wide range of movement. In comparison, the fingers of apes are longer, more powerful, and more curved, making them extremely well adapted for hanging and swinging from branches. Apes also have very short thumbs, which limits their ability to manipulate small objects. Paleoanthropologists speculate as to whether the long and dexterous thumbs of australopiths allowed them to use tools more efficiently than do apes.
The anatomy of australopiths shows a number of adaptations for bipedalism, in both the upper and lower body. Adaptations in the lower body included the following: The australopith ilium, or pelvic bone, which rises above the hip joint, was much shorter and broader than it is in apes. This shape enabled the hip muscles to steady the body during each step. The australopith pelvis also had a bowl-like shape, which supported the internal organs in an upright stance. The upper legs angled inward from the hip joints, which positioned the knees to better support the body during upright walking. The legs of apes, on the other hand, are positioned almost straight down from the hip, so that when an ape walks upright for a short distance, its body sways from side to side. Australopiths also had shorter and less flexible toes than do apes. The toes worked as rigid levers for pushing off the ground during each bipedal step.
Other adaptations occurred above the pelvis. The australopith spine had an S-shaped curve, which shortened the overall length of the torso and gave it rigidity and balance when standing. By contrast, apes have a relatively straight spine. The australopith skull also had an important adaptation related to bipedalism. The opening at the bottom of the skull through which the spinal cord attaches to the brain, called the foramen magnum, was positioned more forward than it is in apes. This position set the head in balance over the upright spine.
Australopiths clearly walked upright on the ground, but paleoanthropologists debate whether the earliest humans also spent a significant amount of time in the trees. Certain physical features indicate that they spent at least some of their time climbing in trees. Such features include their curved and elongated fingers and elongated arms. However, their fingers, unlike those of apes, may not have been long enough to allow them to brachiate through the treetops. Study of fossil wrist bones suggests that early australopiths had the ability to lock their wrists, preventing backward bending at the wrist when the body weight was placed on the knuckles of the hand. This could mean that the earliest bipeds had an ancestor that walked on its knuckles, as African apes do.
Compared with apes, humans have very small canine teeth. Apes—particularly males—have thick, projecting, sharp canines that they use for displays of aggression and as weapons to defend themselves. The oldest known bipeds, who lived at least 6 million years ago, still had large canines by human standards, though not as large as in apes. By 4 million years ago australopiths had developed the human characteristic of having smaller, flatter canines. Canine reduction might have related to an increase in social cooperation among humans and an accompanying decrease in the need for males to make aggressive displays.
The australopiths can be divided into an early group of species, known as gracile australopiths, which arose prior to 3 million years ago; and a later group, known as robust australopiths, which evolved after 3 million years ago. The gracile Australopiths — of which several species evolved between 4.5 million and 3 million years ago — generally had smaller teeth and jaws. The later-evolving robusts had larger faces with large jaws and molars (cheek teeth). These traits indicate powerful and prolonged chewing of food, and analyses of wear on the chewing surface of robust australopith molar teeth support this idea. Some fossils of early australopiths have features resembling those of the later species, suggesting that the robusts evolved from one or more gracile ancestors.
Paleoanthropologists recognize at least eight species of early australopiths. These include the three earliest established species, which belong to the genera Sahelanthropus, Orrorin, and Ardipithecus, a species of the genus Kenyanthropus, and four species of the genus Australopithecus.
The oldest known australopith species is Sahelanthropus tchadensis. Fossils of this species were first discovered in 2001 in northern Chad, Central Africa, by a research team led by French paleontologist Michel Brunet. The researchers estimated the fossils to be between 7 million and 6 million years old. One of the fossils is a cracked yet nearly complete cranium that shows a combination of apelike and humanlike features. Apelike features include small brain size, an elongated brain case, and areas of bone where strong neck muscles would have attached. Humanlike features include small, flat canine teeth, a short middle part of the face, and a massive brow ridge (a bony, protruding ridge above the eyes) similar to that of later human fossils. The opening where the spinal cord attaches to the brain is tucked under the brain case, which suggests that the head was balanced on an upright body. It is not certain that Sahelanthropus walked bipedally, however, because bones from the rest of its skeleton have yet to be discovered. Nonetheless, its age and humanlike characteristics suggest that the human and African ape lineages had divided from one another by at least 6 million years ago.
In addition to reigniting debate about human origins, the discovery of Sahelanthropus in Chad significantly expanded the known geographic range of the earliest humans. The Great Rift Valley and South Africa, from which almost all other discoveries of early human fossils came, are apparently not the only regions of the continent that preserve the oldest clues of human evolution.
Orrorin tugenensis lived about 6 million years ago. This species was discovered in 2000 by a research team led by French paleontologist Brigitte Senut and French geologist Martin Pickford in the Tugen Hills region of central Kenya. The researchers found more than a dozen early human fossils dating between 6.2 million and 6 million years old. Among the finds were two thighbones that possess a groove indicative of an upright stance and bipedal walking. Although the finds are still being studied, the researchers consider these thighbones to be the oldest evidence of habitual two-legged walking. Fossilized bones from other parts of the skeleton show apelike features, including long, curved finger bones useful for strong grasping and movement through trees, and apelike canine and premolar teeth. Because of this distinctive combination of ape and human traits, the researchers gave a new genus and species name to these fossils, Orrorin tugenensis, which in the local language means “original man in the Tugen region.” The age of these fossils suggests that the divergence of humans from our common ancestor with chimpanzees occurred before 6 million years ago.
In 1994 an Ethiopian member of a research team led by American paleoanthropologist Tim White discovered human fossils estimated to be about 4.4 million years old. White and his colleagues gave their discovery the name Ardipithecus ramidus. Ramid means “root” in the Afar language of Ethiopia and refers to the closeness of this new species to the roots of humanity. At the time of this discovery, the genus Australopithecus was scientifically well established. White devised the genus name Ardipithecus to distinguish this new species from other australopiths because its fossils had a very ancient combination of apelike and humanlike traits. More recent finds indicate that this species may have lived as early as 5.8 million to 5.2 million years ago. It has been suggested, however, that these older fossils may represent a related species called Ardipithecus kadabba.
The teeth of Ardipithecus ramidus had a thin outer layer of enamel — a trait also seen in the African apes but not in other australopith species or most older fossil apes. This trait suggests a fairly close relationship with an ancestor of the African apes. In addition, the skeleton shows strong similarities to that of a chimpanzee but has slightly reduced canine teeth and adaptations for bipedalism.
In 1965 a research team from Harvard University discovered a single arm bone of an early human at the site of Kanapoi in northern Kenya. The researchers estimated this bone to be 4 million years old, but could not identify the species to which it belonged or return at the time to look for related fossils. It was not until 1994 that a research team, led by British-born Kenyan palaeanthropologist Meave Leakey, found numerous teeth and fragments of bone at the site that could be linked to the previously discovered fossil. Leakey and her colleagues determined that the fossils were those of a very primitive species of australopith, which was given the name Australopithecus anamensis. Researchers have since found other A. anamensis fossils at nearby sites, dating between about 4.2 million and 3.9 million years old. The skull of this species appears apelike, while its enlarged tibia (lower leg bone) indicates that it supported its full body weight on one leg at a time, as in regular bipedal walking.
Australopithecus anamensis was quite similar to another, much better-known species, A. afarensis, a gracile australopith that thrived in eastern Africa between about 3.8 million and 3 million years ago. The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered by American palaeanthropologist Donald Johanson in 1974 at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Scientists have identified several hundred fossils of A. afarensis from Hadar, including a collection representing at least 13 individuals of both sexes and various ages, all from a single site.
One of the most complete specimens of A. afarensis found so far was announced in 2006. A team led by Ethiopian scientist Zeresenay Alemseged unearthed the partial skeleton of a three-year-old female at Dikika in the Afar region of Ethiopia. Nicknamed 'Selam,' the Dikika child dates from around 3.3 million years ago. The well-preserved bones provide previously undocumented details of the skull and skeleton. Some features such as the shape of the shoulder blades, the long, curved fingers, and the semi-circular ear canals involved in balance are more ape-like, suggesting an adaptation for climbing trees. However, the leg bones and feet indicate an ability to walk upright even at an early age. The shape of the brain was preserved and its size indicates the species grew to adulthood more slowly than chimpanzees, a characteristic of later hominids, including modern humans. The hyoid bone that supports the tongue was found as well. The bone is crucial to speech in modern humans but the shape in the Dikika child is like that found in modern great apes, and not humans.
Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli. This site, dated at 3.6 million years old, is best known for its spectacular trails of bipedal human footprints. Preserved in hardened volcanic ash, these footprints were discovered in 1978 by a research team led by British paleoanthropologist Mary Leakey. They provide irrefutable evidence that australopiths regularly walked bipedally.
Paleoanthropologists have debated interpretations of the characteristics of A. afarensis and its place in the human family tree. One controversy centers on the Laetoli footprints, which some scientists believe show that the foot anatomy and gait of A. afarensis did not exactly match those of modern humans. This observation may indicate that early australopiths did not live primarily on the ground or at least spent a significant amount of time in the trees. The skeleton of Lucy also indicates that A. afarensis had longer, more powerful arms than most later human species, suggesting that this species was adept at climbing trees.
Another controversy has to do with the scientific classification of the A. afarensis fossils. Compared with Lucy, who stood only 1.1 m (3.5 ft) tall, other fossils identified as A. afarensis from Hadar and Laetoli came from individuals who stood up to 1.5 m (5 ft) tall. This great difference in size leads some scientists to suggest that the entire set of fossils now classified as A. afarensis actually represents two species. Most scientists, however, believe the fossils represent one highly dimorphic species—that is, a species that has two distinct forms (in this case, two sizes). Supporters of this view note that both large (presumably male) and small (presumably female) adults occur together in one site at Hadar.
In 1974 American palaeanthropologist Donald Johanson discovered the skeleton of “Lucy,” a 3.2-million-year-old female of the early human species Australopithecus afarensis, at Hadar, Ethiopia. Until the late 1990s, Lucy’s was the most complete skeleton of an australopithecine ever found. Australopithecines were primitive humans that first evolved over 4.4 million years ago. Lucy’s pelvis and leg bones, similar to those of modern humans, indicate that she regularly walked upright.
A third controversy arises from the claim that A. afarensis was the common ancestor of both later australopiths and the modern human genus, Homo. While this idea remains a strong possibility, the similarity between this and another australopith species—one from southern Africa, named Australopithecus africanus—makes it difficult to decide which of the two species gave rise to the genus Homo.
Australopithecus africanus thrived in the Transvaal region of what is now South Africa between about 3.3 million and 2.5 million years ago. Australian-born anatomist Raymond Dart discovered this species—the first known Australopiths — in 1924 at Taung, South Africa. The specimen, that of a young child, came to be known as the Taung Child. For decades after this discovery, almost no one in the scientific community believed Dart’s claim that the skull came from an ancestral human. In the late 1930s teams led by Scottish-born South African paleontologist Robert Broom unearthed many more A. africanus skulls and other bones from the Transvaal site of Sterkfontein.
A. africanus generally had a more globular braincase and less primitive-looking face and teeth than did A. afarensis. Thus, some scientists consider the southern species of early australopith to be a likely ancestor of the genus Homo. According to other scientists, however, certain heavily built facial and cranial features of A. africanus from Sterkfontein identify it as an ancestor of the robust australopiths that lived later in the same region. In 1998 a research team led by South African paleoanthropologist Ronald Clarke discovered an almost complete early australopith skeleton at Sterkfontein. This important find may resolve some of the questions about where A. africanus fits in the story of human evolution.
Working in the Lake Turkana region of northern Kenya, a research team led by paleontologist Meave Leakey uncovered in 1999 a cranium and other bone remains of an early human that showed a mixture of features unseen in previous discoveries of early human fossils. The remains were estimated to be 3.5 million years old, and the cranium’s small brain and earhole were similar to those of the very earliest humans. Its cheekbone, however, joined the rest of the face in a forward position, and the region beneath the nose opening was flat. These are traits found in later human fossils from around 2 million years ago, typically those classified in the genus Homo. Noting this unusual combination of traits, researchers named a new genus and species, Kenyanthropus platyops, or “flat-faced human from Kenya.” Before this discovery, it seemed that only a single early human species, Australopithecus afarensis, lived in East Africa between 4 million and 3 million years ago. Yet Kenyanthropus indicates that a diversity of species, including a more humanlike lineage than A. afarensis, lived in this time period, just as in most other eras in human prehistory.
The human fossil record is poorly known between 3 million and 2 million years ago, which makes recent finds from the site of Bouri, Ethiopia, particularly important. From 1996 to 1998, a research team led by Ethiopian paleontologist Berhane Asfaw and American paleontologist Tim White found the skull and other skeletal remains of an early human specimen about 2.5 million years old. The researchers named it Australopithecus garhi; the word garhi means “surprise” in the Afar language. The specimen is unique in having large incisors and molars in combination with an elongated forearm and thighbone. Its powerful arm bones suggest a tree-living ancestry, but its longer legs indicate the ability to walk upright on the ground. Fossils of A. garhi are associated with some of the oldest known stone tools, along with animal bones that were cut and cracked with tools. It is possible, then, that this species was among the first to make the transition to stone toolmaking and to eating meat and bone marrow from large animals.
By 2.7 million years ago the later, robust australopiths had evolved. These species had what scientists refer to as megadont cheek teeth—wide molars and premolars coated with thick enamel. Their incisors, by contrast, were small. The robusts also had an expanded, flattened, and more vertical face than did gracile australopiths. This face shape helped to absorb the stresses of strong chewing. On the top of the head, robust australopiths had a sagittal crest (ridge of bone along the top of the skull from front to back) to which thick jaw muscles attached. The zygomatic arches (which extend back from the cheek bones to the ears), curved out wide from the side of the face and cranium, forming very large openings for the massive chewing muscles to pass through near their attachment to the lower jaw. Altogether, these traits indicate that the robust australopiths chewed their food powerfully and for long periods.
Other ancient animal species that specialized in eating plants, such as some types of wild pigs, had similar adaptations in their facial, dental, and cranial anatomy. Thus, scientists think that the robust australopiths had a diet consisting partly of tough, fibrous plant foods, such as seed pods and underground tubers. Analyses of microscopic wear on the teeth of some robust australopith specimens appear to support the idea of a vegetarian diet, although chemical studies of fossils suggest that the southern robust species may also have eaten meat.
Scientists originally used the word robust to refer to the late australopiths out of the belief that they had much larger bodies than did the early, gracile australopiths. However, further research has revealed that the robust australopiths stood about the same height and weighed roughly the same amount as Australopithecus afarensis and A. africanus.
The earliest known robust species, Australopithecus aethiopicus, lived in eastern Africa by 2.7 million years ago. In 1985 at West Turkana, Kenya, American palaeanthropologist Alan Walker discovered a 2.5-million-year-old fossil skull that helped to define this species. It became known as the “black skull” because of the color it had absorbed from minerals in the ground. The skull had a tall sagittal crest toward the back of its cranium and a face that projected far outward from the forehead. A. aethiopicus shared some primitive features with A. afarensis—that is, features that originated in the earlier East African australopith. This may indicate that A. aethiopicus evolved from A. afarensis.
Australopithecus boisei, the other well-known East African robust australopith, lived over a long period of time, between about 2.3 million and 1.4 million years ago. In 1959 Mary Leakey discovered the original fossil of this species—a nearly complete skull — at the site of Olduvai Gorge in Tanzania. Kenyan-born palaeanthropologist Louis Leakey, husband of Mary, originally named the new species Zinjanthropus boisei (Zinjanthropus translates as “East African man”). This skull — dating from 1.8 million years ago—has the most specialized features of all the robust species. It has a massive, wide and dished-in face capable of withstanding extreme chewing forces, and molars four times the size of those in modern humans. Since the discovery of Zinjanthropus, now recognized as an australopith, scientists have found great numbers of A. boisei fossils in Tanzania, Kenya, and Ethiopia.
The southern robust species, called Australopithecus robustus, lived between about 1.8 million and 1.3 million years ago in the Transvaal, the same region that was home to A. africanus. In 1938 Robert Broom, who had found many A. africanus fossils, bought a fossil jaw and molar that looked distinctly different from those in A. africanus. After finding the site of Kromdraai, from which the fossil had come, Broom collected many more bones and teeth that together convinced him to name a new species, which he called Paranthropus robustus (Paranthropus meaning “beside man”). Later scientists dated this skull at about 1.5 million years old. In the late 1940s and 1950 Broom discovered many more fossils of this species at the Transvaal site of Swartkrans.
Many scientists believe that robust australopiths represent a distinct evolutionary group of early humans because these species share features associated with heavy chewing. According to this view, Australopithecus aethiopicus diverged from other australopiths and later gave rise to A. boisei and A. robustus. Paleoanthropologists who strongly support this view think that the robusts should be classified in the genus Paranthropus, the original name given to the southern species. Thus, these three species are sometimes referred to as P. aethiopicus, P. boisei, and P. robustus.
Other paleoanthropologists believe that the eastern robust species, A. aethiopicus and A. boisei, may have evolved from an early australopith of the same region, perhaps A. afarensis. According to this view, A. africanus gave rise only to the southern species, A. robustus. Scientists refer to such a case—in which two or more independent species evolve similar characteristics in different places or at different times—as parallel evolution. If parallel evolution occurred in australopiths, the robust species would make up two separate branches of the human family tree.
The last robust australopiths died out about 1.4 million years ago. At about this time, climate patterns around the world entered a period of fluctuation, and these changes may have reduced the food supply on which robusts depended. Interaction with larger-brained members of the genus Homo, such as Homo erectus, may also have contributed to the decline of late australopiths, although no compelling evidence exists of such direct contact. Competition with several other species of plant-eating monkeys and pigs, which thrived in Africa at the time, may have been an even more important factor. But the reasons why the robust australopiths became extinct after flourishing for such a long time are not yet known for sure.
Scientists have several ideas about why australopiths first split off from the apes, initiating the course of human evolution. Virtually all hypotheses suggest that environmental change was an important factor, specifically in influencing the evolution of bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis.
The global climate cooled and became drier between 8 million and 5 million years ago, near the end of the Miocene Epoch. According to the savanna hypothesis, this climate change broke up and reduced the area of African forests. As the forests shrunk, an ape population in eastern Africa became separated from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to its drier environment, which contained larger areas of grassy savanna.
The expansion of dry terrain favored the evolution of terrestrial living, and made it more difficult to survive by living in trees. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators—activities that also may have required the ability to communicate well. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans.
