Body Proportions in Australopithecus afarensis and africanus

Henry M. McHenry
Department of Anthropology
University of California
Davis, California 95616 USA
(916) 752-1588
(916) 752-8885 (FAX)
hmmchenry@ucdavis.edu

Lee R. Berger
Department of Anatomical Sciences
University of the Witwatersrand
7 York Rd., Parktown 2193, Johannesburg, South Africa
(011) 647-2017
(011) 643-4318
055LRBS@CHIRON.WITS.AC.ZA

Key Words: Australopithecus, Postcrania, Sterkfontein, Hadar, Human Evolution, Paleoanthropology, Limb Proportions, Body Weight

Running Title: Proportions in A. afarensis and africanus

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. Joint sizes of the forelimb specimens match modern humans weighing between 41 and 62 kg. The hindlimb specimens correspond in size to modern humans weighing 27 to 45 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 to hindlimb 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 MY) is the most primitive with strongly sloping symphysis, large canine roots, etc., A. afarensis (3.0-3.6 MY) is less primitive, and A. africanus (2.6-2.8 MY) 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 and 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 anamensis evolved independent of the lineage leading to Homo and do not imply a close phylogenetic link with Homo. Both of these explanations or any other phylogeny imply homoplasy.