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Human Evolution, the biological and cultural development of the species Homo sapiens, or human beings. A large number of fossil bones and teeth have been found at various places throughout Africa, Europe, and Asia. Tools of stone, bone, and wood, as well as fire hearths, campsites, and burials, also have been discovered and excavated. As a result of these discoveries, a picture of human evolution during the past 4 to 5 million years has emerged.

It is widely agreed upon that original routes of human migration began with emigrations from Africa into the Mideast, Asia, and Europe, and only much later from Asia to the Americas. Exactly when and how migrations occurred is highly debated. Nonetheless, it is clear that people encountered and overcame substantial geographic and climactic barriers, including deserts, mountain ranges, bodies of water, and glaciers (especially during periodic Ice Ages).

HUMAN PHYSICAL TRAITS  
Humans are classified in the mammalian order Primates; within this order, humans, along with our extinct close ancestors, and our nearest living relatives, the African apes, are sometimes placed together in the family Hominidae because of genetic similarities, although classification systems more commonly still place great apes in a separate family, Pongidae. If the single grouping, Hominidae, is used, the separate human line in the hominid family is distinguished by being placed in a subfamily, Homininae, whose members are then called hominines—the practice that is followed in this article. An examination of the fossil record of the hominines reveals several biological and behavioral trends characteristic of the hominine subfamily.

The skull of Homo neanderthalensis (left) differs considerably from that of anatomically modern humans, or Homo sapiens sapiens (right). Neandertals had thick-walled skulls, sloping foreheads, and heavy brow ridges. This contrasts with the thin-walled skulls, high foreheads, and flat faces of modern humans. Neandertals also had more pronounced and powerful jaws but less of a chin than do modern humans.

Bipedalism  
Two-legged walking, or bipedalism, seems to be one of the earliest of the major hominine characteristics to have evolved. This form of locomotion led to a number of skeletal modifications in the lower spinal column, pelvis, and legs. Because these changes can be documented in fossil bone, bipedalism usually is seen as the defining trait of the subfamily Homininae.

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.

Brain Size and Body Size  
Much of the human ability to make and use tools and other objects stems from the large size and complexity of the human brain. Most modern humans have a braincase volume of between 1300 and 1500 cc (between 79.3 and 91.5 cu in). In the course of human evolution the size of the brain has more than tripled. The increase in brain size may be related to changes in hominine behavior. Over time, stone tools and other artifacts became increasingly numerous and sophisticated. Archaeological sites, too, show more intense occupation in later phases of human biological history.

Humans have undergone major anatomical changes over the course of evolution. This illustration depicts Australopithecus afarensis (center), the earliest of the three species; Homo erectus (left), an intermediate species; and Homo sapiens sapiens (right), a modern human. H. erectus and 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 an almost flat face beneath the front of the braincase.

In addition, the geographic areas occupied by our ancestors expanded during the course of human evolution. Earliest known from eastern and southern Africa, they began to move into the tropical and subtropical areas of Eurasia sometime after a million years ago, and into the temperate parts of these continents about 500,000 years ago. Much later (perhaps 50,000 years ago) hominines were able to cross the water barrier into Australia. Only after the appearance of modern humans did people move into the New World, some 30,000 years ago. It is likely that the increase in human brain size took place as part of a complex interrelationship that included the elaboration of tool use and toolmaking, as well as other learned skills, which permitted our ancestors to be increasingly able to live in a variety of environments.

The human skull has changed dramatically during the past 3 million years. As the skull evolved from Australopithecus to Homo (sapiens) sapiens, the capacity of the cranium increased (to accommodate the growth of the brain), the face flattened, the chin recessed, and the size of the teeth decreased. Scientists believe that the incredible growth in the size of the brain may be related to the increasing sophistication of hominine behavior. Anthropologists also theorize that the brain evolved a high capacity for learning and reasoning, and after that cultural, not physical, evolution changed the way humans live.

The earliest hominine fossils show evidence of marked differences in body size, which may reflect a pattern of sexual dimorphism in our early ancestors. The bones suggest that females may have been 0.9 to 1.2 m (3 to 4 ft) in height and about 27 to 32 kg (about 60 to 70 lb) in weight, while males may have been somewhat more than 1.5 m (about 5 ft) tall, weighing about 68 kg (about 150 lb). The reasons for this body size difference are disputed, but may be related to specialized patterns of behavior in early hominine social groups. This extreme dimorphism appears to disappear gradually sometime after a million years ago.