Critics of the savanna hypothesis argue against it on several grounds, but particularly for two reasons. First, discoveries by a French scientific team of australopith fossils in Chad, in Central Africa, suggests that the environments of East Africa may not have been fully separated from those farther west. Second, recent research suggests that open savannas were not prominent in Africa until sometime after 2 million years ago.
Criticism of the savanna hypothesis has spawned alternative ideas about early human evolution. The woodland-mosaic hypothesis proposes that the early australopiths evolved in patchily wooded areas — a mosaic of woodland and grassland — that offered opportunities for feeding both on the ground and in the trees, and that ground feeding favoured bipedalism.
The variability hypothesis suggests that early australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Scientists have found that this range of habitats existed at the time when the early australopiths evolved. So the development of new anatomical characteristics—particularly bipedalism — combined with an ability to climb trees, may have given early humans the versatility to live in a variety of habitats.
Scientists also have many ideas about which benefits of bipedalism may have influenced its evolution. Ideas about the benefits of regular bipedalism include that it freed the hands, making it easier to carry food and tools; allowed early humans to see over tall grass to watch for predators; reduced exposure of the body to hot sun and increased exposure to cooling winds; improved the ability to hunt or use weapons, which became easier with an upright posture; and made extensive feeding from bushes and low branches easier than it would have been for a quadruped. Scientists do not overwhelmingly support any one of these ideas. Recent studies of chimpanzees suggest, though, that the ability to feed more easily might have particular relevance. Chimps move on two legs most often when they feed from the ground on the leaves and fruits of bushes and low branches. Chimps cannot, however, walk in this way over long distances.
Bipedalism in early humans would have enabled them to travel efficiently over long distances, giving them an advantage over quadrupedal apes in moving across barren open terrain between groves of trees. In addition, the earliest humans continued to have the advantage from their ape ancestry of being able to escape into the trees to avoid predators. The benefits of both bipedalism and agility in the trees may explain the unique anatomy of australopiths. Their long, powerful arms and curved fingers probably made them good climbers, while their pelvis and lower limb structure was reshaped for upright walking.
People belong to the genus Homo, which first evolved at least 2.3 million to 2.5 million years ago. The earliest members of this genus differed from the australopiths in at least one important respect—they had larger brains than did their predecessors.
The evolution of the modern human genus can be divided roughly into three periods: early, middle, and late. Species of early Homo resembled gracile australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of middle Homo began perhaps between 2 million and 1.8 million years ago, overlapping with the end of early Homo. Species of middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from middle to late Homo probably occurred sometime around 200,000 years ago. Species of late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution.
The origin of the genus Homo has long intrigued paleoanthropologists and prompted much debate. One of several known species of australopiths, or one not yet discovered, could have given rise to the first species of Homo. Scientists also do not know exactly what factors favoured the evolution of a larger and more complex brain—the defining physical trait of modern humans.
Louis Leakey originally argued that the origin of Homo related directly to the development of toolmaking—specifically, the making of stone tools. Toolmaking requires certain mental skills and fine hand manipulation that may exist only in members of our own genus. Indeed, the name Homo habilis (meaning “handy man”) refers directly to the making and use of tools.
However, several species of australopiths lived at the same time as early Homo, making it unclear which species produced the earliest stone tools. Recent studies of australopith hand bones have suggested that at least one of the robust species, Australopithecus robustus, could have made tools. In addition, during the 1960s and 1970s researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that australopiths and the apes that preceded them probably also made some kinds of tools.
According to some scientists, however, early Homo probably did make the first stone tools. The ability to cut and pound foods would have been most useful to these smaller-toothed humans, whereas the robust australopiths could chew even very tough foods. Furthermore, early humans continued to make stone tools similar to the oldest known kinds for a time long after the gracile australopiths died out.
Some scientists think that a period of environmental cooling and drying in Africa set the stage for the evolution of Homo. According to this idea, many types of animals suited to the challenges of a drier environment originated during the period between about 2.8 million and 2.4 million years ago, including the first species of Homo. A toolmaking human might have had an advantage in obtaining alternative food sources as vegetation became sparse in increasingly dry environments. The new foods might have included underground roots and tubers, as well as meat obtained through scavenging or hunting. However, some scientists disagree with this idea, arguing that the period during which Homo evolved fluctuated between drier and wetter conditions, rather than just becoming dry. In this case, the making and use of stone tools and an expansion of the diet in early Homo—as well as an increase in brain size—may all have been adaptations to unpredictable and fluctuating environments. In either case, more scientific documentation is necessary to strongly support or refute the idea that early Homo arose as part of a larger trend of rapid species extinction and the evolution of many new species during a period of environmental change.
Paleoanthropologists generally recognize two species of early Homo—Homo habilis and H. rudolfensis (although other species may also have existed). The record is unclear because most of the early fossils that scientists have identified as species of Homo—rather than robust australopiths who lived at the same time—occur as isolated fragments. In many places, only teeth, jawbones, and pieces of skull—without any other skeletal remains—indicate that new species of smaller-toothed humans had evolved as early as 2.5 million years ago. Scientists cannot always tell whether these fossils belong to late-surviving gracile australopiths or early representatives of Homo. The two groups resemble each other because Homo likely descended directly from a species of gracile australopith.
Between 1960 and 1963, at Olduvai Gorge, Tanzania, a team led by Louis and Mary Leakey discovered the remains of an early human that seemed distinctly different from the australopiths. In 1964 Louis Leakey, South African palaeanthropologist Philip Tobias, and British primate researcher John Napier concluded that these remains represented a new species, which they named Homo habilis. The scientists placed the species in the genus Homo because its brain was estimated to be significantly larger than that of any known australopith. Other scientists questioned whether the amount of brain enlargement was sufficient for inclusion of the species in Homo, and even whether H. habilis was different from Australopithecus africanus, as the teeth of the two species look similar. However, scientists now widely accept both the genus and species names designated by the Olduvai team. According to recent estimates, H. habilis had a brain volume that ranged from 590 to 690 cu cm (36 to 42 cu in), well above the range for australopithecines.
H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species somewhat resemble those of australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust australopiths.
A fragmented skeleton of a female from Olduvai shows that she stood only about 1 m (3.3 ft) tall, and the ratio of the length of her arms to her legs was greater than that in the australopith Lucy. At least in the case of this individual, therefore, H. habilis had very apelike body proportions. However, H. habilis had more modern-looking feet and hands capable of producing tools. Some of the earliest stone tools from Olduvai have been found with H. habilis fossils, suggesting that this species made and used the tools at this site.
Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have indicated that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with some other fossils — exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size actually exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa.
This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana). The best-known fossils of H. rudolfensis come from the area surrounding this lake and date from about 1.9 million years ago. Paleoanthropologists have not determined the entire time range during which H. rudolfensis may have lived.
This species had a larger face and body than did H. habilis. The cranial capacity of H. rudolfensis averaged about 750 cu cm (46 cu in). Scientists need more evidence to know whether the brain of H. rudolfensis in relation to its body size was larger than that proportion in H. habilis. A larger brain-to-body-size ratio can indicate increased mental abilities. H. rudolfensis also had fairly large teeth, approaching the size of those in robust australopiths. The discovery of even a partial fossil skeleton would reveal whether this larger form of early Homo had apelike or more modern body proportions. Scientists have found several modern-looking thighbones that date from between 2 million and 1.8 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m (5 ft) and 52 kg (114 lb).
By about 1.9 million years ago, the period of middle Homo had begun in Africa. Until recently, paleoanthropologists recognized one species in this period, Homo erectus. Many now recognize three species of middle Homo: H. ergaster, H. erectus, and H. heidelbergensis. However, some still think H. ergaster is an early African form of H. erectus, or that H. heidelbergensis is a late form of H. erectus.
The skulls and teeth of early African populations of middle Homo differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. H. ergaster makes a better candidate for an ancestor of the modern human line because Asian H. erectus has some specialized features not seen in some later humans, including our own species. H. heidelbergensis has similarities to both H. erectus and the later species H. neanderthalensis, although it may have been a transitional species between middle Homo and the line to which modern humans belong.
Homo ergaster probably first evolved in Africa around 2 million years ago. This species had a rounded cranium with a brain size of between 700 and 850 cu cm (49 to 52 cu in), a prominent brow ridge, small teeth, and many other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including relatively thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.5 million years ago.
The most important fossil of this species yet found is a nearly complete skeleton of a young male from West Turkana, Kenya, which dates from about 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also determined from patterns of tooth eruption and bone growth that the boy had died when he was between 9 and 12 years old.
The Turkana boy, as the skeleton is known, had elongated leg bones and arm, leg, and trunk proportions that essentially match those of a modern human, in sharp contrast with the apelike proportions of H. habilis and Australopithecus afarensis. He appears to have been quite tall and slender. Scientists estimate that, had he grown into adulthood, the boy would have reached a height of 1.8 m (6 ft) and a weight of 68 kg (150 lb). The anatomy of the Turkana boy indicates that H. ergaster was particularly well adapted for walking and perhaps for running long distances in a hot environment (a tall and slender body dissipates heat well) but not for any significant amount of tree climbing.
The oldest humanlike fossils outside of Africa have also been classified as H. ergaster, dated around 1.75 million years old. These finds, from the Dmanisi site in the southern Caucasus Mountains of Georgia, consist of several crania, jaws, and other fossilized bones. Some of these are strikingly like East African H. ergaster, but others are smaller or larger than H. ergaster, suggesting a high degree of variation within a single population.
H. ergaster, H. rudolfensis, and H. habilis, in addition to possibly two robust australopiths, all might have coexisted in Africa around 1.9 million years ago. This finding goes against a traditional paleoanthropological view that human evolution consisted of a single line that evolved progressively over time — an australopith species followed by early Homo, then middle Homo, and finally H. sapiens. It appears that periods of species diversity and extinction have been common during human evolution, and that modern H. sapiens has the rare distinction of being the only living human species today.
Although H. ergaster appears to have coexisted with several other human species, they probably did not interbreed. Mating rarely succeeds between two species with significant skeletal differences, such as H. ergaster and H. habilis. Many Paleoanthropologists now believe that H. ergaster descended from an earlier population of Homo—perhaps one of the two known species of early Homo—and that the modern human line descended from H. ergaster. Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for a few million years. The earliest human species known to have spread in large numbers beyond the African continent was first discovered in Southeast Asia. In 1891 Dutch physician Eugène Dubois found the cranium of an early human on the Indonesian island of Java. He named this early human Pithecanthropus erectus, or “erect ape-man.” Today paleoanthropologists refer to this species as Homo erectus.
H. erectus appears to have evolved in Africa from earlier populations of H. ergaster, and then spread to Asia sometime between 1.8 million and 1.5 million years ago. The youngest known fossils of this species, from the Solo River in Java, may date from as recently as 53,000 to 27,000 years ago (although that dating is controversial). So H. erectus was a very successful species—both widespread, having lived in Africa and much of Asia, and long-lived, having survived for possibly more than 1.5 million years.
H. erectus had a low and rounded braincase that was elongated from front to back, a prominent brow ridge, and an adult cranial capacity of 800 to 1,250 cu cm (50 to 80 cu in), an average twice that of the australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Although it had much smaller teeth than did the australopiths, it had a heavy and strong jaw.
In the 1920's and 1930's German anatomist and physical anthropologist Franz Weidenreich excavated the most famous collections of H. erectus fossils from a cave at the site of Zhoukoudian (Chou-k’ou-tien), China, near Beijing (Peking). Scientists dubbed these fossil humans Sinanthropus pekinensis, or Peking Man, but others later reclassified them as H. erectus. The Zhoukoudian cave yielded the fragmentary remains of over 30 individuals, ranging from about 500,000 to 250,000 years old. These fossils were lost near the outbreak of World War II, but Weidenreich had made excellent casts of his finds. Further studies at the cave site have yielded more H. erectus remains.
Other important fossil sites for this species in China include Lantian, Yuanmou, Yunxian, and Hexian. Researchers have also recovered many tools made by H. erectus in China at sites such as Nihewan and Bose, and other sites of similar age (at least 1 million to 250,000 years old).
Ever since the discovery of H. erectus, scientists have debated whether this species was a direct ancestor of later humans, including H. sapiens. The last populations of H. erectus—such as those from the Solo River in Java — may have lived as recently as 53,000 to 27,000 years ago, at the same time as did populations of H. sapiens. Modern humans could not have evolved from these late populations of H. erectus, a much more primitive type of human. However, earlier East Asian populations could have given rise to H. sapiens.
Many paleoanthropologists believe that early humans migrated into Europe by 800,000 years ago, and that these populations were not Homo erectus. A growing number of scientists refer to these early migrants into Europe — who predated both Neanderthal and H. sapiens in the region — as H. heidelbergensis. The species name comes from a 500,000-year-old jaw found near Heidelberg, Germany.
Scientists have found few human fossils in Africa for the period between 1.2 million and 600,000 years ago, during which H. heidelbergensis or its ancestors first migrated into Europe. Populations of H. ergaster (or possibly H. erectus) appear to have lived until at least 800,000 years ago in Africa, and possibly until 500,000 years ago in northern Africa. When these populations disappeared, other massive-boned and larger-brained humans—possibly H. heidelbergensis - appear to have replaced them. Scientists have found fossils of these stockier humans at sites in Bodo, Ethiopia; Saldanha (also known as Elandsfontein), South Africa; Ndutu, Tanzania; and Kabwe, Zimbabwe.
Scientists have come up with at least three different interpretations of these African fossils. Some scientists place the fossils in the species H. heidelbergensis and think that this species gave rise to both the Neanderthal (in Europe) and H. sapiens (in Africa). Others think that the European and African fossils belong to two distinct species, and that the African populations—which, in this view, were not H. heidelbergensis but a separate species—gave rise to H. sapiens. Yet other scientists advocate a long-held view that H. erectus and H. sapiens belong to a single evolving lineage, and that the African fossils belong in the category of archaic H. sapiens (archaic meaning not fully anatomically modern).
The fossil evidence does not clearly favour any of these three interpretations over another. A growing number of fossils from Asia, Africa, and Europe have features that are intermediate between early H. ergaster and H. sapiens. This kind of variation makes it hard to decide how to identify distinct species and to determine which group of fossils represents the most likely ancestor of later humans.
Humans evolved in Africa and lived only there for as long as 4 million years or more, so scientists wonder what finally triggered the first human migration out of Africa (a movement that coincided with the spread of early human populations throughout the African continent). The answer to this question depends, in part, on knowing exactly when that first migration occurred. Some studies claim that sites in Asia and Europe contain crude stone tools and fossilized fragments of humanlike teeth that date from more than 1.8 million years ago. Although these claims remain unconfirmed, small populations of humans may have entered Asia prior to 1.8 million years ago, followed by a more substantial spread between 1.6 million and 1 million years ago. Early humans reached northeastern Asia by around 1.4 million years ago, inhabiting a region close to the perpetually dry deserts of northern China. The first major habitation of central and western Europe, on the other hand, does not appear to have occurred until between 1 million and 500,000 years ago.
Scientists once thought that advances in stone tools could have enabled early humans such as Homo erectus to move into Asia and Europe, perhaps by helping them to obtain new kinds of food, such as the meat of large mammals. If African human populations had developed tools that allowed them to hunt large game effectively, they would have had a reliable source of food wherever they went. In this view, humans first migrated into Eurasia based on a unique cultural adaptation.
By 1.5 million years ago, early humans had begun to make new kinds of tools, which scientists call Acheulean. Common Acheulean tools included large handaxes and cleavers. While these new tools might have helped early humans to hunt, the first known Acheulean tools in Africa date from later than the earliest known human presence in Asia. Also, most East Asian sites over 200,000 years old contain only simply shaped cobble and flake tools. In contrast, Acheulean tools were more finely crafted, larger, and more symmetrical. Thus, the earliest settlers of Eurasia did not have a true Acheulean technology, and advances in toolmaking alone cannot explain the spread out of Africa.
Another possibility is that the early spread of humans to Eurasia was not unique, but rather part of a wider migration of meat-eating animals, such as lions and hyenas. The human migration out of Africa occurred during the early part of the Pleistocene Epoch, between 1.8 million and 780,000 years ago. Many African carnivores spread to Eurasia during the early Pleistocene, and humans could have moved along with them. In this view, H. erectus was one of many meat-eating species to expand into Eurasia from Africa, rather than a uniquely adapted species. Relying on meat as a primary food source might have allowed many meat-eating species, including humans, to move through many different environments without having to quickly learn about unfamiliar and potentially poisonous plants.
However, the migration of humans to eastern Asia may have occurred gradually and through lower latitudes and environments similar to those of Africa. If East African populations of H. erectus moved at only 1.6 km (1 mi) every 20 years, they could have reached Southeast Asia in 150,000 years. Over this amount of time, humans could have learned about and begun relying on edible plant foods. Thus, eating meat may not have played a crucial role in the first human migrations to new continents. Careful comparison of animal fossils, stone tools, and early human fossils from Africa, Asia, and Europe will help scientists to better determine what factors motivated and allowed humans to venture out of Africa for the first time.
The origin of our own species, Homo sapiens, is one of the most hotly debated topics in paleoanthropology. This debate centers on whether or not modern humans have a direct relationship to H. erectus or to the Neanderthal, a well-known, more modern group of humans who evolved within the past 250,000 years. Paleoanthropologists commonly use the term anatomically modern Homo sapiens to distinguish people of today from these similar predecessors.
Traditionally, paleoanthropologists classified as Homo sapiens any fossil human younger than 500,000 years old with a braincase larger than that of H. erectus. Thus, many scientists who believe that modern humans descend from a single line dating back to H. erectus use the name archaic Homo sapiens to refer to a wide variety of fossil humans that predate anatomically modern H. sapiens. The term archaic denotes a set of physical features typical of Neanderthal and other species of late Homo prior to modern Homo sapiens. These features include a combination of a robust skeleton, a large but low braincase (positioned somewhat behind, rather than over, the face), and a lower jaw lacking a prominent chin. In this sense, Neanderthal are sometimes classified as a subspecies of archaic H. sapiens—H. sapiens neanderthalensis. Other scientists think that the variation in archaic fossils actually falls into clearly identifiable sets of traits, and that any type of human fossil exhibiting a unique set of traits should have a new species name. According to this view, the Neanderthal belong to their own species, H. neanderthalensis.
