Human Evolution

Human Evolution
I INTRODUCTION
Human Evolution, biological and cultural development of humans and related species. The main physical evidence of our evolution is a large number of fossil bones and teeth that 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, have also been discovered and excavated. As a result of these discoveries in archaeology and anthropology (see Biological Anthropology; Social Anthropology), a picture of human evolution duuring the past 4 million to 5 million years has emerged.
II HUMAN PHYSICAL TRAITS
Human beings (members of the genus Homo) are classified in the mammalian order Primates; within this order, human beings, along with our extinct close relatives, 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 inn 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 behavioural tr

rends characteristic of the hominine subfamily.
A 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 neck, lower spinal column, pelvis, and legs. Because these changes can be documented in fossil bone, bipedalism is usually seen as the defining trait of the subfamily.
B Brain Size and Body Size
Sophisticated human behaviour, such as our ability to make and to use tools, is related to the large size and complexity of the human brain. Most modern human beings have a braincase volume of between 1,200 and 1,500 cm3. In the course of human evolution the size of the brain has more than tripled. The inncrease in brain size may be related to changes in hominine behaviour. Over time, stone tools and other artefacts became increasingly numerous and sophisticated. Archaeological sites, too, show more intense occupation in later phases of human biological history.
In addition, the geographical areas occupied by our ancestors expanded during the course of human evolution. As far as is known, the earliest examples were in north-central, eastern, and southern Africa, and they began to move into the tropical and subtropical areas of Eu
urasia at least 1.5 million years ago, and into the temperate parts of these continents at least 500,000 years ago. Much later (perhaps 60,000 years ago) human beings were able to cross the water barrier into Australia. Only long after the appearance of modern human beings did people move into the New World, probably less than 30,000 years ago. It is likely that the increase in human brain size took place as part of a complex interrelationship that included increasing social complexity and 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.
Some of the earliest hominine fossils show evidence of large differences in body size, which may reflect a pattern of sexual dimorphism in our early possible 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 (60 to 70 lb) in weight, while males may have been somewhat more than 1.5 m (5 ft) tall, weighing about 68 kg (150 lb). The reasons for this body size difference are disputed, but may be related to specialized patterns of behaviour in early hominine social groups. This extreme dimorphism seems to have disappeared gr
radually in the last million years.
C Face and Teeth
Another 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, tusk-like 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 overall in size over time, although not consistently. 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 human beings is located below, rather than in front of, the large, expanded braincase.
III HUMAN ORIGINS
The fossil evidence for possible immediate ancestors of modern human beings is divided into the genera Ardipithecus, Australopithecus, Paranthropus, and Homo, and begins about 5.8 million years ago. The nature of the hominine evolutionary tree before that is uncertain.
Between 7 million and 20 million years ago, primitive ape-like animals were widely distributed on th
he African and, later, 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 human beings remain matters of active debate among scientists. One of these fossil apes, known as Sivapithecus, appears to share some distinguishing features with the living Asian great ape, the orang-utan, whose direct ancestor it might be. None of these fossils, however, offer 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 those of human beings indicate 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 5 million to 8 million years ago. It is, therefore, quite possible that the known hominine fossil record, which begins about 5.8 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 apes split off from those leading to modern people and hominine/human evolution can be said to have begun.
A Ardipithecus and Australopithecines
The fossil evidence for human evolution may begin with a poorly known creature called Ardipithecus ramidus. This species is known from Ethiopian sites near Aramis, and is represented by teeth, jaw fragments, and part of a skeleton, dating from about 4.4 and 5.8 million years ago. Until this material has been published in more detail, it will remain unclear whether Ardipithecus lies on the hominine line, or might instead represent a fossil ape. If the latter possibility turns out to be the case, the earliest definite hominines are represented by the somewhat later australopithecines (“southern apes”).