Modern human beings, like gorillas, tarsiers, and chimpanzees, are primates. Sometime along the course of primate evolution, human development diverged from that of gorillas and other primates. Although many similarities exist between other primates, particularly gorillas and chimpanzees, and modern humans, fundamental differences attest to the divergence in development. This illustration of the skulls of a modern gorilla and a modern human depict some of these differences. The gorilla possesses larger canine teeth and a protruding jaw as compared with members of the hominid line.

Face and Teeth  
The third major trend in hominine development is the gradual decrease in the size of the face and teeth. All the great apes are equipped with large, tusklike canine teeth that project well beyond the level of the other teeth. The earliest hominine remains possess canines that project slightly, but those of all later hominines show a marked reduction in size. Also, the chewing teeth—premolars and molars—have decreased in size over time. Associated with these changes is a gradual reduction in the size of the face and jaws. In early hominines, the face was large and positioned in front of the braincase. As the teeth became smaller and the brain expanded, the face became smaller and its position changed; thus, the relatively small face of modern humans is located below, rather than in front of, the large, expanded braincase.

HUMAN ORIGINS  
The fossil evidence for immediate ancestors of modern humans is divided into the genera Australopithecus and Homo, and begins about 5 million years ago. The nature of the hominine evolutionary tree before that is uncertain.

Between 7 and 20 million years ago, primitive apelike animals were widely distributed on the African and, later, on the Eurasian continents. Although many fossil bones and teeth have been found, the way of life of these creatures, and their evolutionary relationships to the living apes and humans, remain matters of active debate among scientists. One of these fossil apes, known as Sivapithecus, appears to share many distinguishing features with the living Asian great ape, the orangutan, whose direct ancestor it may well be. None of these fossils, however, offers convincing evidence of being on the evolutionary line leading to the hominid family generally or to the human subfamily in particular.

Comparisons of blood proteins and the DNA of the African great apes with that of humans indicates that the line leading to modern people did not split off from that of chimpanzees and gorillas until comparatively late in evolution. Based on these comparisons, many scientists believe a reasonable time for this evolutionary split is 6 million to 8 million years ago. It is, therefore, quite possible that the known hominine fossil record, which begins about 5 million years ago, extends back virtually to the beginnings of the human line. Future fossil discoveries may permit a more precise placement of the time when the direct ancestors of the modern African ape split off from those leading to modern people and human evolution can be said to begin.

Australopithecus afarensis is the earliest known hominid species. The lighter portions of the skull cast represent actual bone fragments, while the dark areas represent what the entire skull might have looked like. Australopithecus is believed to have lived from 3 or 4 million years ago to 1.5 million years ago.

Australopithecus  
The fossil evidence for human evolution begins with the australopithecines. Fossils of this genus have been discovered in a number of sites in eastern and southern Africa. Dating from more than 4 million years ago (fragmentary remains are tentatively identified from about 5 million years ago), the genus seems to have become extinct about 1.5 million years ago. All the australopithecines were efficiently bipedal and therefore indisputable hominines. In details of their teeth, jaws, and brain size, however, they differ sufficiently among themselves to warrant division into five species: Australopithecus anamensis,A. afarensis, A. africanus, A. robustus, and A. boisei.

The earliest known australopithecine is A. anamensis. Discovered in 1995 on the shores of Lake Turkana in Kenya, the remains of A. anamensis are estimated to be between 3.9 million and 4.2 million years old. Its fossilized teeth and jaws resemble a chimpanzee’s, although the canine teeth are larger and less angled. However, A. anamensis had a distinctly hominine body adapted for walking upright.

A. afarensis, lived in eastern Africa between 3 and 4 million years ago. Found in the Afar region of Ethiopia and in Tanzania, this australopithecine had a brain size a little larger than those of chimpanzees (about 400 to 500 cc/about 24 to 33.6 cu in). Some individuals possessed canine teeth somewhat more projecting than those of later hominines. No tools of any kind have been found with A. afarensis fossils.

Between about 2.5 million and 3 million years ago, A. afarensis apparently evolved into a later australopithecine, A. africanus. Known primarily from sites in southern Africa, A. africanus possessed a brain similar to that of its predecessor. However, although the size of the chewing teeth remained large, the canines, instead of projecting, grew only to the level of the other teeth. As with A. afarensis, no stone tools have been found in association with A. africanus fossils.

By about 2.6 million years ago, the fossil evidence reveals the presence of at least two, and perhaps as many as four, separate species of hominines. An evolutionary split seems to have occurred in the hominine line, with one segment evolving toward the genus Homo, and finally to modern humans, and the others developing into australopithecine species that eventually became extinct. The latter include the robust australopithecines, A. robustus, limited to southern Africa, and A. boisei, found only in eastern Africa. The robust australopithecines represent a specialized adaptation because their principal difference from other australopithecines lies in the large size of their chewing teeth, jaws, and jaw muscles. The robust australopithecines became extinct about 1.5 million years ago.