Neanderthal lived in areas ranging from western Europe through central Asia from about 200,000 to about 28,000 years ago. The name Neanderthal (sometimes spelled Neanderthal) comes from fossils found in 1856 in the Feldhofer Cave of the Neander Valley in Germany (tal—a modern form of thal—means “valley” in German). Scientists realized several years later that prior discoveries—at Engis, Belgium, in 1829 and at Forbes Quarry, Gibraltar, in 1848—also represented Neanderthal. These two earlier discoveries were the first early human fossils ever found.
In the past, scientists claimed that Neanderthal differed greatly from modern humans. However, the basis for this claim came from a faulty reconstruction of a Neanderthal skeleton that showed it with bent knees and a slouching gait. This reconstruction gave the common but mistaken impression that Neanderthal were dim-witted brutes who lived a crude lifestyle.
In the contrary, Neanderthal, like the species that preceded them, walked fully upright without a slouch or bent knees. In addition, their cranial capacity was quite large at about 1,500 cu cm (about 90 cu in), slightly larger on average than that of modern humans. (The difference probably relates to the greater muscle mass of Neanderthal as compared with modern humans, which usually correlates with a larger brain size.)
Compared with earlier humans, Neanderthal had a high degree of cultural sophistication. They appear to have performed symbolic rituals, such as the burial of their dead. Neanderthal fossils — including a number of fairly complete skeletons — are quite common compared to those of earlier forms of Homo, in part because of the Neanderthal practice of intentional burial. Neanderthal also produced sophisticated types of stone tools known as Mousterian, which involved creating blanks (rough forms) from which several types of tools could be made.
Along with many physical similarities, Neanderthal differed from modern humans in several ways. The typical Neanderthal skull had a low forehead, a large nasal area (suggesting a large nose), a forward-projecting nasal and cheek region, a prominent brow ridge with a bony arch over each eye, a non-projecting chin, and an obvious space behind the third molar (in front of the upward turn of the lower jaw).
Neanderthal also had a more heavily built and large-boned skeleton than do modern humans. Other Neanderthal skeletal features included a bowing of the limb bones in some individuals, broad scapulae (shoulder blades), hip joints turned outward, a long and thin pubic bone, short lower leg and arm bones relative to the upper bones, and large surfaces on the joints of the toes and limb bones. Together, these traits made a powerful, compact body of short stature—males averaged 1.7 m (5 ft 5 in) tall and 84 kg (185 lb), and females averaged 1.5 m (5 ft) tall and 80 kg (176 lb).
The short, stocky build of Neanderthal conserved heat and helped them withstand extremely cold conditions that prevailed in temperate regions beginning about 70,000 years ago. The last known Neanderthal fossils come from western Europe and date from approximately 28,000 years ago.
Along the same time as Neanderthal populations grew in number in Europe and parts of Asia, other populations of nearly modern humans arose in Africa and Asia. Scientists also commonly refer to these fossils, which are distinct from but similar to those of Neanderthal, as archaic. Fossils from the Chinese sites of Dali, Maba, and Xujiayao display the long, low cranium and large face typical of archaic humans, yet they also have features similar to those of modern people in the region. At the cave site of Jebel Irhoud, Morocco, scientists have found fossils with the long skull typical of archaic humans but also the modern traits of a somewhat higher forehead and flatter midface. Fossils of humans from East African sites older than 100,000 years — such as Ngaloba in Tanzania and Eliye Springs in Kenya — also seem to show a mixture of archaic and modern traits.
One of the most unusual branches of the human family tree was discovered on the Indonesian island of Flores in 2003 and first described in 2004. A research team digging in a cave, Liang Bua, uncovered the nearly complete skeleton of what appeared to be a miniature human that lived as recently as 18,000 years ago. The specimen, believed to be an adult female, was estimated to stand only about 1 m (3.3 ft) tall. Its brain, estimated at 380 cu cm (23 cu in), was as small as those of chimpanzees and the smallest australopiths. It had fairly large brow ridges, and its teeth were large relative to the rest of the skull. Despite being extremely small-brained, it apparently made simple stone tools. On the basis of these unique traits, the researchers assigned the skeleton to a new species, Homo floresiensis. The researchers concluded that H. floresiensis was probably descended from H. erectus, although this continues to be debated. The diminutive stature and small brain of H. floresiensis may have resulted from island dwarfism—an evolutionary process that results from long-term isolation on a small island with limited food resources and a lack of predators. Pygmy elephants on Flores, now extinct, showed the same adaptation.
The oldest known footprints of an anatomically modern human are embedded in rock north of Cape Town, South Africa. Geologist David Roberts and paleoanthropologist Lee Berger announced the discovery of the footprints in August 1997. A human being made the footprints about 117,000 years ago by walking through wet sand, which eventually hardened into rock.
The oldest known fossils that possess skeletal features typical of modern humans date from 195,000 years ago. Several key features distinguish the skulls of modern humans from those of archaic species. These features include a much smaller brow ridge, if any; a globe-shaped braincase; and a flat or only slightly projecting face of reduced size, located under the front of the braincase. Among all mammals, only humans have a face positioned directly beneath the frontal lobe (forward-most area) of the brain. As a result, modern humans tend to have a higher forehead than did Neanderthal and other archaic humans. The cranial capacity of modern humans ranges from about 1,000 to 2,000 cu cm (60 to 120 cu in), with the average being about 1,350 cu cm (80 cu in).
Scientists have found both fragmentary and nearly complete cranial fossils of early anatomically modern Homo sapiens from the sites of Singha, Sudan; Omo, Ethiopia; Klasies River Mouth, South Africa; and Skhul Cave, Israel. Based on these fossils, many scientists conclude that modern H. sapiens had evolved in Africa by 195,000 years ago and started spreading to diverse parts of the world beginning on a route through the Near East sometime before 90,000 years ago.
Paleoanthropologists are engaged in an ongoing debate about where modern humans evolved and how they spread around the world. Differences in opinion rest on the question of whether the evolution of modern humans took place in a small region of Africa or over a broad area of Africa and Eurasia. By extension, opinions differ as to whether modern human populations from Africa displaced all existing populations of earlier humans, eventually resulting in their extinction.
Those who think modern humans originated only in Africa and then spread around the world support what is known as the out of Africa hypothesis. Those who think modern humans evolved over a large region of Eurasia and Africa support the so-called multi-regional hypothesis.
Researchers have conducted many genetic studies and carefully assessed fossils to determine which of these hypotheses agrees more with scientific evidence. The results of this research do not entirely confirm or reject either one. Therefore, some scientists think a compromise between the two hypotheses is the best explanation. The debate between these views has implications for how scientists understand the concept of race in humans. The question raised is whether the physical differences among modern humans evolved deep in the past or relatively recently.
According to the out of Africa hypothesis, also known as the replacement hypothesis, early populations of modern humans from Africa migrated to other regions and entirely replaced existing populations of archaic humans. The replaced populations would have included the Neanderthal and any surviving groups of Homo erectus. Supporters of this view note that many modern human skeletal traits evolved relatively recently—within the past 200,000 years or so—suggesting a single, common origin. In addition, the anatomical similarities shared by all modern human populations far outweigh those shared by pre-modern and modern humans within particular geographic regions. Furthermore, biological research indicates that most new species of organisms, including mammals, arise from small, geographically isolated populations.
According to the multi-regional hypothesis, also known as the continuity hypothesis, the evolution of modern humans began when Homo erectus spread throughout much of Eurasia around 1 million years ago. Regional populations retained some unique anatomical features for hundreds of thousands of years, but they also mated with populations from neighbouring regions, exchanging heritable traits with each other. This exchange of heritable traits is known as gene flow.
Through gene flow, populations of H. erectus passed on a variety of increasingly modern characteristics, such as increases in brain size, across their geographic range. Gradually this would have resulted in the evolution of more modern looking humans throughout Africa and Eurasia. The physical differences among people today, then, would result from hundreds of thousands of years of regional evolution. This is the concept of continuity. For instance, modern East Asian populations have some skull features that scientists also see in H. erectus fossils from that region.
Some critics of the multi-regional hypothesis claim that it wrongly advocates a scientific belief in race and could be used to encourage racism. Supporters of the theory point out, however, that their position does not imply that modern races evolved in isolation from each other, or that racial differences justify racism. Instead, the theory holds that gene flow linked different populations together. These links allowed progressively more modern features, no matter where they arose, to spread from region to region and eventually become universal among humans.
Scientists have weighed the out of Africa and multiregional hypotheses against both genetic and fossil evidence. The results do not unanimously support either one, but weigh more heavily in favour of the out of Africa hypothesis.
Geneticists have studied the amount of difference in the DNA (deoxyribonucleic acid) of different populations of humans. DNA is the molecule that contains our heritable genetic code. Differences in human DNA result from mutations in DNA structure. Mutations may result from exposure to external elements such as solar radiation or certain chemical compounds, while others occur naturally at random.
Geneticists have calculated rates at which mutations can be expected to occur over time. Dividing the total number of genetic differences between two populations by an expected rate of mutation provides an estimate of the time when the two shared a common ancestor. Many estimates of evolutionary ancestry rely on studies of the DNA in cell structures called mitochondria. This DNA is referred to as mtDNA (mitochondrial DNA). Unlike DNA from the nucleus of a cell, which codes for most of the traits an organism inherits from both parents, mtDNA inheritance passes only from a mother to her offspring. MtDNA also accumulates mutations about ten times faster than does DNA in the cell nucleus (the location of most DNA). The structure of mtDNA changes so quickly that scientists can easily measure the differences between one human population and another. Two closely related populations should have only minor differences in their mtDNA. Conversely, two very distantly related populations should have large differences in their mtDNA.
MtDNA research into modern human origins has produced two major findings. First, the entire amount of variation in mtDNA across human populations is small in comparison with that of other animal species. This means that all human mtDNA originated from a single ancestral lineage — specifically, a single female — fairly recently and has been mutating ever since. Most estimates of the mutation rate of mtDNA suggest that this female ancestor lived about 200,000 years ago. In addition, the mtDNA of African populations varies more than that of peoples in other continents. This suggests that the mtDNA of African populations has changed for a longer time than it has in populations of any other region, and that all living people inherited their mtDNA from one woman in Africa, who is sometimes called the Mitochondrial Eve. Some geneticists and anthropologists have concluded from this evidence that modern humans originated in a small population in Africa and spread from there.
MtDNA studies have weaknesses, however, including the following four. First, the estimated rate of mtDNA mutation varies from study to study, and some estimates put the date of origin closer to 850,000 years ago, the time of Homo erectus. Second, mtDNA makes up a small part of the total genetic material that humans inherit. The rest of our genetic material — about 400,000 times more than the amount of MtDNA — came from many individuals living at the time of the African Eve, conceivably from many different regions. Third, the time at which modern mtDNA began to diversify does not necessarily coincide with the origin of modern human biological traits and cultural abilities. Fourth, the smaller amount of modern mtDNA diversity outside of Africa could result from times when European and Asian populations declined in numbers, perhaps due to climate changes.
Despite these criticisms, many geneticists continue to favor the out of Africa hypothesis of modern human origins. Studies of nuclear DNA also suggest an African origin for a variety of genes. Furthermore, in a remarkable series of studies in the late 1990s, scientists recovered mtDNA from the first Neanderthal fossil found in Germany and two other Neanderthal fossils. In each case, the mtDNA does not closely match that of modern humans. This finding suggests that at least some Neanderthal populations had diverged from the line to modern humans by 500,000 to 600,000 years ago. This also suggests that Neanderthal represent a separate species from modern H. sapiens. In another study, however, mtDNA extracted from a 62,000-year-old Australian H. sapiens fossil was found to differ significantly from modern human mtDNA, suggesting a much wider range of mtDNA variation within H. sapiens than was previously believed. According to the Australian researchers, this finding lends support to the multi-regional hypothesis because it shows that different populations of H. sapiens, possibly including Neanderthal, could have evolved independently in different parts of the world.
With genetic research, fossil evidence also does not entirely support or refute either of the competing hypotheses of modern human origins. However, many scientists see the balance of evidence favoring an African origin of modern H. sapiens within the past 200,000 years. The oldest known modern-looking skulls come from Africa and date from perhaps 195,000 years ago. The next oldest come from the Near East, where they date from about 90,000 years ago. Fossils of modern humans in Europe do not exist from before about 40,000 years ago. In addition, the first modern humans in Europe — often referred to as Cro-Magnon people — had elongated lower leg bones, as did African populations that were adapted to warm, tropical climates. This suggests that populations from warmer regions replaced those in colder European regions, such as the Neanderthal.
Fossils also show that populations of modern humans lived at the same time and in the same regions as did populations of Neanderthal and Homo erectus, but that each retained its distinctive physical features. The different groups overlapped in the Near East and Southeast Asia for between about 30,000 and 50,000 years. The maintenance of physical differences for this amount of time implies that archaic and modern humans either could not or generally did not interbreed. To some scientists, this also means that the Neanderthal belong to a separate species, H. neanderthalensis, and that migratory populations of modern humans entirely replaced archaic humans in both Europe and eastern Asia.
On the other hand, fossils of archaic and modern humans in some regions show continuity in certain physical characteristics. These similarities may indicate multi-regional evolution. For example, both archaic and modern skulls of eastern Asia have flatter cheek and nasal areas than do skulls from other regions. By contrast, the same parts of the face project forward in the skulls of both archaic and modern humans of Europe. Assuming that these traits were influenced primarily by genetic inheritance rather than environmental factors, archaic humans may have given rise to modern humans in some regions or at least interbred with migrant modern-looking humans.
Each of the competing major hypotheses of modern human origins has its strengths and weaknesses. Genetic evidence appears to support the out of Africa hypothesis. In the western half of Eurasia and in Africa, this hypothesis also seems the better explanation, particularly in regard to the apparent replacement of Neanderthal by modern populations. At the same time, the multiregional hypothesis appears to explain some of the regional continuity found in East Asian populations.
Therefore, many paleoanthropologists advocate a theory of modern human origins that combines elements of the out of Africa and the multiregional hypotheses. Humans with modern features may have first emerged in Africa or come together there as a result of gene flow with populations from other regions. These African populations may then have replaced archaic humans in certain regions, such as western Europe and the Near East. But elsewhere — especially in East Asia—gene flow may have occurred among local populations of archaic and modern humans, resulting in distinct and enduring regional characteristics.
All three of these views—the two competing positions and the compromise—acknowledge the strong biological unity of all people. In the multi-regional hypothesis, this unity results from hundreds of thousands of years of continued gene flow among all human populations. According to the out of Africa hypothesis, on the other hand, similarities among all living human populations result from a recent common origin. The compromise position accepts both of these as reasonable and compatible explanations of modern human origins.
The scientific classification of primates reflects evolutionary relationships between individual species and groups of species. Strepsirhini (meaning ‘turned-nosed’) primates - of which the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians - evolved earliest and are the most primitive forms of primates. The earliest monkeys and apes evolved from ancestral haplorhine (meaning ‘simple-nosed’) primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors.
Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, Janurary the discovery of the most ancient hominid (humanlike)species yet uncovered, which they have named Australopithecus afarensis. The fossils on which they base this claim are about three million to four million years old and were found during the 1970's at two widely separated localities in East Africa. The majority were collected at Hadar, a remote region of the Afar Depression of Ethiopia, by Johanson; the others were uncovered in northern Tanzania at Laetolil, 30 miles south of Olduvai Gorge, by anthropologist Mary Leakey. The Hadar material consists of bones from at least 35 individuals and includes the best preserved australopithecine skeleton yet found. Nicknamed "Lucy" by Johanson, this skeleton is about 40 percent complete and is evidently of a female who stood about 3.5-4 feet tall and lived some three million years ago. The Laetolil fossils, closer to four million years old, are astonishingly similar in many ways to the Hadar material. Because of the remarkable completeness and good preservation of both fossil collections, we are afforded a previously unavailable glimpse of early human evolution. Analysis suggests that these creatures had rather small brains, no larger than those of the gorilla, but the leg and pelvic bones clearly indicate that A. afarensis walked on two legs like humans. In these respects the newly described fossils do not differ substantially from previously described australopithecine species, which date from about 1.5 million to 2.5 million years ago. All previously recognized hominids, however, show larger cheek teeth (molars) and smaller front teeth (incisors and canines) than the apes. A. afarensis, in contrast, shows very broad incisors and large, projecting canines, somewhat more like those of an apes. The appearance of such primitive dental characteristics in an australopithecine has profound implications for evolutionary history. The most widely held theory states that the evolutionary lines leading to modern humans and apes diverged some 12 million to 15 million years ago, when apes from which humans are descended moved out of the trees and began to exploit the resources of open grasslands for food. This change in habitats is thought to have produced the characteristic humanlike denotation, which is more efficient at chewing tough food, such as seeds, roots, and tubers, than is the denotation of the apes. Fossil teeth and jaws of a human type characterize a creature called Ramapithecus, which lived around ten million years ago and is generally considered a human ancestor. However, the primitive, more primordial ape of A. afarensis has now cast doubt on the status of Ramapithecus as an ancestral hominid and made unclear the ultimate reason for the differentiation of human ancestors from the apes.
Researchers in South Africa having discovered what they believe are the oldest and best-preserved skulls and skeletons of one of humanity's earliest ancestors, according to a report published in the December 9, 1998, issue of The South African Journal of Science. Paleontologists said the fossilized remains would be as much as two million years older than the oldest previously known complete hominid skeletons. The new finding is expected to reveal much about the anatomy and evolution of early humans, and may rank among the most important breakthroughs in Paleoanthropology (the archaeological study of early human evolution).“It is one of many key elements from ape to man,” said Ronald J. Clarke, a paleoanthropologist at the University of Witwatersrand in Johannesburg, South Africa, who led the team that made the discovery. The skeleton was discovered in the fossil-rich Sterkfontein Caves, near Krugersdorp in northeastern South Africa. The skeleton is of a small, adult hominid who was about 1.2-m (4-ft) tall and weighed about 32 k. (70 lb.). Clarke's team dated the bones at 3.2 million to 3.6 million years old.
The bones are believed to belong to a species of australopithecine, an early hominid that had human and apelike features. However, most of the bones remain embedded in rock within the cave. Paleontologists can identify fully and study the skeleton until it is removed, a process that Clarke said could take a year or longer. Clarke made the discovery after unexpectedly finding four hominid foot bones in a box of unsorted fossils at the university in 1994. Another search of boxes in a university storage room in May 1997 revealed more foot and lower leg bones. To Clarke's astonishment, all of the bones appeared to belong to the same hominid.
