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 bipedalism3,4,5,6,7,8,9,10. 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.
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We thank L. Aiello, E. Delson, B. Demes, J. Fleagle, F. Grine, W. Jungers, S. Larson, D. Lieberman, O. Pearson, D. Pilbeam, J. Polk, E. Sarmiento, R. Susman and B. Wood for providing valuable comments; F. Grine, L. Gordon, R. Thorington, R. Potts, A. Walker, and C. Ward for providing access to specimens in their care; and The Henry Luce Foundation.
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