In August 1960, Dr. Louis Leakey, a British-Kenyan paleoanthropologist, discovered the skull shown here on the right next to the skull of a chimpanzee, in the Olduvai Gorge in northern Tanzania, Africa. Leakey later identified the skull as that of Homo habilis, meaning "handy man," because he believed that his specimen had been a toolmaker. Today, while anthropologists generally agree that the specimen in fact used tools, they disagree whether the fossils Leakey found should be called Homo habilis or Australopithecus habilis. Comparisons between Leakey’s fossils and those of australopithecines seem to indicate that Homo habilis was in fact more advanced, but some anthropologists believe that Homo habilis, when compared with modern humans, is too far removed to be considered of the Homo genus.

The Genus Homo  
Although scientists do not agree, many believe that after the evolutionary split that led to the robust australopithecines, A. africanus evolved into the genus Homo. If so, this evolutionary transition occurred between 1.5 and 2 million years ago. Fossils dating from this period display a curious mixture of traits. Some possess relatively large brains—several almost 800 cc (about 49 cu in)—and large, australopithecine-sized teeth. Others have small, Homo-sized teeth but also small, australopithecine-sized brains. A number of fossil skulls and jaws from this period, found in Tanzania and Kenya in eastern Africa, have been placed in the category H. habilis, meaning "handy man," because some of the fossils were found associated with stone tools. H. habilis possessed many traits that link it both with the earlier australopithecines and with later members of the genus Homo. It seems likely that this species represents the evolutionary transition between the australopithecines and later hominines.

Fossils of tools and the skeletons of large mammals found near fossils of Homo erectus, or "upright man," suggest that these human ancestors led more complicated lives than earlier species. Although the anatomical structure of this species resembles that of modern humans, anthropologists have found that the human brain underwent many changes during the evolution from Homo erectus to modern humans. Anthropologists believe that Homo erectus lived between 1.5 million and 0.3 million years ago. This skull belonged to a female Homo erectus of the Beijing type.

The earliest evidence of stone tools comes from sites in Africa dated to about 2.5 million years ago. These tools have not been found in association with a particular hominine species. By 1.5 to 2 million years ago, sites in various parts of eastern Africa include not only many stone tools, but also animal bones with scratch marks that experiments have shown could only be left by humanlike cutting actions. These remains constitute evidence that by this time early hominines were eating meat, but whether this food was obtained by hunting or by scavenging is not known. Also unknown at present is how much of their diet came from gathered vegetable foods and insects (dietary items that do not preserve well), and how much came from animal tissue. It is also not known whether these sites represent activities by members of the line leading to Homo, or if the robust australopithecines were also making tools and eating meat.

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.

A new species of large brained, small-toothed hominines emerged in Africa around 1.7 million years ago. A similar species appeared in Asia around the same time or slightly later. Until the 1990s scientists classified both species as Homo erectus. However, most scientists now believe that they actually constitute two separate hominine species. The African specimens are now usually referred to as Homo ergaster, while the designation H. erectus is reserved for the Asian fossils. Most scientists believe that H. ergaster arose first in Africa and then spread rapidly into Asia, where it evolved into H. erectus. The relationship between these two species remains controversial, however. Recent discoveries and new dating methods have led some paleoanthropologists to claim that H. erectus inhabited China between 1.7 million and 1.9 million years ago. Some scientists have suggested that the oldest hominine fossils found in Asia may be evolutionary predecessors to H. erectus.

Throughout the time of H. erectus and H. ergaster the major trends in human evolution continued. The brain sizes of early H. ergaster fossils are not much larger than those of previous hominines, ranging from 750 to 800 cc (45.8 to 48.8 cu in). Later H. erectus skulls possess brain sizes in the range of 1100 to 1300 cc (67.1 to 79.3 cu in), within the size variation of Homo sapiens.

A number of archaeological sites dating from the time of H. erectus reveal a greater sophistication in toolmaking than was found at the earlier sites. Evidence found at the cave site of "Peking Man" in northern China, suggests that H. erectus used fire. Fossils of bones from large mammals such as elephants found near H. erectus sites suggest that H. erectus had developed sophisticated hunting skills. Such data suggests that many hominine behaviors were becoming more complex and efficient.