Clarke's initial discovery, announced in 1995, added new evidence to a longstanding debate among anthropologists about the path of early hominid development. Clarke and several of his colleagues argued that the bones of the specimen, dubbed Little Foot, reflected a transition from four-legged tree dwellers to two-legged creatures capable of walking upright. In particular, Clarke said the specimen's humanlike ankles and grasping, an apelike big toe suggested that the creature was a capable tree climber who could easily walk on two legs. Other anthropologists dismissed the idea, however, asserting that humans evolved from plains-dwelling hominids and did not live in trees.
After finding additional bones in 1997, Clarke believed that the rest of the skeleton might be present in Sterkfontein's Silberberg Grotto, where the bones had been originally excavated. Within days of searching, his assistants discovered a piece of fossilized bone protruding from the cave wall that perfectly matched one of Clarke's fossil fragments. Although the excavation is at a preliminary stage, Clarke said the remainder of the skeleton might be present and intact, laying face downward in limestone.
Before Clarke's find, the most comprehensive australopithecine skeleton was an Australopithecus afarensis specimen known as Lucy, discovered by anthropologist Donald Johanson in Ethiopia in 1974 and dated at 3.2 million years old. Lucy, however, is only about 40 percent complete. The oldest known complete hominid skeleton was a species of Homo erectus found in Kenya and dated at 1.5 million years old.
The new discovery is considered extraordinary because the fossil record of early hominids is so fragmentary. Paleontologists have had to piece together knowledge about ancient human species by using bone fragments derived from many individuals, making generalizations about anatomy difficult. Once the bones have been chipped from the grotto rock, scientists will examine the hips and legs to determine whether or not the creature could easily climb trees. In addition, they hope skeletal features will give them clues about the specimen's sex and how these early hominids lived, including their likely diet and possible foraging behaviours.
Scientists also believe the skeleton's intact skull could shed light on another key puzzle of early human evolution: the relation between a brain size and upright locomotion. Many experts believe that it was the ability to walk on two legs - rather than brain size or use of tools - that set the human lineage apart from all other primates
Another mystery scientists will explore is whether the fossils represent either an example of Australopithecus afarensis (like Lucy), of a southern African hominid species, known as Australopithecus africanus, or possibly a new species together. If the species is unrelated to Lucy and is older, then it could force anthropologists to reconsider their views about hominid evolution in Africa. Because of Lucy's age, many scientists now believe that A. afarensis is a common ancestor to all succeeding australopithecine species.
But some paleontologists cautioned that the age of the new find had not yet been conclusively established and could be only about two million year’s old. The most accurate forms of dating require the presence of volcanic ash, which contains radioactive elements that decay in a predicable manner. No such material was present in the cave.
To date the skeleton, Clarke and his team finds distinctive animal fossils near the hominid remains. The age of these animal fossils had already been determined at other datable sites. This technique is not foolproof, however, because movements in the rock layers could make fossils from animals that did not coexist appear next to each other, experts said.
Yet, Spanish paleoanthropologists recently described the fossil remains of several human ancestors from the last Ice Age that were found at a cave site in northern Spain. Did their findings identify a distinct human ancestral species, as the Spaniards suggested? The debate over the paths of human evolution continued as new findings about Human ancestors came of Spanish paleoanthropologists, and have added to the complexity of theories about early humans in Europe with their recent description of the fossil remains of several Ice Age human ancestors found in northern Spain. The researchers suggested that these early humans, who lived more than 780,000 years ago, may have been a separate species that preceded both modern humans and the now-extinct early humans known as Neanderthals.
The researchers said that among the fossils were the facial bones of a boy showing both primitive and modern features and identifying these human ancestors as a distinct species. They suggested the name Homo’s antecessor for the proposed new species. Spanish paleoanthropologist José Bermúdez de Castro of the National Museum of Natural Sciences in Madrid, Spain, and his colleagues described the fossils in the May 30, 1997, issues of the journal Science.
Although anthropologists agreed that the fossil find was very important, most were not ready to accept the ancient humans as representing a new species. Anthropologists pointed out that not only are the dental and facial bones of a boy scant evidence on which to identify a new species, but also there is a chance that some of the boy's features were in an intermediate stage that would have changed when the boy reached adulthood. The Spanish researchers' proposed path of human evolution also was controversial, because it pushes groups of early humans off the direct line leading to modern humans, suggesting that there may have been more dead ends in human evolution than previously thought.
The Spanish scientists first reported finding this group of fossils, the oldest remains of prehumans ever found in Europe, in August 1995. Previously the oldest known Europeans were a group of early humans sometimes classified as a separate species, Homo heidelbergensis. The earliest known specimens from this group date from roughly 500,000 years ago. Using a technique known as Paleomagnetic analysis, the Spanish researchers dated the fossil remains found recently in northern Spain to at least 780,000 years ago, in the Pleistocene Epoch.
Paleomagnetic dating is because the direction of the earth's geomagnetic field has reversed often during the history of the world. The dates of these irregular reversals in geomagnetic polarity have been well documented. Currently the geomagnetic polarity of the earth is facing north, but less than a million years ago it faced south. Internal magnetic traces in the layers of rock found near the fossils shown that the fossils had been buried before the earth's magnetic field last switched direction, from south to north, 780,000 years ago.
The Spanish paleoanthropologists found more than 80 fossils representing at least six individuals, including both juveniles and adults. The fossils were found in a deep pit at a cave site known as Gran Dolina, in the Atapuerca Mountains near the city of Burgos in northern Spain.
The picture of early human residence in Europe is unclear at best. Anthropologists agree that hominids - a family of bipedal primates that includes modern human beings and all extinct species of early humans - have their origins in Africa. The Spanish anthropologists speculated that the early humans they called Homo antecessor may have first evolved in Africa from a primitive human classified by some paleoanthropologists as Homo ergaster and by others as early Homo erectus and that Homo sapiens developed from Homo antecessor in Africa. The researchers further proposed that Homo antecessor migrated to Europe, and that Homo heidelbergensis and then Neanderthals (also called Homo sapiens neanderthals) evolved from this line.
Under this proposed set of relationships, Homo heidelbergensis would not be a common ancestor of Homo sapiens and Neanderthals, as currently believed by many anthropologists. The Spanish anthropologists' model also pushes some later members of A Homo erectus well off the direct line leading to modern human beings. An early Homo erectus appeared about 1.9 million to two million years ago in Africa, and more recent examples of this early human have been found in China and Java, Indonesia. In the Spanish scientists' proposed model, Asian members of A Homo erectus become a side path on human evolution, rather than an intermediate step between Homo ergaster and Homo heidelbergensis.
The human family tree has usually become less linear and more complex over the past decade as anthropologists have made more discoveries.
Archaeological study covers an extremely long span of time and a great variety of subjects. The earliest subjects of archaeological study date from the origins of humanity. These include fossil remains believed to be of human ancestors who lived 3.5 million to 4.5 million years ago. The earliest archaeological sites include those at Hadar, Ethiopia; Olduvai Gorge and Laetoli, Tanzania; East Turkana, Kenya; and elsewhere in East Africa. These sites contain evidence of the first appearance of bipedal (upright-walking), apelike early humans. Laetoli even reveals footprints of humans from 3.6 million years ago. Some sites also contain evidence of the earliest use of simple tools. Archaeologists have also recorded how primitive forms of humans spread out of Africa into Asia about 1.8 million years ago, then into Europe about 900,000 years ago.
The first physically modern humans, The Homo sapiens, appeared in tropical Africa between 200,000 and 150,000 years ago - dates determined by molecular biologists and archaeologists working together. Dozens of archaeological sites throughout Asia and Europe show how people migrated from Africa and settled these two continents during the last Ice Age (100,000 to 15,000 years ago). Archaeological studies have also provided much information about the people who first arrived in the Americas more than 12,000 years ago.
In their search for the original cradle of humanity, anthropologists have long been looking for remains of early man in all corners of the world. In this effort they have eliminated the Americas and Australia from the competition and to convey the honour of having seen man's first emergence to either Africa or Asia. Any find of early man made in these areas takes on, therefore, a particular importance. In 1953 fragments of a human skull without the face were discovered in probably late Pleistocene levels near Hopefield, 76 miles north of Capetown, Africa, close to Saldanha Bay. This Saldanha skull is very big and low, has a strongly receding forehead, tremendous bone ridges across the eyebrows, and its bones are extremely thick. All these features are typical of the most primitive types of early man. Probably the most significant fact is that this new specimen resembles very closely the famous Rhodesian Man known from a skull found in late Pleistocene levels at Broken Hill in 1921. This Rhodesian lowbrow is one of the most puzzling finds of early man ever made, for he combines certain extremely primitive characteristics with some very modern features. He has enormous brow ridges, the heaviest ever found in any type of early man, a very strongly projecting face and unusually broad palate, a strongly receding forehead, and low cranial vaults. But his large skull has a brain volume within the range of recent man, and in spite of the various primitive features, he suffered from a truly modern disease: tooth decay. This plague of modern humans appears in the Neolithic, the period in which pottery was discovered and in which man began to boil his food. Every dentist will tell you that soft food is the greatest enemy of teeth, and thus the boiling of food instead of the earlier roasting caused the frequent occurrence of tooth decay in the Neolithic period. But the Rhodesian Man must have been an unfortunate creature indeed; for apart from the dubious honour of being the first man who needed a dentist, he suffered from mastoiditis and rheumatism, as appears from a careful inspection of his skull and his tibia. And to top it all, Sir Arthur Keith, Britain's most distinguished anthropologist, suggests that this truly sick man suffered from acromegaly, a disease of overgrowth of the head, feet, and hands. Although such a diagnosis has previously been doubted on certain grounds, it is the merit of Saldanha man of really absolving his Rhodesian cousin from at least this latter verdict and saving his face - or rather brain. Since both skulls resemble each other so closely, the large size of the skull and the enormous orbital ridges can no longer be considered as pathological features but must are typical of early man in Africa.
Fossils suggest that he evolutionary line leading to us had achieved an upright posture by around four million years ago, then began to increase in body size and a relative brain size of around 2.5 million years ago. That protohuman generally known as Australopithecus africanus, Homo habilis and Homo erectus, which apparently evolved into each other in the sequence. Although the Homo erectus, the stage reached around 1.7 million years ago, was close to us modern humans in body size, its brain size, but still barely half of ours. Stone tools became common around 2.5 million years ago, but they were merely the crudest of flaked or battered stones. In zoological significance and distinctiveness, The Homo erectus was more than an ape, but still much less than a modern human.
Since the biological regularities of living organisms display an active and intimate engagement with their environment that is categorically different from that of inorganic matter, we can conclude that they represent profound oppositions. Since organic and inorganic matter are constructs that cannot be applied simultaneously in the same situation and yet are both required for a complete description of the situation, they must be as be viewed in compliments. In that, given the lawful regularities displayed by organic and inorganic matter are different. A profound complementary relationship exists between the law of physics and that of biology. For example, a complete description in mathematical physics of all the mechanisms of a DNA molecule would not be a complete description of organic matter for an obvious reason. The quality of life associated with the known mechanism of DNA replication exists except an objectivised description. It seems likened to the seamless web of interactions under which the organism holds with its environment, only to suggest that the laws of nature have accorded for biological regularities. Additionally, as it seems agreed to the behaviours we associate with life, which are not merely those of mathematical physics. Even if we could replicate all of the fundamental mechanisms of biological life by manipulating inorganic matter in the laboratory, this problem would remain. To prove that no laws other than those of mathematical physics are involved, that if we would be obliged to create life without any interaction with an environment in which the life form sustains itself or interacts.
Although most physical scientists probably assume that the mechanism of biological life can be completely explained following the law of mathematical physics, many phenomena associated with life cannot be explained in these terms. For example, the apparent compulsion of individual organisms to perpetuate their gene, ‘selfish’ or not, is obviously a dynamic of biological regularities that is not apparent in an isolated system. This contributive functional dynamic cannot be described as for the biochemical mechanisms of DNA or any other aspect of isolated organic matter. The specific evolutionary path followed by living organisms is unique and cannot be completely described as based on prior applications of the laws of physics.
The more complex organisms that evolve from a symbiotic union that is sometimes called in biology texts factories or machines, but, nonetheless, a machine, as Darwin’s model for the relationship part and whole suggest, is a unity of order and not of substance, and the order that exist in a machine is external to the parts. As the biologist Paul Weiss has pointed out, however, the part-whole relationship that exists within and between cells in complex life forms is not that of a machine.
The whole within the part that sets the boundary conditions of cells is DNA, and a complete strand of the master molecule of life exists in the nucleus of each cell. DNA evolved in an unbroken sequence from the earliest life form, and the evolution of even the most complex life forms cannot be separated from the co-evolution of microbial ancestors. DNA in the average cell codes for the production of about two thousand different enzymes, and each of these enzymes canalizes a particular chemical reaction. The boundary conditions within each cell resonate with the boundary condition of all other cells and maintain the integrity of uniqueness of whole organisms.
Volution, in biology, defines a complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses how, over a time, various plant and animal species branch off to become entirely new species, and how different species are related through complicated family trees those span millions of years.
Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants, insects, birds, and mammals, have the same ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in the past. The animal most closely related to humans, for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to have lived approximately six million to seven million years ago. On the other hand, an ancestor common to humans and reptiles that had existed of some 300 million years ago, as these common ancestors were more distantly related forms that lived even farther in the past. Evolutionary biologists attempt to figure out the history of lineages as they diverge and how differences in characteristics developed over time.
Throughout history, philosophers, religious thinkers, and scientists have attempted to explain the history and variety of life on Earth. During the rise of modern science in western Europe in the 17th and 18th centuries, a predominant view held that God created every organism on Earth almost as it now exists. However, in that time of burgeoning interest in the study of apes and natural history, the beginnings of a modern evolutionary theory began to take shape. Early evolutionary theorists proposed that all of the life on Earth evolved gradually from simple organisms. Their knowledge of science was incomplete, however, and their theories left too many questions unanswered. Most prominent scientists of the day remained convinced that the variety of life on Earth could only result from an act of divine creation.
In the mid-19th century a modern theory of evolution took hold, thanks to British naturalist Charles Darwin. In his book, On the Origin of Species by Means of Natural Selection, Darwin described the evolution of life as a process of natural selection. Life, he suggested, is a competitive struggle to survive, often in the face of limited resources. Living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment, such as changes in the climate. Darwin offered that, within a given population in a given environment, certain individuals possess characteristics that make them more likely to survive and reproduce. These individuals will pass these critical characteristics onto their offspring. The number of organisms with these traits increases as each generation passes on the advantageous combination of traits. Out matched, individuals lacking the beneficial traits gradually decrease in number. Slowly, Darwin argued, natural selection tips the balance in a population toward those with the combination of traits, or adaptations, best fitted in with their environment.
While, On the Origin of Species were an instant sensation and best sellers, Darwin’s theories faced hostile reception by critics giving further information of those railed against his blasphemous ideas. Other critics pointed to questions left unresolved by Darwin’s careful arguments. For instance, Darwin could not explain the mechanism that caused life forms to change from generation to generation.
Hostility gave to a considerable degree the acclaim as scientists vigorously debated, explored, and built on Darwin’s theory of natural selection. As the 20th century unfolded, scientific advances revealed the detailed mechanisms missing from Darwin’s theory. Study of the complex chemistry of all organisms unveiled the structure of genes and how they are duplicated, altered, and passed from generation to generation. New statistical methods helped explain how genes in specific populations change over generations. These new methods provided insight into how populations remain adaptable to changing environmental circumstances and broadened our understanding of the genetic structure of populations. Advances in techniques used to find out the age of fossils provided clues about when extinct organisms existed and details about the circumstances surrounding their extinction. New molecular biology techniques compare the genetic structures of different species, enabling scientists to find specific undetectable evolutionary relationships between species. Today, evolution is recognized as the cornerstone of modern biology. Uniting such diverse scientific fields as cell biology, genetics, palaeontology, and even geology and statistics, the study of evolution reveals an exquisitely complex interaction of the forces that act upon every life form on Earth.
Natural selection is tied to traits that organisms pass from one generation to the next. In humans, these traits include hundreds of features such as eye colour, blood type, and height. Nature offers countless other examples of traits in living things, such as the pattern on a butterfly’s wings, the markings on a snail’s shell, the shape of a bird’s beak, or the colour of a flower’s petals.
Such traits are controlled by specific bits of biochemical instructions known as genes. Genes are composed of individual segments of the long, coiled molecule called deoxyribonucleic acid (DNA). They direct the synthesis of proteins, molecular labourers that serve as the constructive edifices to all building blocks of cells, control chemical reactions, and transport materials to and from cells. Proteins are themselves composed of long chains of amino acids, and the biochemical instructions found in DNA determine the arrangement of amino acids in a chain. The specific sequence of amino acids dictates the structure and resulting function of each protein.
All genetic traits result from different combinations of gene pairs, one gene inherited from the mother and one from the father. Each trait is thus represented by two genes, often in different forms. Different forms of the same gene are called alleles. Traits depend on very precise rules governing how genetic units are expressed through generations. According to the laws governing heredity, when a dominant allele (say, tongue rolling) and a recessive allele (no tongue rolling) combines, the trait will always be dictated by the dominant allele. The no tongue rolling trait, or any other recessive trait, will only occur in an individual introduced by those sustaining of getting hold of the two recessive alleles.
Evolutionary change takes place in populations over many generations. Since individual organisms cannot evolve in a single lifetime, evolutionary science focuses on a population of interbreeding individuals. All populations contain some variations in traits. In humans, for example, some people are tall, some are short, and some are of medium height.
In interbreeding populations, genes are randomly shuffled among members of the population through sexual reproduction, the process that produces genetically unique offspring. Individuals of different sexes develop specialized sex cells called gametes. In humans and other vertebrates (animals with backbones), these gametes are sperm in males and eggs in females. When males and females mate, these sex cells join in fertilization. A series of cell divisions creates individuals with a unique assembly of genes. No individual members of any population (except identical twins, which develop from a single egg) are alike in their genetic makeup. This diversity, called genetic diversity or variation, is essential to evolution. The greater a population’s genetic diversity, the more likely it is to evolve specific traits that enable it to adapt to new environmental pressures, such as climate change or disease. Expressing of some time, an expressing eventful place showing of a distinction of contrast of such pressures might drive a population with a low degree of genetic diversity to extinction.