Fossils of these creatures have been discovered in a number of sites in eastern and southern Africa, and they are divided into three genera, Australopithecus, Paranthropus, and Kenyanthropus. Australopithecus is first known from sites in northern Kenya at about 4 million years ago, and the genus seems to have become extinct about 2 million years ago. Paranthropus originated at least 2.5 million years ago, and persisted until about 1 million years ago. All the australopithecines were regular bipeds, and therefore indisputable hominines. In details of their teeth, jaws, and brain size, however, they differ sufficiently among themselves to warrant division into at least four species of Australopithecus, three species of Paranthropus, and a distinct form called Kenyanthropus platyops. The other species are called Australopithecus anamensis, Australopithecus afarensis, Australopithecus africanus, and Australopithecus garhi; and Paranthropus aethiopicus, Paranthropus robustus, and Paranthropus boisei. It is possible that Australopithecus anamensis gave rise to the later species of Australopithecus, while Paranthopus aethiopicus was ancestral to the later species of Paranthropus.
The australopithecine Australopithecus anamensis is known from fragmentary remains found at sites in Kenya dating from about 4 million years ago. Its back teeth are large, with a thick covering of enamel, while an upper arm and shin bone thought to belong to the same creature are rather human in form. Australopithecus afarensis is represented by many more fossils, including the famous partial skeleton of Lucy, found at Hadar in Ethiopia. The species was present in eastern Africa between 3 million and 4 million years ago, it is deduced from evidence discovered in Kenya, the Afar region of Ethiopia, and Tanzania, and may also have been present further west in Chad. Australopithecus afarensis had a brain size hardly larger than those of chimpanzees (about 350 to 500 cm3). Some individuals possessed somewhat more projecting canine teeth than those of later hominines. No tools of any kind have been found with Australopithecus afarensis fossils, but footprint trails preserved at Laetoli in Tanzania are thought to have been made by this creature. By about 3 million years ago, Australopithecus afarensis may have evolved into a later australopithecine, Australopithecus africanus. Known from sites in southern Africa, Australopithecus 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 Australopithecus afarensis, no stone tools have been found in association with Australopithecus africanus fossils, and, as in the earlier species, hip and limb bones suggest that while these creatures were regular bipeds, they still had ape-like body proportions and other characteristics. Between 1996 and 1998 a collection of fossilized bones belonging to a new species was discovered in Ethiopia. Named Australopithecus garhi, the species had a primitive projecting face with small braincase, but unusually large back teeth. It apparently also had long human-like legs but short ape-like forearms. The findings were dated at 2.5 million years old, a critical juncture between Australopithecus afarensis and early forms of Homo. Broken and crushed animal bones showing cut marks and gouges were found alongside suggesting that Australopithecus garhi used stone tools to butcher animals. However, evidence for whether Australopithecus garhi was a direct ancestor of modern humans or just another evolutionary dead end is still inconclusive. The same is also true for the newly discovered form called Kenyanthropus platyops, found in Kenya and dating from about 3.5 million years ago. It had a distinctively flat face with high cheekbones.
In July 2002 a team of scientists from France and Chad led by Michel Brunet, working at a site in the Djurab desert in northern Chad, announced the discovery of a fossilized skull, consisting of a near-complete cranium and fragmentary lower jaws, estimated to be approximately 6 to 7 million years old. The specimen was given a new genus and species name, Sahelanthropus tchadensis, and may prove to be the oldest-known member of the hominine subfamily. The specimen shows an intriguing combination of ape and hominine characteristics. Its skull and brain size are similar to those of a chimpanzee, but the size of the teeth is smaller, and the canine tips worn, features more indicative of later hominines, including humans. The face of Sahelanthropus tchadensis is shorter and flatter than that of a chimpanzee, yet its massive brow ridge is more pronounced and thicker. In the absence of any part of the skeleton, it is not possible to come to any definite conclusion on the crucial question of its gait and whether or not it was bipedal.
The location of the find, some 2,400 km (1,500 mi) further west of major fossil sites in east Africa, suggests a new area for further exploration. The age of the specimen is currently based on a comparison of fossilized fauna at a site with very similar fauna found in Kenya that has been accurately dated using radiometric dating.
The age and location of Sahelanthropus tchadensis possibly indicates that the divergence of humans from apes may have occurred earlier than previously thought, and that our potential ancestors may have been more widely distributed. Although few doubt the overall importance of Brunet’s discovery and accept that Sahelanthropus tchadensis lies close to the human-chimpanzee divergence, some scientists remain unconvinced that it is the earliest member of the human line. Given the emerging complexity of early human evolution, it is also possible that Sahelanthropus was an ape ancestor, or that it represents one of a number of evolutionary dead ends. Further research is being undertaken and may shed more light on what appears to be a crucial piece in the puzzle of evolution.
