Body proportions of Australopithecus afarensis and A. africanus and the origin of the genus Homo.

Abstract

New discoveries of A. africanus fossils from Member 4 Sterkfontein reveal a body form quite unlike earlier Australopithecus species. The new adult material consists of over 48 fore- and hindlimb specimens and includes an associated partial skeleton, Stw 431. The forelimbs and relatively large: the average size of their joints corresponds to a modern human with body mass of 53 kg. The hindlimbs are much smaller with an average size matching a modern human of only 33 kg. Analyses of the Stw 431 partial skeleton confirm these results. In contrast, A. afarensis and anamensis more closely approximate a human pattern of forelimb joint size. This is an unanticipated complication in our understanding of early human evolution. In general, craniodental morphology tracks time in species of Australopithecus: A. anamensis (3.5-4.1 Ma) is the the most primitive with a strongly sloping symphysis, large canine roots, etc., A. afarensis (3.0-3.6 Ma) is less primitive, and A. africanus (2.6-3.0 Ma) shares many derived characteristics with early Homo (e.g., expanded brain, reduced canine, bicuspid lower third premolar, reduced prognathism, greater flexion of the cranial base, deeper TMJ). the new postcranial material, however, reveals an apparently primitive morphology of relatively large forelimb and small hindlimb joints resembling more the pongid than the human pattern. More pongid-like proportions are also present in the two known associated partial skeletons of H. habilis (OH 62 KNM-ER 3735). This may imply either (1) that A. africanus and H. habilis evolved craniodental characters in parallel with the lineage leading to later Homo, or (2) that fore- to hindlimb proportions of A. afarensis (and perhaps A. anamensis) evolved independent of the lineage leading to Homo and does not imply a close phylogenetic link with Homo. Both of these explanations or any other phylogeny imply homoplasy.