Sexual reproduction ensures that the genes in a population are rearranged in each generation, a process termed recombination. Although the contributive combinations of genes in individuals change with each new generation, the gene frequency, or ratio of different alleles in the entire population, remains constant if no evolutionary forces act on the population. One such force is the introduction of new genes into the genetic material of the population, or gene pool.
When individuals move between one population and another, new genes may be introduced to populations. This phenomenon, known as gene flow, results from chance dispersal and intentional migration. Take, for example, two populations of related wildflowers, one red and one white, separated by a large tract of land. Under normal circumstances, the two groups do not interbreed because the wind does not blow hard enough to carry pollen between the populations so that pollination can occur. If in agreement that it happens of an unusually strong wind that carries pollen from the red wildflower population to the white wildflower population, the gene for red flowers may be introduced to the white population’s gene pool.
Genes themselves are constantly being modified through a process called mutation: a change in the structure of the DNA in an individual's cells. Mutations can occur during replication, the process in which a cell splits itself into two identical copies known as daughter cells. Normally each daughter cell receives an exact copy of the DNA from the parent cell. Occasionally, however, errors occur, resulting in a change in the gene. Such a change may affect the protein that the gene produces and, ultimately, change an individual’s traits. While some mutations occur spontaneously, others are caused by factors in the environment, known as mutagens. Examples of mutagens that affect human DNA include ultraviolet light, X rays, and various chemicals.
Whatever their cause, mutations are a rare but slow and continuous sources of new genes in a gene pool, yet mutations are neutral - that is, they have no effect. Other mutations are detrimental to life, causing the immediate death of any organism that inherits them. Once in a great while, however, a mutation gives an organism an advantageous trait. A single organism with an advantageous trait is only half of the equation, however. For evolution to occur, the forces of natural selection must distribute that trait to other members of a population.
Natural selection sorts out the useful changes in the gene pool. When this happens, populations evolve. Beneficial new genes quickly spread through a population because members who carry them have a greater reproductive success, or evolutionary fitness, and consequently pass the beneficial genes to more offspring. Conversely, genes that are not as good for an organism are eliminated from the population, sometimes quickly and sometimes more gradually, depending on the severity of the gene, because the individuals who carry them do not survive and reproduce with individuals without the bad gene. Over several generations, the gene and most of its carriers are eliminated from the population. Severely detrimental genes may persist at very low levels in a population, however, because they can be reintroduced each generation by mutation.
Natural selection only allows organisms to adapt to their current environment. Should environmental conditions change, new traits may prevail. Moreover, natural selection does not always favour a single version of a trait. Occasionally, multiple versions of the same trait may instill their carriers with equal evolutionary benefit. Nor does natural selection always favour change. If environmental conditions so dictate, natural selection remains unchanged by eliminating extreme versions of a particular trait from the population.
Often, shifts in environmental conditions, such as climate change or the presence of a new disease or predator, can push a population toward one extreme for a trait. In periods of prolonged cold temperatures, for example, natural selection may favour larger animals because they are better able to withstand extreme temperatures. This mode of natural selection, known as directional selection, is evident in cheetahs. About four million years ago, cheetahs were more than twice as heavy as modern cheetahs. Still, quicker and lighter members of the population had greater reproductive success than did larger members of the population. Over time, natural selection takes to be smaller and smaller cheetahs.
Sometimes natural selection acts to preserve the status quo by favouring the intermediate version of a characteristic instead of one of two extremes. An example of this selective force, known as stabilizing selection, was evident in a study of the birth weight of human babies published in the middle of the 20th century. It showed that babies of intermediate weight, about 3.5 kg. (8 lb.), was more likely to survive. Babies with a heftier birth weight had lower chances for survival because they were more likely to cause complications during the delivery process, and lightweight babies were often born premature or with other health problems. Babies of intermediate birth weight, then, were more likely to survive to reproductive age.
Occasionally natural selection favours two extremes, causing alleles for intermediate forms of a trait to become less common in the gene pool. The African Mocker swallowtail butterfly has undergone this form of selection, known as disruptive selection. The Mocker swallowtail evades its predators by resembling poisonous butterflies in its ecosystem. Predators have learned to avoid these poisonous butterflies and to steer away from the look-alike Mocker swallowtails. The Mocker swallowtail has a large range, and in different regions, the Mocker swallowtail looks very different, depending on which species of poisonous butterfly it mimics. In some areas the butterfly displays black markings on a white background; in others the markings float on an orange background. Since a Mocker swallowtail appears poisonous to predators, it has a greater chance of survival and therefore a higher evolutionary fitness. Mocker swallowtails that do not look poisonous have a much lower evolutionary fitness because predators quickly eat them. Disruptive selection, then, favours the extreme colour patterns of white or orange, and nothing between.
In many species, sexual selection results in an accompaniment to the male with elaborate features. Many male birds, such as peacocks, have colourful and showy plumage. Male fiddler crabs have one greatly enlarged claw, and large skin flaps frame the face of the male frilled lizards. Exuberantly some species, males dance elaborate courtship dances designed to display the males’ virility and physical fitness.
Many such traits are a liability to survival, making them counter to the principles of natural selection. For instance, bright colouration and elaborate courtship dances draw the attention of predators. The fiddler crab’s large claw is cumbersome, as are the frilled lizard’s skin flaps. The huge tail feathers of the male peacock give it an awkward, bumbling gait. All these features undoubtedly slow the animals, making them less capable of evading predators or securing prey. Nevertheless, the increased reproductive success these showy characteristics instill makes them worth the risk.
Natural selection is not the only force that changes the ratio of alleles present in a population. Sometimes the frequency of particular alleles may be altered drastically by chance alone. This phenomenon, known as genetic drift, can cause the loss of an allele in a population, even if the allele leads to greater evolutionary fitness. Conversely, genetic drift can cause an allele to become fixed in a population, that is the allele can be found in every member of the population, even if the allele decreases fitness.
Although any population can fall victim to genetic drift, small populations are more vulnerable than larger populations. Imagine a particular allele is present in 25 percent of a population of worms. If a flood occurs and randomly eliminates half the population, the laws of probability predict that approximately 25 percent of the surviving population will carry the allele. In a population of 120,000 worms, this means that about 15,000 of the surviving 60,000 worms will carry the allele. Even if, by chance, the flood claimed the lives of an additional 10 percent of the carriers, thousands of copies of the allele would remain in the population. Still, in a population of only 12 worms, the laws of probability predict that only 1.5 of the surviving six worms would carry the allele. If, by chance, the flood claimed more of the carriers of the allele than the non-carriers, the allele could be eliminated.
The hypothetical flood created what is called a population bottleneck. It reduced the genetic variation in the smaller population such that, even if the group’s number again reached 12 members, its genetic diversity might very well be lower than the genetic diversity of the original population. All of the descendants came from just a few surviving individuals, who carried just a fraction of the alleles present in the former population. Likewise, when a few individuals leave a large population and establish a new one, they bring only a fraction of the genetic diversity of the original population with them. Any descendants of the founding members face the possibility of a drastically reduced genetic diversity. An example of this principle, known as the founder effect, is evident in the Amish community in Pennsylvania. All of the people in this community are descendants of about 200 individuals who established the community after leaving Europe in the early 1700s. One of these founders carried an unusual allele that causes a rare kind of dwarfism. As a result, in the Pennsylvania Amish community today the frequency of this rare allele is one in 14 individuals. In the general population this allele appears in one in 1,000 individuals.
The forces of natural selection and genetic drift continuously influence and change the characteristics of a population. However, most often these forces are not sufficient to create an entirely new species. Different species arise when, for one reason or another, members of a population cease to interbreed. When something prevents populations from mating, they are said to be reproductively isolated from one another. Two reproductively isolated populations cannot randomly exchange genetic material with each other, and as a result, the groups diverge as they evolve independently of one another. In this process, called speciation, the members of each group become so different that they can no longer successfully interbreed. At this point, a new species has formed.
Interbreeding normally continues if a time or goal holds of what might especially be a place where nothing is to stop it. Anything that hinders interbreeding is called an isolating mechanism. Geographic barriers isolate populations, leading to the formation of entirely new species in a process called allopatric speciation. Less frequently, mutations or subtle changes in behaviour prevent individuals living in close proximity from reproducing. This may lead to sympatric speciation, in which two distinct subgroups of a population cease exchanging genetic material and evolve into two or more distinct species.
In sympatric speciation, isolating mechanisms may be triggered by differences in habitat, sexual reproduction, or heredity. Similarly of plants, which may fail to breed together because their flowering seasons are different. Many different types of rain forest orchids, for example, cannot interbreed because they flower on different days. Some animals mate only if they recognize characteristic colour patterns or scents of their own group. Other organisms, particularly birds, are stimulated to breed only after witnessing a song, display, or other courtship ritual that is characteristic in their group.
Sometimes two sub-populations of the same species do not produce offspring with one another, though they come into breeding contact. This may be due, for example, to reproductive incongruities between two sub-populations that cause embryos to die before development and birth. In other instances, if viable offspring are produced, reproductive isolation is still maintained because the offspring are sterile. For example, assess and horses are capable of mating, but their offspring are usually sterile. Both types of reproductive dysfunctions occur when the hereditary factors of the two groups have become incompatible in some way and cannot combine to produce normal offspring.
Speciation may occur even when no isolating mechanism is present. Here, a new species may form through the slow modification of a single group of organisms into an entirely new group. The evolving population gradually changes over generations until the organisms at the end of the line appear very different from the first. Foraminifera, a tiny species of marine animals that live in the Indian Ocean, displays this process, known as vertical or phyletic evolution. From about ten million to six million years ago, the species remained unchanged. These organisms then began a slow and gradual change, lasting about 600,000 years, that left them so unlike their ancestors that biologists consider them an entirely new species.
Whatever the cause of their reproductive isolation, independently evolving populations have a tendency to follow general patterns of evolutionary descent. Most often, environmental factors determine the pattern followed. A gradually cooling climate, for example, may result in a population of foxes developing progressively thicker coats over successive generations. This pattern of gradual evolutionary change occurs in a population of interbreeding organisms evolving together. When two or more populations diverge, they may evolve to be less alike or more alike, depending on the conditions of their divergence.
In the pattern known as divergent evolution, after two segments of a population diverge, each group follows an independent and gradual process of evolutionary change, leading them to grow increasingly different from each other over time. Over many generations, the two segments of the population look less and less like each other and their ancestor species. For example, when the Colorado River formed the Grand Canyon, a geographic barrier developed between two populations of antelope-squirrels. The groups diverged, resulting in two distinct species of antelope squirrel that have different physical characteristics. On the south rim of the canyon is Harris’s antelope squirrel, while just across the river on the north rim is the smaller, white-tailed antelope squirrel.
Sometimes divergence occurs simultaneously among several populations of a single species. In this process, known as adaptive radiation, members of the species quickly disperse to take advantage of the many different types of habitat niches, that is the different ways of obtaining food and shelter in their environment. Such specialization ultimately results in many genetically distinctive but similar-looking species. This commonly occurs when a species colonizes a new habitat in which it has little competition. For example, a flock of one species of birds may arrive on some sparsely populated islands. Finding little competition, the birds may evolve rapidly into several species. Each adapted to one available niche. Charles Darwin noted an instance of adaptive radiation on his visit to the Galápagos Islands off the coast of South America. He surmised that one species of the finch colonized the island’s thousands of years ago and produced the 14 species of finch-like birds that exist there now. Darwin observed that the greatest differences in their appearance lay in the shapes of the bills, adapted for their mode of eating. Some species possessed large beaks for cracking seeds. Others had smaller beaks for eating vegetation, and still others featured long, thin beaks for eating insects.
Sometimes distantly related species evolve in ways that make themselves obtainably appears more closely related. This pattern, known as convergent evolution, occurs when members of distantly related species occupy similar ecological niches. Natural selection favours similar adaptations in each population.
Noticeable examples of convergent evolution are the marsupial mammals of Australia and their placental mammal counterparts on other continents. About 50 million years in the past, the Australian continent separated from the rest of the Earth’s continents. Biologists speculate that few if any placental mammals had migrated to Australia by the time the continents split. They also surmise that neither marsupial mammals, nor their placental counterparts could cross the ocean after the landmasses drifted apart. As a result, the animals evolved entirely independently. Yet many marsupial mammals in Australia strongly resemble many placental mammals found on other continents.
For example, the marsupial mole of Australia looks very much like the placental moles found on other continents, yet these animals have evolved entirely independent of one another. The explanation for the moles’ similar appearances lies in the principles of convergent evolution. Both species evolved to exploit similar ecological niches, and, here, the realm just beneath the surface of the ground. While millions of generations in both marsupial and placental moles, natural selection favoured adaptations suited for a life of burrowing: tube-shaped bodies, broad, shovel-like feet, and short, silky fur that sheds dirt or sand easily. The most striking difference between placental moles and marsupial moles is the colour of their fur. Placental moles are usually dark brown or gray, a colouration that enables them to blend in with the soil in their habitat. Marsupial moles burrow in the golden or reddish sand of Australia, so natural selection produced golden or golden-red fur.
Often two or more organisms in an ecosystem fall into evolutionary steps with one another, each adapting to changes in the other, a pattern known as coevolution. Coevolution is often apparent in flowers and their pollinators. Hummingbirds, for example, have long, narrow beaks and a poor sense of smell, and they are attracted to the colour red. Fuchsias, flowering plants that rely on hummingbirds for pollination, usually have long, slender flowers in various shades of red, and they have almost no fragrance. What at first may be a remarkable coincidence is, in fact, the product of thousands of generations of evolutionary fine-tuning. More likely to attract hummingbirds than fuchsias with different colouration, red-flowered, individuals had greater reproductive success. Hummingbirds tended to spend more time extracting nectar from the flower of fuchsias with shapes that matched the size of their slender beaks, thus increasing the likelihood of successful pollination. Similarly, those hummingbirds with long, slender beaks were best able to collect nectar from the long-necked flower. Over many generations, long-necked hummingbirds became the rule, rather than the exception, in hummingbird populations.
Species do not change overnight, or even during one lifetime. Evolutionary change usually occurs in tiny, almost imperceptible increments over thousands of generations - periods that range from decades to millions of years. To study the evolutionary relationships among organisms, scientists must take complex measures to exert effort in the detection of deriving indirect clues from the fossil record, patterns of animal distribution, comparative anatomy, molecular biology, and finally, direct observation in laboratories and the natural environment.
One way biologists learn about the evolutionary relationships between species is by examining fossils. These ancient remains of living things are created when a dead plant or animal is buried under layers of mud or sand that gradually turns into stone. Over time, the organism remains themselves may turn to stone, becoming preserved within the rock layer in which they came to rest. By measuring radioactivity in the rock in which a fossil is embedded, paleontologists (scientists who study the fossil record) can determine the age of a fossil.
Fossils present a vivid record of the earliest life on Earth, and of a progression over time from simple to more-complex life forms. The earliest fossils, for example, are those of the primitive bacteria. Some of which are 3.5 billion years’ old, and are embedded in more recent layers of rock. The first animal fossils appear as primitive jellyfish that assign of a date from 680 million years into the past. Still more-complex forms, such as the first vertebrates (animals with backbones), are documented by fossils some 570 million year’s old. Fossils also show that the first mammals appeared roughly 200 million years in the past.
Although these ancient forms of life have not existed on Earth for millions of years, scientists have been able, typically, to show a clear evolutionary line between extinct animals and their modern descendants. The horse’s lineage, for example, can be traced back about 50 million years to a four-toed animal about the size of a dog. Fossils provide evidence of several different transitional forms between this ancient horselike animal and the modern species. In another example, the extinct, winged creature Archaeopteryx lived next to 145 million years ago. Its fossil shows the skeleton of a dinosaur and the feathers of a bird. Many paleontologists consider this creature an intermediate step in the evolution of reptilian dinosaurs into modern birds. Fossils show clear evidence that the earliest human species had many apelike features. These features included large, strong jaws and teeth; short stature, long, curved fingers; and faces that protruded outward from the forehead. Later species evolved progressively more humanlike features.
Scientists also learn about evolution by studying how different species of plants and animals are geographically distributed in nature, and how they relate to their environment and to each other. In particular, populations that exist on islands provide living clues of patterns of evolution. The study of these evolutionary relationships, known as island biogeography, has its roots in Darwin’s observations of the adaptive radiation of the Galapagos finches. The Hawaiian Islands provide similar examples, particularly in the species of birds known as honey reapers. Like the Galapagos finches, the honey-creepers of Hawaii evolved from a common ancestor and branched into several species, showing a striking variety of beak shapes adapted for obtaining different food sources in their various niches.
Detailed study of the internal and external features of different living things, a discipline known as comparative anatomy, also provides a wealth of information about evolution. The arm of a human, the flipper of a whale, the foreleg of a horse, and the wing of a bird have different forms and are adapted to different functions. Yet they correspond in some way, and this correspondence extends too many details. In the case of the arm, flipper, foreleg, and wing, for example, each appendage shows a similar bone structure. The study of comparative anatomy has revealed many instances of correspondence within various groups of organisms and these bodily structures are said to be homologous. Evolutionary biologists suggest that such homologous structures originated in a common ancestor. The differences arose as each group diverged from the common ancestor and adapted to different ways of life. The more recent the common ancestor, the more similar the species.
The skeletons of humans, for instance, retain evidence of a tail-like structure that is probably a relic from previous mammalian ancestors. This feature, called the coccyx, or more commonly, the tailbone, has little apparent function in modern humans. Relic features such as the coccyx are called vestigial organs. Another vestigial organ in humans is the appendix, under which a narrow tube attached to the large intestine. In some plant-eating mammals, the appendix is a functioning organ that helps to digest plant material. In humans, however, the organ lacks this purpose and is considerably reduced in size, serving only as a minor source of certain white blood cells that guard against infection.
The field of embryology, the study of how organisms develop from a fertilized egg until they are ready for birth or hatching, also provides evolutionary clues. Scientists have noted that in the earliest stages of development, the embryos of organisms that share a recent common ancestor are very similar in appearance. As the embryos develop, they grow less similar. For example, the embryos of dogs and cats, both members of the mammal order Carnivora, are more similar in the early stages of development than just before birth. The same is true of human and ape embryos, biology in the last few decades, researchers seek evolutionary clues at the smallest level: within the molecules of living organisms. Despite the enormous variety of form and function seen in living things, the underlying genetic code, under which the molecular building material of life displays a striking uniformity. Most living organisms have DNA, and in each case it consists of different pairings of the same building blocks: four nucleotide bases called adenine, thymine, guanine, and cytosine. Using different combinations of these bases, DNA directs the assembly of amino acids into functional proteins. The same uniform code operates within all living things.
