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 dduring 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 hhuman 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 aand behavioural trends 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 mmore than tripled. The increase 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 aand subtropical areas of Eurasia 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 mmay be related to specialized patterns of behaviour in early hominine social groups. This extreme dimorphism seems to have disappeared gradually 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 oof the hominine evolutionary tree before that is uncertain.

Between 7 million and 20 million years ago, primitive ape-like animals were widely distributed on the 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 oof 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 aare 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 lleast 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 ffrom 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 AAfar 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 aa 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 iintriguing 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 ...