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.

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