These molecules contain more than the master plan for living organisms, but each is a record of an organism's evolutionary history. By examining the makeup of such molecules, scientists gain insights into how different species are related. For example, scientists compare the protein cytochrome from different species. In closely related species, the proteins have amino-acid sequences that are very similar, perhaps varying by one or a few amino acids. More distantly related organisms generally have proteins with fewer similarities. The more distant the relationship, the less alike the proteins.
The idea that species become genetically more different as they diverge from a common ancestor laid the groundwork for the concept of the molecular clock. Scientists know that, statistically, neutral mutations tend to accumulate at a regular rate, like ticks of a clock. Therefore, the number of molecular differences in a shared molecule is proportional to the time that has elapsed since the species had the same ancestor. This calculation has provided new knowledge of the evolutionary relationship between modern apes and modern humans. The ‘molecular clock’ concept is controversial, however, and has caused much disagreement between evolutionary scientists who study molecules and those who study fossils. This disagreement arises particularly when the molecular clock time estimates do not agree with the estimates derived from studying the fossil record.
Information about evolutionary processes is also obtained by direct observation of species that undergo rapid modification in only a few generations. One of the most powerful tools in the study of evolutionary mechanisms is also one of the tiniest common fruit flies. These insects have short life spans and, therefore, short generations. This enables researchers to observe and manipulate fruit fly reproduction in the laboratory and learn about evolutionary change in the process.
Scientists also study organisms in their natural environments to learn about evolutionary processes, for example, how insects develop genetic resistance to human-made pesticides, such as DDT. While pesticides are often initially effective in killing crop-destroying pests, sometimes the insect populations bounce back. In every insect population a few individual insects are not affected by the pesticide. The pesticide wipes out most of the population, leaving only the genetically resistant individuals to multiply and flourish. Gradually, resistant individuals predominate in the population, and the pesticide loses its effectiveness. The same phenomenon has been observed in strains of disease-causing bacteria that have become resistant to even the most powerful antibiotics. Bacterial resistance forces scientists to develop new antibacterial compounds continuously. Scientists have hoped that curbing overuse of antibiotics might cause the drugs to become effective again. Recent research, however, suggests that bacteria may retain their resistance to antibiotics over many generations, even if they have not been exposed to the agent.
How life changes and diversifies over time, some evolutionary biologists are trying to understand how life originated on Earth. This too requires the careful examination and interpretation of many indirect clues. In one well-known series of experiments in 1953, the American chemists’ Stanley L. Miller and Harold C. Urey attempted to reproduce the atmosphere of the primitive Earth nearly four billion years ago. They circulated a mixture of gases believed to have been present at the time (hydrogen, methane, ammonia, and water vapour) over water in a sterile glass container. They then subjected the gases to the energy of electrical sparks, simulating the action of lightning on the primitive Earth. After about a week, the fluid turned brown and found to contain amino acids: the constructively stabling blocks of proteins. Subsequent work by these scientists and others also succeeded in producing nucleotides, the all-important constructions to accompany the building blocks of DNA and other nucleic acids. While the artificial generation of these molecules in laboratories did not produce a living organism, this research offers some support that the first building blocks of life could have arisen from raw materials that were present in the environment of the primitive Earth.
Other theories regarding the origin of life on Earth point to outer space. Molecules formerly believed to be produced only by living systems have been found spontaneously to form in great abundance in space. Some scientists speculate that the building blocks of early life might have reached the primitive Earth on meteorites or from the dust of a comet tail.
Once all the raw materials were in place, nucleic acids, proteins, and the other components of simple cells, - it is not clear how the first self-replicating life forms came about. Recent theories centre on the role of a particular nucleic acid - ribonucleic acid (RNA), which, in modern cells, carries out the task of translating the instructions coded in DNA for the assembling of proteins. RNA also acts as a catalyst, that is, to cause other chemical reactions and perhaps most significantly, to make copies of itself. Some scientists believe that the first self-replicating organisms were based on RNA.
According to the fossil record, the first single-celled bacteria appeared some 3.5 billion to 3.9 billion years ago. These microscopic creatures lived in the water, converting the Sun's light into chemical energy. This metabolic process, called photosynthesis, released oxygen gas as a byproduct. Photosynthesis slowly changed the composition of the early atmosphere, adding more oxygen to what scientists believe was a mixture of sulfur and carbon gases and watered vapour. Perhaps two billion years ago, more-complex cells appeared. These were the first eukaryotic cells, containing a nucleus and other organized internal structures. At around the same time, the oxygen in the Earth's atmosphere increased to nearly what it is today, which was yet another step that was crucial to the development of early life. Around one billion years ago, the first multicellular life forms began to appear. The beginning of the Cambrian Period (around 540 million years in the past), known as the Cambrian explosion, marked an enormous expansion in the diversity and complexity of life. Following this great diversification, plant life found its way to land, while the first fishes evolved, ultimately producing amphibians. Later came reptiles and, later still, mammals. The tumult of evolution was in full swing, as it remains today.
The origins of life on Earth have been a source of speculation among philosophers, religious thinkers, and scientists for thousands of years. Many human civilizations used rich and complex creation stories and myths to explain the presence of living organisms. Ancient Greek philosophers and scientists were among the earliest to apply the principles of modern science to the mysterious complexity and variety of life around them. During early Christian times, ancient Greek ideas gave way to Creationism, the view that a single God created the universe, the world, and all life on Earth. For the next 1,500 years, evolutionary science was at a standstill. The dawn of the Renaissance in the early 14th century brought a renewed interest in science and medicine. Advances in anatomy highlighted physical similarities in the features of widely different organisms. Fossils provided evidence that life on this planet was vastly different millions of years ago. With each development came new ideas and theories about the nature of life.
The Greek philosopher Anaximander, who lived in the 500's Bc, is generally credited as the earliest evolutionist. Anaximander believed that the Earth first existed in a liquid state. Further, he believed that humans evolved from fishlike aquatic beings who left the water once they had developed sufficiently to survive on land. Greek scientist Empedocles speculated in 400 Bc that plant life arose first on Earth, followed by animals. Empedocles proposed that humans and animals arose not as complete individuals but as various body parts that joined randomly to form strange, fantastic creatures. Some of these creatures, being unable to reproduce, became extinct, while others thrived. Outlandish as his ideas seem today, Empedocles’ thinking anticipates the fundamental principles of natural selection.
The Greek philosopher and scientist Aristotle, who lived in the
300's Bc, referred to a ‘ladder of nature’, a progression of life forms from lower too higher, but his ladder was a static hierarchy of levels of perfection, not an evolutionary concept. Each rung on this ladder was occupied by organisms of higher complexity than the rung before it, with humans occupying the top rung. Aristotle acknowledged that some organisms are incapable of meeting the challenges of nature and so cease to exist. As he saw it, successful creatures possessed a gift, or perfecting principle, that enabled them to rise to meet the demands of their world. Creatures without the perfecting principle died out. In Aristotle’s view it was this principle - not evolution, which accounted for the progression of forms in nature.
Many centuries later, the idea of a perfect and unchanging natural world. The product of divine creation was predominant, not only in religion and philosophy but in science. Gradually, however, as knowledge accumulated from seemingly disparate areas, the beginnings of modern evolutionary theory began to take shape. A key figure in this regard was the Swedish naturalist Carolus Linnaeus, who became known as the father of modern taxonomy, the science of classifying organisms.
In his major work Systema Naturae (The System of Nature), first published in 1735, Linnaeus devised a system of classification of organisms that is still in use today. This system places living things within increasingly specific categories based on common attributions - from a general grouping (kingdom) down to the specific individual (species). Using this system, Linnaeus named nearly 10,000 plant and animal species in his lifetime. Not an evolutionist by any means, Linnaeus believed that each species was created by God and was incapable of change. Nevertheless, his orderly groupings of living things provided important insights for later theorists. Perhaps the most prominent of those who embraced the idea of progressive change in the living world was the early 19th-century French biologist Jean-Baptiste Lamarck. Whom of which as, Lamarck's theory, now known as Lamarckism and based in part on his study of the fossils of marine invertebrates, was that species do change over time. He believed, furthermore, that animals evolve because unfavourable conditions produce needs that animals try to satisfy. For example, short-necked ancestors of the modern giraffe voluntarily stretched their necks to reach leaves high in trees during times when food was scarce. Proponents of Lamarckism thought that this voluntary employment slightly changed the hereditary characteristics controlling neck growth: The giraffe then transmitted these alterations to its offspring as what Lamarck called acquired characteristics. Modern scientists know that adaptation and natural selection are far more complicated than Lamarck supposed, having nothing to do with an animal's voluntary efforts. Nonetheless, the idea of acquired characteristics, with Lamarck as its most famous proponent, persisted for many years.
French naturalist and paleontologist Georges Cuvier feuded with Lamarck. Unearthing the fossils of mastodons and other disappeared or vanquished species. Cuvier produced proof of long-extinct life forms on Earth. Unlike Lamarck, however, Cuvier did not believe in evolution. Instead, Cuvier believed that floods and other cataclysms destroyed such ancient species. He suggested that after each cataclysmic event, God created a new set of organisms.
At around the same time that Cuvier and Lamarck were squabbling, British economist Thomas Robert Malthus proposed ideas extremely influential in evolutionary theory. In his 1798 work, An Essay on the Principle of Population Malthus theorized that the human population would increase at a much greater rate than its food sources. This theory introduced the key idea of competition for limited resources, that is, there is not enough food, water, and living space to go around, and organisms must somehow compete with each other to obtain resources necessary for survival. Another key idea came from Scottish geologist Charles Lyell, who supplied a deeper understanding of Earth’s history. In his book Principles of Geology (1830), Lyell set forth his case that the Earth was millions of years old rather than only a few thousand years old, as was maintained by those who accepted the biblical story of divine creation as fact.
In 1831, Charles Darwin, who was intending to become a country minister, had an opportunity to sail as ship’s naturalist aboard the HMS Beagle on a five-year, around-the-world mapmaking voyage. During the journey, as the ship anchored off South America and other distant shores, Darwin had the opportunity to travel inland and make observations of the natural world. In the Galápagos Islands, he noted how species on the various islands were similar but distinct from one another. He also observed fossils and other geological evidence of the Earth's great age. The observation’s Darwin made on that voyage seemed to suggest the evolution, rather than the creation, of the many local forms of life.
In 1837, shortly after returning to England, Darwin began a notebook of his observations and thoughts on evolution. Although Darwin had developed the major components of his theory of evolution by natural selection in a 1842 unpublished paper circulated among his friends, he was unwilling to publish the results until he could present as complement by which its case of the possibility. He laboured for almost 20 additional years on his theory of evolution and on its primary mechanism, natural selection. In 1858 he received a letter from British naturalist Alfred Russel Wallace, a professional collector of wildlife specimens. Much to Darwin’s surprise, Wallace had independently hit upon the idea of natural selection to explain how species are modified by adapting to different conditions. Not wanting Darwin to be unfairly deprived of his share of the credit for the theory. Some of Darwin's scientific colleagues presented extracts of Darwin's work along with Wallace's paper at a meeting of the Linnean Society, a London-based science organization, in June 1858. Wallace's paper stimulated Darwin to finish his work and get it into print. Darwin published, On the Origin of Species by Means of Natural Selection on November 24, 1859. All 1,250 copies of the first printing were sold on that day.
Darwin’s book and the theory it popularized evolution through natural selection, which set off a storm of controversy. Some protest came from the clergy and other religious thinkers. Other objections came from scientists. Many scientists continued to believe in Lamarckism, the idea that living things could consciously strive to accumulate modifications during a lifetime and could pass these traits onto their offspring. Other scientists objected to the seemingly random quality of natural selection. If natural selection depended upon random combinations of traits and variations, critics asked, how could it account for such refined and complex structures as the human eye? Perhaps the most serious question, one for which Wallace and Darwin had no answer - concerning the inheritance of traits. How exactly were traits passed along to offspring?
Darwin did not know it, but the answer was nearby - although it would not be acknowledged in his lifetime. In the Augustinian monastery at Brünn (now Brno in the Czech Republic), Austrian monk Gregor Mendel experimented with the breeding of garden peas, observing how their traits were passed down through generations. In crossbreeding pea plants to produce different combinations of traits’ - colour, height, smoothness, and other characteristics, - Mendel noted that although a given trait might not appear in every generation, the trait did not disappear. Mendel discovered that the expression of traits hinged on whether the traits were dominant or recessive, and on how these dominant and recessive traits combined. He learned that contrary to what most scientists believed at the time. The mixing of traits in sexual reproduction did not result in a random blending. Traits were passed along in discrete units. These units are now known as genes. Mendel created hundreds of experiments and produced precise statistical models and principles of heredity, now known as Mendel’s Laws, showing how dominant and recessive traits are expressed over generations. However, no one appreciated the significance of Mendel’s work until after his death. However, his work ultimately gave birth to the modern field of genetics?
In 1900, the Dutch botanist Hugo Marie de Vries and others independently discovered Mendel’s laws. The following year, de Vries's book The Mutation Theory challenged Darwin's concept of gradual changes over long periods by proposing that evolution occurred in abrupt, radical steps. Having observed new varieties of the evening primrose plant coming into existence in a single generation, de Vries had subsequently determined that sudden change, or mutation, in the genetic material was responsible. As the debate over evolution continued in the early 20th century, some scientists came to believe that mutation, and not natural selection, was the driving force in evolution. In the face of these mutationists, Darwin's central theory threatened to fall out of favour.
As the science of genetics advanced during the 1920's and 1930's, several key scientists forged a link between Mendel's laws of inheritance and the theory of natural selection proposed by Darwin and Wallace. British mathematician Sir Ronald Fisher, British geneticist J.B.S. Haldane, and American geneticist Sewall Wright pioneered the field of population genetics. By mathematically annualizing the genetic variation in entire populations, these scientists showed that natural selection, and not just mutation, could result in evolutionary change.
Further investigation into population genetics and such fields as palaeontology, taxonomy, biogeography, and the biochemistry of genes eventually led to what is called the modern synthesis. This modern view of evolution integrated discoveries and ideas from many different disciplines. In so doing, this view reconciled the many disparate ideas about evolution into the all-encompassing evolutionary science studied today. The modern synthesis was advanced in such books as Genetics and the Origin of Species, published in 1937 by the Russian-born American geneticist Theodosius Dobzhansky; Evolution: The Modern Synthesis (1942) by British biologist Sir Julian Huxley; and Systematics and the Origin of Species (1942) by German-born American evolutionary biologist Ernst Mayr. In 1942, American paleontologist George Gaylord Simpson showed from the fossil record that rates and modes of evolution are correlated: New kinds of organisms arise when their ancestors invade a new niche, and evolve rapidly to exploit the conditions in the new environment best. In the late 1940's American botanist G. Ledyard Stebbins showed that plants display evolutionary patterns similar to those of animals, and especially that plant evolution has shown diverse adaptive responses to environmental pressures and opportunities.
In addition, biologists reviewed a broad range of genetic, ecological, and anatomical evidence to show that observation and experimental evidence strongly supported the modern synthesis. The theory has formed the basis of evolutionary science since the 1950s. It has also led to an effort to classify organisms according to their evolutionary history, and their physical similarities. Modern scientists use the principles of genetics and molecular biology to study relationships first proposed by Carolus Linnaeus more than 200 years ago.
In 1953, American biochemist James Watson and British biophysicist Francis Crick described the three-dimensional shape of DNA, the molecule that contains hereditary information in nearly all living organisms. In the following decade, geneticists developed techniques to compare DNA and proteins from different organisms rapidly. In one such procedure, electrophoresis, geneticists evaluate different specimens of DNA or proteins by observing how they behave in the presence of a slight electric charge. Such techniques opened entirely new ways to study evolution. For the first time geneticists could quantitatively determine, for example, the genetic change that occurs during the formation of new species.
Electrophoresis and other biochemical techniques also proved to geneticists that populations varied extensively at the molecular level. They learned that much of the population variation at the molecular or biochemical level has no apparent benefit. In 1968 Japanese geneticist Motoo Kimura proposed that much of the variation at the molecular level results not from the forces of natural selection, but from chance mutations that do not affect an organism's fitness. Not all scientists agree with the neutral gene theory.
In recent decades, another branch of evolutionary theory has appeared, as researchers have explored the possibility that not only physical traits, but behaviour itself, might be inherited. Behavioural geneticists have studied how genes influence behaviour, and more recently, the role of biology in social behaviour has been explored. This field of investigation, known as Sociobiology, was inaugurated in 1975 with the publication of the book Sociobiology: The New Synthesis by American evolutionary biologist Edward O. Wilson. In this book, Wilson proposed that genes influence much of the animals and humanizing behaviours, and, least of mention, that these characteristics are also subject to natural selection.
Sociobiologists examine animal behaviours called altruistic, that is, unselfish, or demonstrating concern for the welfare of others. When birds feed on the ground, for example, one individual may notice a predator and sound an alarm. In so doing, the bird also calls the predator’s attention to itself. What can account for the behaviour of such a sentry, who seems to derive no evolutionary benefit from its unselfish behaviour and so seem to defy the laws of natural selection?
Darwin was aware of altruistic social behaviour in animals, and of how this phenomenon challenged his theory of natural selection. Among the different types of bees in a colony, for example, worker bees are responsible for collecting food, defending the colony, and caring for the nest and the young, but they are sterile and create no offspring. Only by her, that the bee-hive area of infactoring takes apart that which only the queen bee has inherently given that which she could reproduce. If natural selection rewards those who have the highest reproductive success, how could sterile worker bees come about by natural selection when worker bees devote themselves to others and do not reproduce?
Scientists now recognize that among social insects, such as bees, wasps, and ants, the sterile workers are more closely related genetically to one another and to their fertile sisters, the queens, than brothers and sisters are among other organisms. By helping to protect or nurture their sisters, the sterile worker’s bees preserve their own genes more so than if they reproduced themselves. Thus, the altruistic behaviour evolved by natural selection.