By about 2.5 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 towards the genus Homo, and finally to modern human beings, and the others developing into species of Paranthropus that eventually became extinct. The latter forms are also known as the robust australopithecines, with the species Paranthropus robustus limited to southern Africa, and Paranthropus aethiopicus and Paranthropus boisei found in eastern Africa. The robust australopithecines seem to represent a specialized adaptation for eating hard food such as seeds and nuts 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 million years ago.
B The Genus Homo
Although scientists do not agree, some believe that after the evolutionary split that led to the robust australopithecines, Australopithecus africanus evolved into the genus Homo. Others believe that humans may have briefly shared an ancestry with the early robust australopithecine line. Although fossil evidence is lacking, the evolutionary transition to Homo probably occurred between about 2.3 million and 2.7 million years ago. Fossils dating from immediately after this period display a curious mixture of traits. Some possess relatively large brains—several almost 800 cm3—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, Kenya, Malawi, and Ethiopia, have been placed in the category Homo habilis, meaning “Handy Man”, because some of the fossils were found associated with early stone tools. Homo habilis possessed many traits that link it both with the earlier australopithecines and with later members of the genus Homo. However, some experts believe that the variation in Homo habilis fossils is so great that at least two separate species are actually represented, the smaller one still called Homo habilis, and the larger one Homo rudolfensis (after Lake Rudolf, an old name for Lake Turkana in northern Kenya).
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 million 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 human-like 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, and how much came from animal tissue. It is also not clear whether these sites represent activities by members of the line leading to Homo, or if the robust australopithecines were also making tools and eating at least some meat.
Fossil evidence of a large-brained, small-toothed form, from northern Kenya and dating from 1.8 million to 1.9 million years ago, has been placed in the species Homo erectus, or a more primitive ancestral form, Homo ergaster (“Work Man”). The first part of the time span of Homo ergaster/erectus was apparently limited to eastern Africa. However, very soon after this, descendent forms of Homo erectus are known from the tropical areas of the Old World, and from Georgia in western Asia. A number of archaeological sites dating from the time of Homo erectus reveal a greater sophistication in toolmaking than was found at the earlier sites—for example, at the Zhoukoudian (“Peking Man”) cave site in northern China, there is possible evidence of the use of fire. Animal fossils found with artefacts thought to have been produced by Homo erectus are sometimes of large mammals such as elephants. These data suggest that hominine behaviour was becoming more complex, with a greater range of capabilities, although it is not clear whether Homo erectus was mainly a scavenger rather than a hunter.
Through the time of Homo erectus certain trends in human evolution continued. The brain sizes of early Homo erectus fossils overlap with those of previous hominines, ranging from 600 to 1,000 cm3. Later Homo erectus skulls possess brain sizes in the range of 1,000 to 1,250 cm3, within the lower end of the size variation of Homo sapiens. Recent research suggests that Homo erectus may have survived in parts of Asia long after it had become extinct, or evolved into a new species, elsewhere. A number of Homo erectus fossils from two sites in Java, Indonesia, were recently dated using associated buffalo teeth to only 30,000 to 50,000 years ago.
B1 Homo heidelbergensis
By about 800,000 years ago, some Homo erectus populations in Africa, and probably also in Europe, had changed sufficiently to be recognized as a new species, Homo heidelbergensis, named after a 500,000-year-old jawbone found near the town of Heidelberg in Germany. Members of this species had, on average, a larger brain size (1,100 to 1,400 cm3), a less projecting face, a more prominent nose, and a more inflated braincase than Homo erectus fossils. Homo heidelbergensis is known from Africa, Europe, and possibly China, between about 300,000 and 800,000 years ago. After about 300,000 years, it apparently split into two descendant species, Homo sapiens and Homo neanderthalensis (“Neanderthal Man”), the former appearing in Africa, and the latter in Europe and western Asia.