A Neandertal skull, top right, and several bones were found at the La-Chapelle-aux-Saints rock shelter in southwestern France in 1908. Another Neandertal skull, bottom right, was found at the nearby La Ferrassie site in 1909. The remains found at La-Chapelle-aux-Saints, marked by arthritis and disease, did much to reinforce a conception of the Neandertal as a slouching, degenerate human form. Scientists now believe Neandertals were a strongly built and intelligent species that thrived in Europe for several hundred thousand years.

Early Homo sapiens  
Between 200,000 and 300,000 years ago, H. ergaster evolved into H. sapiens. Because of the gradual nature of human evolution at this time, it is difficult to identify precisely when this evolutionary transition occurred, and certain fossils from this period are classified as late H. erectus by some scientists and as early H. sapiens by others.

Although placed in the same genus and species, these early H. sapiens are not identical in appearance with modern humans. New fossil evidence suggests that modern man, H. sapiens sapiens, first appeared more than 90,000 years ago. There is some disagreement among scientists on whether the hominine fossil record shows a continuous evolutionary development from the first appearance of H. sapiens to modern humans. This disagreement has especially focused on the place of Neandertals (or Neandertals), often classified as H. sapiens neanderthalensis, in the chain of human evolution. Fossil evidence indicates that the Neandertals (named for the Neander Valley in Germany, where one of the earliest skulls was found) occupied parts of Europe and the Middle East as early as 120,000 years ago until about 30,000 years ago, when they disappeared from the fossil record.

The dispute over the Neandertals also involves the question of the evolutionary origins of modern human populations, or races. Although a precise definition of the term race is not possible (because modern humans show continuous variation from one geographic area to another), widely separate human populations are marked by a number of physical differences. The majority of these differences represent adaptations to local environmental conditions, a process that some scientists believe began with the spread of H. erectus to all parts of the Old World. In their view, human development since H. erectus has been one continuous, in-position evolution; that is, local populations have remained, changing in appearance over time. The Neandertals and other early H. sapiens are seen as descending from H. erectus and are ancestral to modern humans.

Other scientists view racial differentiation as a relatively recent phenomenon. In their opinion, the features of the Neandertals—a low, sloping forehead, large brow ridge, and a large face without a chin—are too primitive for them to be considered the ancestors of modern humans. They place the Neandertals on a side branch of the human evolutionary tree that became extinct. In 1997 a team of scientists added strong evidence to support this view. They managed for the first time to analyze mitochondrial DNA—a DNA form inherited only from the mother and particularly useful for determining ancient ancestral relations—from a Neandertal skull. The analysis showed that the lines leading to Neandertals and modern Homo sapiens began to diverge over 500,000 years ago and that Neandertals and modern humans did not interbreed.

Also according to this theory, the origins of modern humans can be found in southern Africa or the Middle East. Evolving perhaps 90,000 to 200,000 years ago, these humans are thought to have spread to all parts of the world, supplanting the local, earlier H. sapiens populations. In addition to some fragmentary fossil finds from southern Africa, support for this theory comes from comparisons of mitochondrial DNA taken from women around the world, representing a worldwide distribution of ancestors. These studies suggest that humans derived from a single generation in sub-Saharan Africa or southeastern Asia. Because of the tracing through the material line, this work has come to be called the "Eve" hypothesis; its results are not accepted by most anthropologists, who consider the human race to be much older.

Whatever the outcome of this scientific disagreement, the evidence shows that early H. sapiens groups were highly efficient at exploiting the sometimes harsh climates of Ice Age Europe. Further, for the first time in human evolution, hominines began to bury their dead deliberately, the bodies sometimes being accompanied by stone tools, by animal bones, and even by flowers.

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.

Modern Humans  
Although the evolutionary appearance of biologically modern peoples did not dramatically change the basic pattern of adaptation that had characterized the earlier stages of human history, some innovations did take place. In addition to the first appearance of the great cave art of France and Spain (See Cave Dwellers), some anthropologists have argued that it was during this time that human language originated, a development that would have had profound implications for all aspects of human activity. About 10,000 years ago, one of the most important events in human history took place—plants were domesticated, and soon after, animals as well. This agricultural revolution set the stage for the events in human history that eventually led to civilization.

Modern understanding of human evolution rests on known fossils, but the picture is far from complete. Only future fossil discoveries will enable scientists to fill many of the blanks in the present picture of human evolution. Employing sophisticated technological devices as well as the accumulated knowledge of the patterns of geological deposition, anthropologists are now able to pinpoint the most promising locations for fossil hunting more accurately. In the years ahead this will result in an enormous increase in the understanding of human biological history.

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