Evolutionary theory has undergone many further refinements in recent years. One such theory challenges the central idea that evolution goes on by gradual change. In 1972 the American paleontologist’s Stephen Jay Gould and Niles’ Eldredge proposed the theory of punctuated equilibria. According to this theory, trends in the fossil record cannot be attributed to gradual transformation within a lineage, but result from quick bursts of rapid evolutionary change. In Darwinian theory, new species arise by gradual, but not necessarily uniform, accumulation of many small genetic changes over long periods of geologic time. In the fossil record, however, new species generally appear suddenly after long periods of the stasis - that is, no change. Gould and Eldredge recognized that speciation more likely occurs in small, isolated, peripheral populations than in the main population of the species, and that the unchanging nature of large populations contributes to the stasis of most fossil species over millions of years. Occasionally, when conditions are right, the equilibrium state becomes ‘punctuated’ by one or more speciation events. While these events probably require thousands or tens of thousands of years to establish effective reproductive isolation and distinctive characteristics, this is but an instant in geologic time compared with an average life span of more than ten million years for most fossil species. Proponents of this theory envision a trend in evolutionary development to be more like climbing a flight of stairs (punctuations followed by stasis) than rolling up an inclined plane.
In the last several decades, scientists have questioned the role of extinction in evolution. Of the millions of species that have existed on this planet, more than 99 percent are extinct. Historically, biologists regarded extinction as a natural outcome of competition between newly evolved adaptively superior species and they are older, more primitive ancestors. Recently, however, paleontologists have discovered that many different, unrelated species living in, and large ecosystems tend to become extinct at nearly the same time. The cause is always some sort of climate change or catastrophic event that produces conditions too severe for most organisms to endure. Moreover, new species evolve after the wave of extinction removes many species that previously occupied a region for millions of years. Thus extinction does not result from evolution, but causes it.
Scientists have identified several instances of mass extinction, when species apparently died out on a huge scale. The greatest of these episodes occurred during the end of the Permian Period, by some odd 245 million years ago. Then, according to estimates, more then 95 percent of species, nearly all life on the planet - died out. Another extensively studied, but extinction took place at the boundary of the Cretaceous Period and the Tertiary Period, roughly 65 million years ago, when the dinosaurs disappeared. In all, more than 20 global mass extinctions have been identified. Some scientists theorize that such events may even be cyclical, occurring at regular intervals.
In that made or broke into the genetic chain no less the chromosomal cells that carry the DNA and inhibiting functions in the transmission of hereditary information, for which the helical hereditary information is necessary for cell growth.
Other theories have entered on abrupt changes in the levels of the world’s oceans, for example, or on the effect of changing salinity on early sea life. Another theory blames catastrophic events for mass extinction. Strong evidence, for example, supports the theory that a meteorite some 10 km. (6 mi.) in diameter struck the Earth 65 million years in the past. The dust cloud from the collision, according to this impact theory, shrouded the Earth for months, blocking the sunlight that plants need to survive. Without plants to eat, the dinosaurs and many other species of land animals were wiped out.
Extinction as a cause of evolution rather than the result of it is perhaps best shown as for our own ancestors, - ancient mammals. During the time of the dinosaurs, mammals made up only several the animals that roamed the planet. The demise of dinosaurs provided an opportunity for mammals to expand their numbers and ultimately to become the dominant land animal. Without the catastrophe that took place 65 million years into the past, mammals may have remained in the shadow of the dinosaurs is not exclusively a natural phenomenon. For thousands of years, as the human species has grown in number and technological sophistication, we have demonstrated our power to cause extinction and to upset the world's ecological balance. In North America alone, for example, about 40 species of birds and more than 35 species of mammals have become extinct in the last few hundred years, mostly from human activity. Humans drive plants and animals to extinction by relentlessly hunting or harvesting them, by destroying and replacing their habitat with farms and other forms of development, by introducing foreign species that hunt or compete with local species, and by poisoning them with chemicals and other pollutants.
The rain forests of South America and other tropical regions offer a particularly troubling scenario. Upwards of 50 million acres of rain forest disappear every year as humans raze trees to make room for agriculture and livestock. Given that a single acre of rain forest may contain thousands of irreplaceable species of plant and animal life, the threat to biodiversity is severe. The conservation of wildlife is now an international concern, as evidenced by treaties and agreements enacted at the 1992 Earth Summit in Rio De Janeiro, Brazil. In the United States, federal laws protect endangered species. The problem, nonetheless, of dwindling biodiversity seems certain to worsen as the human population continues to expand, and no one knows for sure how it will affect evolution.
Advances in medical technology may also affect natural selection. The study from the mid-20th century showing that babies of medium birth weights were more likely to survive than their heavier or lighter counterparts would be difficult to reproduce today. Advances in neonatal medical technology have made it possible for small or premature babies to survive in a great deal higher of numbers.
Recent genetic analysis shows the human population contains harmful mutations in unprecedented levels. Researchers attribute this to genetic drift acting on small human populations throughout history. They also expect that improved medical technology may exacerbate the problem. Better medicine enables more people to survive to reproductive age, even if they carry mutations that in past generations would have caused their early death. The genetic repercussions of this are still unknown, but biologists speculate that many minor problems, such as poor eyesight, headaches, and stomach upsets may be attributable to our collection of harmful mutations.
Humans have also developed the potential to affect evolution at the most basic level, - the genes. The techniques of genetic engineering have become commonplace. Scientists can extract genes from living things, alter them by combining them with another segment of DNA, and then place this recombinant DNA back inside the organism. Genetic engineering has produced pest-resistant crops and larger cows and other livestock. To an increasing extent, genetic engineers fight human disease, such as cancer and heart disease. The investigation of gene therapy, in which scientists substitute functioning copies of a given gene for a defective gene, is an active field of medicine, and that in this way the tinkering with genetic material will affect evolutionary remains, yet to be determined.
The most contentious debates over evolution have involved religion. From Darwin's day to the present, members of some religious faiths have perceived the scientific theory of evolution to be in direct and objectionable conflict with religious doctrine regarding the creation of the world. Most religious denominations, however, see no conflict between the scientific study of evolution and religious teachings about creation. Christian Fundamentalists and others who believe literally in the biblical story of creation choose to reject evolutionary theory because it contradicts the book of Genesis, which describes how God created the world and all its plant and animal life in six days. Many such people maintain that the Earth is comparatively young - perhaps 6,000 to 8,000 years old - and that humans and all the worlds’ species have remained unchanged since their recent creation by a divine hand.
Opponents of evolution argue that only a divine intelligence, and not some comparatively random, undirected process, could have created the variety of the world's species, not to mention an organism as complex as a human being. Some people are upset by the oversimplification that humans evolved from monkeys. In the eyes of some, a divine being placed humans apart from the animal world. Proponents of this view find any attempt to place humans within the context of natural history deeply insulting.
For decades, the teaching of evolution in schools has been a flash point in the conflict between religious fundamentalism and science. During the 1920's, Fundamentalists lobbied against the teaching of evolution in public schools. Four states - Arkansas, Mississippi, Oklahoma, and Tennessee - passed laws outlawing public-school instruction in the principles of Darwinian evolution. In 1925 John Scopes, a biology teacher in Dayton, Tennessee, assigned his students readings about Darwinism, in direct violation of state law. Scopes was arrested and placed on trial. In what was the major trial of its time, American defence attorney Clarence Darrow represented Scopes, while American politician William Jennings Bryan argued for the prosecution. Ultimately, Scopes was convicted and customarily received a small fine. However, the ‘Monkey Trial,’ as it became called, was seen as a victory for evolution, since Darrow, in cross examining Bryan, succeeded in pointing out several serious inconsistencies in Fundamentalists belief.
Laws against the teaching of evolution were upheld for another 40 years, until the Supreme Court of the United States, in a 1968 decision in the case Epperson V. Arkansas, ruled that such laws were an unconstitutional violation of the legally required separation of church and state. Over the next few years, Fundamentalists responded by de-emphasizing the religious content in their doctrine and instead casting their arguments as a scientific alternative to evolution called creation science, now also called intelligent design theory. In response to Fundamentalist pressure, 26 states debated laws that would require teachers to spend equal amounts of time teaching creation science and evolution. Only two states, Arkansas and Louisiana, passed such laws. The Arkansas law was struck down in federal district court, while proponents of the Louisiana law appealed all the way to the Supreme Court. In its 1987 decision in Edwards v Aquillard, the Court struck down such equal time laws, ruling that creation science is a religious idea and thus an illegal violation of the church - state separation. Despite these rulings, school board members and other government officials continue to grapple with the long-standing debate between creation and evolution scientists. Even so, efforts to permit the teaching of intelligent design theory in public schools have been unsuccessfully as scientists have sought - and found-evidence for evolution. The fossil record demonstrates that life on this planet was vastly different millions of years ago. Fossils, furthermore, provide evidence of how species change over time. The study of comparative anatomy has highlighted physical similarities in the features of widely different species - proof of common ancestry. Bacteria that mutate and develop resistance to antibiotics, along with other observable instances of adaptation, demonstrate evolutionary principles at work. The study of genes, proteins, and other molecular evidence has added to the understanding of evolutionary descent and the relationship among all living things. Research in all these areas has led to overwhelming support for evolution among scientists.
Nevertheless, evolutionary theory is still, in some cases, the cause of misconception or misunderstanding. People often misconstrue the phrase ‘survival of the fittest’. Some people interpret this to mean that survival is the reward for the strongest, the most vigorous, or the most dominant. In the Darwinian sense, however, fitness does not necessarily mean strength so much as the capacity to adapt successfully. This might mean developing adaptations for more efficiently obtaining food, or escaping predators, or enduring climate change - in short, for thriving in a given set of circumstances.
Yet it bears repeating that organisms do not change their characteristics in direct response to the environment. The key is genetic variation within a population, - and the potential for new combinations of traits. Nature will select those individuals that have developed the ideal characteristics with which to flourish in a given environment or niche. These individuals will have the greatest degree of reproductive success, passing their successful traits onto their descendants.
Another misconception is that evolution always progresses to better creatures. In fact, if species become too narrowly adapted to a given environment, they may ultimately lose the genetic variation necessary to survive sudden changes. Evolution, in such cases, will lead to extinction.
Once upon a time, in Human Evolution, now considered as pensively the process though which a lengthy period of change is admissively given by people who have originated from apelike ancestors. Scientific evidence shows that the physical and behavioural traits shared by all people evolved over a period of at least six million years.
One of the earliest defining human traits, Bipedalism, - walking on two legs as the primary form of locomotion - undergoing an evolution of more than four million years ago. Other important human characteristics’ - such as a large and complex brain, the ability to make and use tools, and the capacity for language - developed more recently. Many advanced traits’, - including complex symbolic expression, such as art, and elaborate cultural diversity - emerged mainly during the past 100,000 years.
Our closest living relatives are three surviving species of great apes: the gorilla, the common chimpanzee, and the pygmy chimpanzee (also known as bonobo). Their confinement to Africa, along with abundant fossils evidence, indicates that the earliest stages of human evolution were also played out in Africa, human history, as sometimes separate from the history of animals, took the initiative in that location about seven million years ago (estimated range from five to nine million years ago). Around that time, a population of African apes split into several populations, of which one proceeded to evolve into modern gorillas, a second into the two modern chimps, and the third into humans. The gorilla line apparently split before the split between the chimp and the human lines.
Fossils indicate that the evolutionary line leading to us had achieved a substantially upright posture by around four million years ago, then began to increase in body size and in relative brain size around 2.5 million years ago. That protohuman is generally known as Australopithecus africaanus. Homo habilis, and Homo erectus, which apparently evolved into each other in that sequence. Although the Homo erectus, the stage extends to around 1.7 million years ago, was close to us modern humans in body size, its brain size was still barely half of ours. Stone tools became common around 2.5 million years ago, but they were merely the crudest of flaked or battered stones. In zoological significance and distinction, The Homo erectus was more than an ape, but still much less than a modern human.
All of that human history, for the first five or six million years after our origins about seven million years ago, remained confined to Africa. The first human ancestor to spread beyond Africa was The Homo erectus, as it is attested by fossils discovered on the Southeast Asian island of Java and conventionally known as Java man the oldest Java ‘man’: fossils’ - of course, they may have belonged to a Java woman, - have usually been argued that they date from about a million years ago. However, it has recently been argued that they date from 1.8 million years ago. (Strictly speaking, the name Homo erectus belongs to these Javan fossils, and the African fossils classified as Homo erectus may warrant a different name). At present, the earliest unquestioned evidence for humans in Europe stems from around half a million years ago, but there are claims of an earlier presence. One would assume that the colonization of Asia also permitted the simultaneous colonization of Europe, since Eurasia is a single landmass not bisected by major barriers.
Nearly half a million years ago, human fossils had diverged from older Homo erectus skeletons in, they’re enlarged, rounder, and fewer angular skulls. African and European skulls of half a million years ago were sufficiently similar to skulls of a modern that they are classified in our species, Homo sapiens, instead of in Homo erectus. This distinction is arbitrary, since Homo erectus evolved into Homo sapiens. However, these early Homo sapiens still differed from us in skeletal details, had brains significantly smaller than ours, and were grossly different from us in their artifacts and behaviour. Modern stone-tool-making peoples, such as Yali’s great grandparents, would have scorned the stone tools of a half million years ago as very crude. The only significant addition to our ancestor’s cultural repertoire that can be documented with confidence around that time was the use of fire.
No art, bone tools, or anything else has come down to us from an early Homo sapiens except their skeletal remains, and those crude stone tools, there were still no humans in Australia, because it would have taken boats to get there from Southern Asia. There were also no humans anywhere in the Americas, because that would have required the occupation of the nearest part of the Eurasian continent (Siberia), and possibly boat-building skills as well. (The present, shallow Bering Strait separating Siberia from Alaska, alternated between a strait and a broad intercontinental bridge of dry land, as sea level repeatedly rose and fell during the Ice Ages). Nevertheless, boat building and survival in cold Siberia were both far beyond the capabilities of an early Homo sapiens. After half a million years ago, the human population of Africa and western Eurasia proceeded to diverge from each other and from East Asia populations in skeletal details. The population of Europe and western Asia between 130,000 and 40,000 years ago is recreated by especially many skeletons’ known as Neanderthals and sometimes classified as some separate spacies, Homo neanderthansis. Despite being depicted in innumerable cartoons as apelike living in caves, Neanderthals have brains larger that our own. They were also the fist humans to leave behind strong evidence of religious, or perhaps, some ceremonial rituals glorifying or taking care of their dead. Yet their stone tools were still crude by comparison with modern New Guinean’s polished stone axes and were usually not yet made in standardized diverse shapes, each with a clearly recognizable function.
The few preserved African skeletal fragments contemporary with the Neanderthals are more similar to our modern skeletons than do Neanderthal skeletons. Even fewer preserved East Asian skeletal fragments are known, but they appear different again from both Africans and Neanderthals. As for the lifestyle at that time, the best-preserved evidence comes from stone artifacts and animal bones accumulated at southern African sites. Although those Africans of 100,000 years ago had more modern skeletons than did their Neanderthal contemporized, they made especially the same crude stone toots as Neanderthals, still lacking standardized shapes. They had no preserved art. To judge from the bone evidence of animal species under which their targeted prey and hunting skills were unimpressive and mainly directed at easy-to-kill, not-at-all-dangerous animals. They were not yet in the business of slaughtering buffalo, pig, and other dangerous prey. They could not even catch fish: their sites immediately on the seacoast lack fish bones and fishhook. They and their Neanderthal contemporaries still rank as less than fully human.
While Neanderthals lived in glacial times and were adapted to the cold, they penetrated no farther north than northern German y and Kiev. Since Neanderthals apparently lacked needles, sewn clothing, warm houses, and other technology essential to survival in the coldest climates. Anatomically modern peoples who did posses such technology had expanded into Siberia by around 20,000 years ago (there are the usual much older disputed claims). That expansion may have been responsible for the extinction of Eurasia’s wooly mammoth and wooly rhinoceros likewise, to note, while the settlements of Australia/New Guinea, humans now occupied three of the five habitable continents, least that we omit Antartica because it was not reached by humans until the 19th century and has never had any self-supporting human population. That left only two continents, North America and South America. For obvious reason that reaching the Americas from the Old world required boats (for which either there is no evidence even in Indonesia until 40,000 years ago and none in Europe until much later) to cross by sea, or else it required the occupation of Siberia (unoccupied until about 20,000 years ago) to cross the Bering Strait. However, it is uncertain when, between about 14,000 and 35,000 years ago, the Americas were first colonized.
Meanwhile, human history at last took off around 50,000 years ago, while of the easiest definite signs had come from East African sites with standardized stone tools and the first preserved jewellery (ostrich-shell beads). Similar developments soon appear in the Near East and in southeastern Europe, then (some 40,000 years ago) in southwestern Europe, where abundant artefacts are associated with fully modern skeletons of people termed Cro-Magnons. Thereafter, the garbage preserved at archaeological sites rapidly becomes ever more interesting and leaves no doubt that we are dealing with biologically and behaviourally modern human, however.
Cro-Magnon’s garbage heaps yield not only stone tools but also tools of bone, whose suitability for shaping (for instance, into fishhooks) had apparently gone unrecognized by previous humans. Tools were produced in diverse. Distinctive shapes do modernly that their function as needles, awls, engraving tools, and so on are obvious to us. Instead of only single-piece tools such as hand-held scrapers, and multipiece tools made their appearance. Recognizable multipiece weapons at Cro-Magnon sites include harpoons, spear-throwers, and eventually bow and arrows, the precursors of rifles and other multipiece modern weapons. Those efficient means of killing at a safe distance permitted the hunting of dangerous prey as rhinos and elephants, while the invention of rope for nets, lines, and snares allowed the addition of fish and bird to our diet. Remains of horses and sewn clothing testify to a greater improved ability to survive in cold climates, and remains of jewellery and carefully buried skeletons indicate revolutionary aesthetic and spiritual development.
Of the Cro-Magnons’ products preserved, the best known are their artworks: Their magnificent cave paintings, statues, and musical instruments, which we still appreciate as art today. Anyone who has experienced firsthand the overwhelming power of the life-sized painted bulls and hoses in the Lascaux Cave of southern France will understand, if not imagine, that their creators must have been as modern in their minds as they were in their skeletons.
Obviously, some momentous change took place in our ancestors’ capabilities between about 100,000 and 50,000 years ago. Presenting us with two major unresolved questions, regarding its triggering cause and its geographic location. As for its case, it can be argued for the perfection of the voiced box and hence for the anatomical basis of modern language, on which the exercise of human creativity is so dependent. Others have suggested instead that a change in brain organization around that time, without a change in brain size, made modern language possible.
