1. Department of Anthropology, The George Washington University, 2110 G Street, NW, Washington, DC 20052, USA

Correspondence to: Brian G. Richmond Correspondence and requests for materials should be addressed to B.G.R. (e-mail: Email: brich@gwu.edu)

Abstract

Bipedalism has traditionally been regarded as the fundamental adaptation that sets hominids apart from other primates. Fossil evidence demonstrates that by 4.1 million years ago¹, and perhaps earlier², hominids exhibited adaptations to bipedal walking. At present, however, the fossil record offers little information about the origin of bipedalism, and despite nearly a century of research on existing fossils and comparative anatomy, there is still no consensus concerning the mode of locomotion that preceded bipedalism^{3, 4, 5, 6, 7, 8, 9, 10}. Here we present evidence that fossils attributed to Australopithecus anamensis (KNM-ER 20419)¹¹ and A. afarensis (AL 288-1)¹² retain specialized wrist morphology associated with knuckle-walking. This distal radial morphology differs from that of later hominids and non-knuckle-walking anthropoid primates, suggesting that knuckle-walking is a derived feature of the African ape and human clade. This removes key morphological evidence for a Pan–Gorilla clade, and suggests that bipedal hominids evolved from a knuckle-walking ancestor that was already partly terrestrial.

1. Department of Anthropology, The George Washington University, 2110 G Street, NW, Washington, DC 20052, USA

Correspondence to: Brian G. Richmond Correspondence and requests for materials should be addressed to B.G.R. (e-mail: Email: brich@gwu.edu)