C Modern Human Origins
However, this view of the origins of modern human beings (Homo sapiens) is still in dispute. Disagreement has focused especially on the place of Neanderthals in the chain of human evolution. The Neanderthals (named from the Neander Valley in Germany, where the first skeleton was recognized) occupied parts of Europe and western Asia from at least 200,000 years ago until about 30,000 to 40,000 years ago, when they began to disappear from the fossil record.
The dispute over the Neanderthals also involves the question of the evolutionary origins of modern human populations, sometimes called races. Although a precise definition of the term “race” is not possible (because modern human beings show continuous variation from one geographical area to another), widely separate human populations are marked by a number of physical differences, dealt with in the classification of races. Some of these differences represent adaptations to local environmental conditions, a process that a few scientists believe began with the spread of Homo erectus to non-African parts of the Old World over a million years ago. In their view, human development since Homo erectus has been one continuous evolution across the whole inhabited world; that is, local populations have remained, changing in appearance over time. The Neanderthals and equivalent peoples in other areas are seen as descending from Homo erectus and ancestral to the various regional forms (“races”) of modern human beings.
Other scientists view racial differentiation as a relatively recent phenomenon. In their opinion, the relatively late dates of the Neanderthals, combined with their distinct features—a low, sloping forehead, large brow ridge, and a long face dominated by a projecting nose—means that they could not have been ancestors of modern human beings, who already existed in Africa and western Asia about 100,000 years ago. They place the Neanderthals and similarly late-surviving primitive peoples in Asia on side-branches of the human evolutionary tree that became extinct. According to this theory, the origins of modern human beings can be found in Africa or the Middle East. Evolving between about 130,000 and 200,000 years ago, these early modern human beings later spread to most parts of the world, supplanting the local archaic populations where these still survived. In addition to various fossil finds from Africa, support for this theory comes from comparisons of DNA in present-day populations. These show that all modern human beings are closely related genetically, suggesting a recent common ancestry, but that non-African peoples (for example, Europeans and Native Americans) are most closely related to each other, suggesting that they descended from an ancestral population that had split from an African one 100,000 years ago or less.
Recent finds in Ethiopia lend more weight to the “out of Africa” theory of human evolution. Partial skulls of two adults and a child were discovered in 2003 at Herto and have been dated at 160,000 years old: the oldest definite record of Homo sapiens. However, due to the lack of fossil discoveries on the vast African continent the question of whether early modern humans originated in East Africa or developed over a wider area is still unresolved.
Whatever the outcome of this scientific debate, the evidence shows that both Neanderthal and Homo sapiens groups were effective 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 and animal bones.
D Modern Human Beings
With the appearance of biologically modern peoples, some important changes in anatomy and behaviour took place. The whole skeleton became more lightly built, the braincase became higher and rounder, the brow-ridges smaller, and the chin more prominent. Tools became more specialized, with the first effective use of difficult materials such as bone, antler, and ivory. Art appeared, especially famous from the walls of caves in France and Spain, produced from 35,000 years ago until the end of the ice age, about 12,000 years ago (see Palaeolithic Art). Some anthropologists have also argued that it was only in the last 100,000 years that full human language originated, a development that would have had profound implications for all aspects of human activity. About 12,000 years ago, one of the most important events in human history took place—plants and animals began to be domesticated. This revolution of agriculture and animal husbandry set the stage for the events in human history that led to the development of towns and cities, and a continuing growth in human population numbers.
Modern understanding of human evolution rests on known fossils, archaeology, and the study of living humans and our nearest relatives. The picture is far from complete, however. 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 addition, genetic studies, including the extraction of ancient DNA from fossils, will become ever more important in reconstructing the pattern of modern human origins. The history of hundreds of different genes is now being studied from comparisons of the DNA of living people, and DNA has even been recovered from several Neanderthal fossils. In general, the evidence supports a recent African origin for modern human beings, and, in the case of the Neanderthals, they are confirmed as a separate lineage from our own, one that began to differentiate from our line of evolution about 500,000 years ago. Such genetic studies hold the promise of an enormous increase in our understanding of human prehistory in the years ahead.

Created by: Marius Lukosius, 05-15-2005

Leave a Comment