As this occurring leap, and its location, did it take place primarily in one geographic area, in one group of humans, who were thereby enabled to expand and replace the former human populations of other parts of the world? Or did it occur in parallel in different regions, in each of which the human populations living today would be descendants of the populations living there before the connective leap? The conventionally advanced-looking human skulls from Africa around 100,000 years ago have been taken to support the former view, within occurring specifically in Africa. Molecular studies (of so-called mitochondrial DNA) were initially also interpreted about an African origin of modern humans, though the meaning of those molecular findings is currently in doubt. On the other hand, skulls of humans living in China and Indonesia hundreds of thousands of years ago are considered by some physical anthropologists to exhibit features still found in modern Chinese and in Aboriginal Australians, respectfully. If true, that in finding would suggest parallel evolution and multiregional origins of modern humans, rather than origins in a single Garden of Eden. The issue remains unresolved.
The evidence for a localized origin of modern humans, followed by their spread and then their replacement of other types of humans elsewhere, seems strongly for Europe. Some 40,000 years ago, into Europe came the Cro-Magnons, with their modern skeleton, superior weapons, and other advanced cultural traits. Within a few thousand years there were no more Neanderthals, who had been evolving as the sole occupants of Europe for hundreds of thousands of years. The sequence strongly suggests that the modern Cro-Magnon somehow used their far superior technology, and their language skills or brains, to infect, kill, or displace the Neanderthals, leaving behind no evidence of hybridization between Neanderthals and Cro-Magnons.
Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa-chimpanzees (including bonobos, or so-called pygmy chimpanzees). Gorilla’s, - share a common ancestor that lived sometime between eight million and six million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between six million and two million years ago come entirely from Africa.
We should be reminded of the ways in which big domestic mammals were crucial to those human societies possessing them. Most notably, they provided meat, milk products, fertilizer, land transportation, leather, military assault, plow traction, and wool, and germs that killed previously unexposed peoples.
In addition, of course, small domestic mammals and domestic birds and insects have also been useful to humans. Many birds were domesticated for meat, eggs, and feathers: the chicken in China, various duck and goose species in parts of Eurasia, turkeys in Mesoamerica, guinea fowl in Africa, and the Muscovy duck in South America. Wolves were domesticated in Eurasia and North America to become our dogs used as hunting companions, sentinels, pets, and, in some societies, food. Rodent and other small mammals domesticated for food include the rabbit in Europe, the guinea pig in the Andes, a giant rat in West Africa, and possibly a rodent called the hutia on Caribbean islands. Ferrets were domesticated in Europe to hunt rabbits, and cats were domesticated in North Africa and Southern Asia to hunt rodent pests. Small mammals domesticated as recently as the 19th and 20th century include foxes, mink, and chinchillas grown for fur and hamsters as pets. Eve n some insects have been domesticated, not ably Europe’s honeybee and China’s silkworm moth, kept for hone y and silk, respectively.
Many of these small animals thus yielded food, clothing or warmth, but none of them pulled plows or wagons, non e bore riders, none except dogs pulled sleds nor became war machines, and nine of them have been as important for food as have big domesticated mammals.
Most scientists distinguish among 12 to 19 different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species’, - probably most of them left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.
Early humans first migrated out of Africa into Asia probably between two million and 1.7 million years ago. They entered Europe somewhat later, generally within the past one million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.
The scientific study of human evolution is called palaeanthropology. Palaeanthropology is a Studfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behaviour. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, palaeanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, and the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world became. Yet many people have come to reconcile such beliefs with the scientific evidence.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring, - that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, and two-party scientific name. In this system, modern humans are classified as Homo sapiens.
The mechanism for evolutionary change resides in genes’ - the basic units of heredity. Genes affect how the body and behaviour of an organism develop during its life. The information contained in genes can change - a process known as mutation. The way particular genes are expressed, - how they affect the body or behaviour of an organism can also change. Over time, genetic change can alter a species overall way of life, such as what it eats, how it grows, and where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population - a group of organisms of the same species that share a particular local habitat. Many factors can favour new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure) physiology (bodily functions, such as digestion, and behaviour). Over long periods, evolution dramatically transformed humans and their ways of life.
Geneticists estimate that the human line began to diverge from that of the African apes between eight million and five million years ago (paleontologists have dated the earliest human fossils to at least six million years ago). This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago.
Humans belong to the scientific order named Primates, a group of more than 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities and some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, and the significance of each step in human evolution.
All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) appearance; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.
The scientific classification of primates reflects evolutionary relationships between individual species and groups of species. Strepsirhine (meaning ‘turned-nosed’) primate’s, - of which the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians evolved earliest and are the most primitive forms of primates. The earliest monkeys and apes evolved from ancestral haplorhine (meaning ‘simple-nosed’) primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors.
Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, and humans - who share many traits not found in other primates - together make up the suborder Anthropoidea. Apes and humans together make up the superfamily Hominoidea, a grouping that emphasizes the close relationship among the species of these two groups.
Strepsirhines are the most primitive types of living primates. The last common ancestors of Strepsirhines and other mammals creatures similar to tree shrews and classified as Plesiadapiformes,
- evolved at least 65 million years ago. The earliest primates evolved about 55 million years ago, and fossil species similar to lemurs evolved during the Eocene Epoch (about 55 million to 38 million years ago). Strepsirhines share all of the basic characteristics of primates, although their brains are not particularly large or complex and they have a more elaborate and sensitive olfactory system (sense of smell) than do other primates.
Tarsiers are the only living representatives of a primitive group of primates that ultimately led to monkeys, apes, and humans. Fossil species called omomyids, with some traits similar to those of tarsiers, evolved near the beginning of the Eocene, followed by early tarsier -like primates. While the omomyids and tarsiers are separate evolutionary branches (and there are no living omomyids), they share features concerning a reduction in the olfactory system, a trait shared by all haplorhine primates, including humans.
The anthropoid primates are divided into New World (South America, Central America, and the Caribbean Islands) and Old World (Africa and Asia) groups. New World monkeys - such as marmosets, capuchins, and spider monkeys, - belong to the infra-order of platyrrhine (broad-nosed) anthropoids. Old World monkeys and apes belong to the infra-order of catarrhine (downward-nosed) anthropoids. Since humans and apes together make up the hominoids, humans are also catarrhine anthropoids.
The first catarrhine primates evolved between 50 million and 33 million years ago. Most primate fossils from this period have been found in a region of northern Egypt known as Al Fayy? Um (or the Fayum). A primate group known as Propliopithecus, one lineage of which is sometimes called Aegyptopithecus, had primitive catarrhine features - that is, it had many basic features that Old World monkeys, apes, and humans share today. Scientists believe, therefore, that Propliopithecus resembles the common ancestor of all later Old World monkeys and apes. Thus, Propliopithecus may also be considered an ancestor or a close relative of an ancestor of humans evolved during the Miocene Epoch (24 million to five million years in the past). Among the oldest known hominoids is a group of primates known by its genus name, Proconsul. Species of Proconsul had features that suggest a close link to the common ancestor of apes and humans, - for example, the lack of a tail. The species Proconsul heseloni lived in the trees of dense forests in eastern Africa about 20 million years ago. An agile climber, it had the flexible backbone and narrow chest characteristic of monkeys, but also a wide range of movement in the hip and thumb, traits characteristic of apes and humans.
Large ape species had originated in Africa by 23 million or 22 million years ago. By 15 million years ago, some of these species had migrated to Asia and Europe over a land bridge formed between the Africa-Arabian and Eurasian continents, which had previously been separated.
Early in their evolution, the large apes underwent several radiations - periods when new and diverse species branched off from common ancestors. Following Proconsul, the ape genus Afropithecus evolved about 18 million years ago in Arabia and Africa and diversified into several species. Soon afterward, three other ape genera evolved, - Griphopithecus of western Asia about 16.5 million years ago, the earliest ape to have spread from Africa; Kenyapithecus of Africa about 15 million years ago; and Dryopithecus of Europe about 12 million years ago. Scientists have not yet determined which of these groups of apes may have caused the common ancestor of modern African apes and humans.
Scientists do not all agree about the appropriate classification of hominoids. They group the living hominoids into either two or three families: Hylobatidae, Hominidae, and sometimes Pongidae. Hylobatidae consists of the small or so-called lesser apes of Southeast Asia, commonly known as gibbons and siamangs. The Hominidae (hominids) includes humans and, according to some scientists, the great apes. For those who include only humans among the Hominidae, all of the great apes, including the orangutans of Southeast Asia, belong to the family Pongidae.
In the past only humans were considered to belong to the family Hominidae, and the term hominid referred only to species of humans. Today, however, genetic studies support placing all of the great apes and humans together in this family and the placing of African apes - chimpanzees and gorillas - together with humans at an even lower level, or subfamily
According to this reasoning, the evolutionary branch of Asian apes leading to orangutans, which separated from the other hominid branches nearly 13 million years ago, belongs to the subfamily Ponginae. The ancestral and living representatives of the African ape and human branches together belong to the subfamily Homininae (sometimes called hominines). Lastly, the line of early and modern humans belongs to the tribe (classificatory level above genus) Hominini, or hominins.
This order of classification corresponds with the genetic relationships between ape and human species. It groups humans and the African apes together at the same level in which scientists group together, for example, all types of foxes, all buffalo, or all flying squirrels. Within each of these groups, the species are very closely related. However, in the classification of apes and humans the similarities among the name’s hominoid, hominid, hominine, and hominin can be confusing. In this context, the term early human refers to all species of the human family tree since the divergence from a common ancestor with the African apes. Popular writing often still uses the term hominid to mean the same thing.
About 98.5 percent of the genes in people and chimpanzees are identical, making chimps the closest living biological relatives of humans. This does not mean that humans evolved from chimpanzees, but it does indicate that both species evolved from a common ape ancestor. Orangutans, the great apes of Southeast Asia, differ much more from humans genetically, indicating a more distant evolutionary relationship.
Modern humans have several physical characteristics reflective of an ape ancestry. For instance, people have shoulders with a wide range of movement and fingers capable of strong grasping. In apes, these characteristics are highly developed as adaptations for brachiation, - swinging from branch to branch in trees. Although humans do not brachiate, the general anatomy from that earlier adaptation remains. Both people and apes also have larger brains and greater cognitive abilities than do most other mammals.
Human social life, too, shares similarities with that of African apes and other primates, - such as baboons and rhesus monkeys - that live in large and complex social groups. Group behaviour among chimpanzees, in particular, strongly resembles that of humans. For instance, chimps form long-lasting attachments with each other; participate in social bonding activities, such as grooming, feeding, and hunting; and form strategic coalitions with each other in order to increase their status and power. Early humans also probably had this kind of elaborate social life.
In whatever manner, modern humans fundamentally differ from apes in many significant ways. For example, as intelligent as apes are, people’s brains are much larger and more complex, and people have a unique intellectual capacity and elaborate forms of culture and communication. In addition, only people habitually walk upright, can precisely manipulate very small objects, and have a throat structure that makes speech possible.
By around six million years ago in Africa, an apelike species had evolved with two important traits that distinguished it from apes: (1) small canine, or eye, teeth (teeth next to the four incisors, or front teeth) and (2) Bipedalism, - that is, walking on two legs as the primary form of locomotion. Scientists refer to these earliest human species as australopithecines, or Australopiths for short. The earliest australopith species known today belong to three genera: Sahelanthropus, Orrorin, and Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as ‘southern ape’, concerning South Africa, where the first known australopith fossils were found.
The Great Rift Valley, a region in eastern Africa in which past movements in Earth’s crust have exposed ancient deposits of fossils, has become famous for its australopith finds. Countries in which scientists have found australopith fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, Australopiths ranged widely over the African continent.
Fossils from several different early australopith species that lived between four million and two million years ago clearly show a variety of adaptations that marks the transition from ape too human. The very early period of this transition, before four million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features.
Fossils reveal much about the physical build and activities of early Australopiths, but not everything about outward physical features such as the colour and texture of skin and hair, or about certain behaviours, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes - particularly the African apes - to understand better how early Australopiths might have looked and behaved, and how the transition from ape too human might have occurred. For example, Australopiths probably resembled the great apes in characteristics such as the shape of the face and the hair on the body. Australopiths also had brains roughly equal in size to those of the great apes, so they probably had apelike mental abilities. Their social life probably resembled that of chimpanzees.
Most of the distinctly human physical qualities in Australopiths related to their bipedal stance. Before Australopiths, no mammal had ever evolved an anatomy for habitual upright walking. Australopiths also had small canine teeth, as compared with long canines found in most other catarrhine primates.
Other characteristics of Australopiths reflected their ape ancestry. They had a low cranium behind a projecting face, and a brain size of 390 to 550 cu. cm. (24 to 34 cu. in.) - between an ape’s brain. The body weight of Australopiths, as estimated from their bones, ranged from 27 to 49 kg. (60 to 108 lb.), and they stood 1.1 to 1.5 m. (3.5 to 5 ft.) tall. Their weight and height compare closely to those of chimpanzees (chimp height measured standing). Some australopith species had a large degree of sexual dimorphism - males were much larger than females - a trait also found in gorillas, orangutans, and other primates.
Australopiths also had curved fingers and long thumbs with a wide range of movement. In comparison, the fingers of apes are longer, more powerful, and more curved, making them extremely well adapted for hanging and swinging from branches. Apes also have very short thumbs, which limits their ability to manipulate small objects. Paleoanthropologists speculate about whether the long and dexterous thumbs of Australopiths allowed them to use tools more efficiently than do apes.
The anatomy of Australopiths shows several adaptations for Bipedalism, in both the upper and lower body. Adaptations in the lower body included the following: The australopithilium, or pelvic bone, which rises above the hip joint, was much shorter and broader than it is in apes. This shape enabled the hip muscles to steady the body during each step. The australopith pelvis also had a bowl-like shape, which supported the internal organs in an upright stance. The upper legs angled inward from the hip joints, which positioned the knees better to support the body during upright walking. The legs of apes, on the other hand, are positioned almost straight down from the hip, so that when an ape walks upright for a short distance, its body sways from side to side. Australopiths also had short and fewer flexible toes than do apes. The toes worked as rigid levers for pushing off the ground during each bipedal step.
Other adaptations occurred above the pelvis. The australopith spine had a S-shaped curve, which shortened the overall length of the torso and gave it rigidity and balance when standing. By contrast, apes have a relatively straight spine. The australopith skull also had an important adaptation related to Bipedalism. The opening at the bottom of the skull through which the spinal cord attaches to the brain, called the foramen magnum, was positioned more forward than it is in apes. This position set the head in balance over the upright spine.
Australopiths clearly walked upright on the ground, but paleoanthropologists debate whether the earliest humans also spent a significant amount of time in the trees. Certain physical features indicate that they spent at least some of their time climbing in trees. Such features included their curved and elongated fingers and elongated arms. However, their fingers, unlike those of apes, may not have been long enough to allow them to brachiate through the treetops. Study of fossil wrist bones suggests that early Australopiths could lock their wrists, preventing backward bending at the wrist when the body weight was placed on the knuckles of the hand. This could mean that the earliest bipeds had an ancestor that walked on its knuckles, as African apes do.
Compared with apes, humans have very small canine teeth. Apes - particularly males - have thick, projecting, sharp canines that they use for displays of aggression and as weapons to defend themselves. The oldest known bipeds, who lived at least six million years ago, still had large canines by human standards, though not as large as in apes. By four million years ago Australopiths had developed the human characteristic of having smaller, flatter canines. Canine reduction might have related to an increase in social cooperation between humans and an accompanying decrease in the need for males to make aggressive displays.
The Australopiths can be divided into an early group of species, known as gracile Australopiths, which arose before three million years ago; and a later group, known as robust Australopiths, which evolved after three million years ago. The gracile Australopiths, - of which several species evolved between 4.5 million and three million years in the past, - generally had smaller teeth and jaws. The later-evolving robust had larger faces with large jaws and molars (cheek teeth). These traits indicate powerful and prolonged chewing of food, and analyses of wear on the chewing surface of robust australopith molar teeth’s support this idea. Some fossils of early Australopiths have features resembling those of the later species, suggesting that the robustus evolved from one or more gracile ancestors.
Paleoanthropologists recognize at least eight species of early Australopiths. These include the three earliest established species, which belong to the genuses’ Sahelanthropus, Orrorin, and Ardipithecus, a species of the genus Kenyanthropus, and four species of the genus Australopithecus.
The oldest known australopith species is Sahelanthropus tchadensis. Fossils of this species were first discovered in 2001 in northern Chad, Central Africa, by a research team led by French paleontologist Michel Brunet. The researchers estimated the fossils to be between seven million and six million years old. One of the fossils is a cracked yet nearly completes cranium that shows a combination of apelike and humanlike features. Apelike features include small brain size, an elongated brain case, and areas of bone where strong neck muscles would have attached. Humanlike features are to include small, flat canine teeth, a short middle part of the face, and a massive brow ridge (a bony, protruding ridge above the eyes) similar to that of later human fossils. The opening where the spinal cord attaches to the brain is tucked under the brain case, which suggests that the head was balanced on an upright body. It is not certain that Sahelanthropus walked bipedally, however, because bones from the rest of its skeleton have yet to be discovered. Nonetheless, its age and humanlike characteristics suggest that the human and African ape lineages had divided from one another by at least six million years ago.
In addition to reigniting debate about human origins, the discovery of Sahelanthropus in Chad significantly expanded the known geographic range of the earliest humans. The Great Rift Valley and South Africa, from which most other discoveries of early human fossils came, are apparently not the only regions of the continent that preserve the oldest clues of human evolution.
Orrorin tugenensis lived about six million years ago. This species was discovered in 2000 by a research team led by French paleontologist Brigitte Senut and French geologist Martin Pickford in the Tugen Hills region of central Kenya. The researchers found more than a dozen early human fossils dating between 6.2 million and six million years old. Among the finds were two thighbones that possess a groove indicative of an upright stance and bipedal walking. Although the finds are still being studied, the researchers consider these thighbones to be the oldest evidence of habitual two-legged walking. Fossilized bones from other parts of the skeleton show apelike features, including long, curved finger bones useful for strong grasping and movement through trees, and apelike canine and premolar teeth. Because of this distinctive combination of ape and human traits, the researchers gave a new genus and species name to these fossils, Orrorin tugenensis, which in the local language means ‘original man in the Tugen region. The age of these fossils suggests that the divergence of humans from our common ancestor with chimpanzees occurred before six million years ago.
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