Nature 410, 324 - 325 (2001) © Macmillan Publishers Ltd.

Palaeoanthropology: Did our ancestors knuckle-walk?

MIKE DAINTON

Department of Human Anatomy and Cell Biology, New Medical School, University of Liverpool, Ashton Street, Liverpool L69 3GE, UK
e-mail: mdainton@liverpool.ac.uk

All African apes walk on their knuckles. There is no evidence for this behaviour in the earliest hominids, however, which conflicts with molecular DNA evidence suggesting that chimpanzees are more closely related to humans than to gorillas. On the basis of a multivariate analysis of four traits of the proximal wrist joint, Richmond and Strait1 claim that African apes and early hominids do share a common knuckle-walking ancestor. I propose that these traits are not uniquely associated with knuckle-walking and question the basis of their conclusion. It is still possible that no human ancestor knuckle-walked and that this behaviour evolved independently in different species.

Although such an ancestor would counter objections to an exclusive human–chimpanzee clade, it would not prove that knuckle-walking evolved only once in the ancestry of African apes and humans. Richmond and Strait1 discount the possibility that knuckle-walking evolved independently in gorillas and in a common human–chimpanzee ancestor, an idea supported by the different ontogenetic development of gorilla and chimpanzee carpus load-bearing morphology2 and by terrestrial orang-utans who normally walk on the dorsum of their fingers, albeit in a less efficient3 fist-walking posture4, 5, and occasionally knuckle-walk6, 7. The entire great-ape lineage may thus be predisposed towards knuckle-walking when terrestrial quadrupedalism becomes a major selective factor6, 7, undermining the claim that the four cited traits can resolve the phylogenetic relationships of chimpanzees, humans and gorillas1.

Richmond and Strait1 assume that because some features of the present-day distal radius fulfil one of the mechanical requisites of African-ape knuckle-walking (limiting extension at the radiocarpal joint), then they must have arisen in response to the requirements of knuckle-walking. But it is debatable whether form, function and adaptive significance should be so linked within a modern evolutionary framework8, 9. Claims for knuckle-walking adaptations in the wrist need careful evaluation as this behaviour describes a novel posture of the metacarpals and phalanges in relation to a substrate4, 5. Digitigrade, semidigitigrade or fist-walking postures may make similar demands on the wrist joints to knuckle-walking4 and so could also have given rise to stabilizing and extension-limiting mechanisms at the radiocarpal joint.

Extension-limiting structures allow contact between the hand and substrate to be maintained more efficiently when climbing large vertical supports4 and so may have arisen in a climbing ancestor of humans and great apes10. >From this perspective, Richmond and Strait1 only demonstrate that behaviours requiring limited extension at the radiocarpal joint were important in hominid ancestry. Methodological considerations, however, suggest that the proposed extension-limiting mechanism of the distal radius may not have been suitably quantified.

A clearly delimited scaphoid notch is not always evident in the hominoid species included in the analysis1, so the measurements may not be replicable. It is unclear why the angle between the scaphoid and lunate facets of the radius may be significant in quantifying the extension-limiting mechanisms at this joint. This dimension should discriminate between African and Asian apes, with a narrower angle in the latter. The greater radio-ulnar curvature of the distal radial articulation in orang-utans and gibbons is atypical for anthropoid primates, and probably facilitates the rotary movement of the proximal carpal joint needed for forearm suspensory locomotion4, 11. But it is not explained why increased rotary movement should preclude an extension-limiting mechanism at the radiocarpal joint1. Part of the discrimination between African apes, Asian apes and hominids (their Fig. 2) may therefore reflect differences in suspensory behaviour and have nothing to do with knuckle-walking.

The first two canonical axes of Fig. 2b of ref. 1 do not separate the A. afarensis radii from orang-utans any more than from chimpanzees. The variables that strongly contribute to the remaining axis are not identified, so it is unclear why the Mahalonobis D2 distance is greater between orang-utans and A. afarensis than between chimpanzees and A. afarensis (their Fig. 2c, based on all canonical axes), a difference that bears on the conclusion that A. afarensis had a knuckle-walking ancestry.

Although hominids may have evolved from a knuckle-walker, I do not believe that Richmond and Strait's analysis1 has proved this. I cannot, therefore, support related interpretations regarding the phylogenetic affinity1 and locomotor mode12 of early hominids, or of the chimpanzee-like characteristics of their immediate ancestor1.

References
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