The oldest known hominids are the Australopithecus found in Africa, where their fossil record extends from ca. 4 to approximately 1 million years ago. Four Australopithecus species are recognized in the literature: A. afarensis, A. africanus, A. robutus, and A. boisei. Two more species are recent discoveries and not yet fully published: A. anamensis and A. aethiopicus. All species are clearly bipedal and thus hominids.

Australopithecine evolutionary origins remain vague, because of a sparse African fossil record between 8 and 4 million years ago. A possible ancestor to some or all Australopithecus is the Ardipithecus ramidus , the oldest known hominid that is dated to 4.4 million years ago. In the nineteenth century Thomas Huxley and Charles Darwin postulated that people originated in Africa, since it was the home of their closest living relatives, the chimpanzee and the gorilla. The relationships of the Australopithecus species to each other and to the earliest widely recognized species of Homo, H. habilis, remain controversial. The continuing hominid fossil discoveries in Africa are slowly revealing the basic morphological and behavioral course of early human evolution, and confirm the prediction of Huxley and Darwin. So far, the australopithecines and earliest Home have been found only in Africa, between Taung, South Africa, and Hadar, Ethiopia.

Known Australopithecus sites are concentrated in two areas from southern and eastern Africa. In South Africa, the sites are all caves containing a mixed stone and fossil matrix. Origin of these fossils included transport to the caves by carnivores, and thus yield little or no evidence for hominid behavior. Since 1959 most Australopithecus research is in the Rift Valley, including Olduvai Gorge, from northern Tanzania to central Ethiopia in Eastern Africa. In this region, the Australopithecus sites are along ancient streams and lakes. Many of the Eastern African sites are found in stratigraphic position to volcanic layers whose age can be determined. One caution should be made: both the australopithecines and early Homo surely ranged widely within tropical and subtropical Africa. The known fossil concentrations reflect the occurrence of good to excellent bone preservation, and the australopithecines surely traveled into areas where conditions for fossil preservation did not exist.