Fossils, feet and the evolution of human bipedal locomotion

Abstract

We review the evolution of human bipedal locomotion with a particular emphasis on the evolution of the foot. We begin in the early twentieth century and focus particularly on hypotheses of an ape-like ancestor for humans and human bipedal locomotion put forward by a succession of Gregory, Keith, Morton and Schultz. We give consideration to Morton's (1935) synthesis of foot evolution, in which he argues that the foot of the common ancestor of modern humans and the African apes would be intermediate between the foot of Pan and Hylobates whereas the foot of a hypothetical early hominin would be intermediate between that of a gorilla and a modern human. From this base rooted in comparative anatomy of living primates we trace changing ideas about the evolution of human bipedalism as increasing amounts of postcranial fossil material were discovered. Attention is given to the work of John Napier and John Robinson who were pioneers in the interpretation of Plio-Pleistocene hominin skeletons in the 1960s. This is the period when the wealth of evidence from the southern African australopithecine sites was beginning to be appreciated and Olduvai Gorge was revealing its first evidence for Homo habilis. In more recent years, the discovery of the Laetoli footprint trail, the AL 288-1 (A. afarensis) skeleton, the wealth of postcranial material from Koobi Fora, the Nariokotome Homo ergaster skeleton, Little Foot (Stw 573) from Sterkfontein in South Africa, and more recently tantalizing material assigned to the new and very early taxa Orrorin tugenensis, Ardipithecus ramidus and Sahelanthropus tchadensis has fuelled debate and speculation. The varying interpretations based on this material, together with changing theoretical insights and analytical approaches, is discussed and assessed in the context of new three-dimensional morphometric analyses of australopithecine and Homo foot bones, suggesting that there may have been greater diversity in human bipedalism in the earlier phases of our evolutionary history than previously suspected.

Fig. 1

The temporal distribution of known hominin taxa.

Fig. 2

(a) Dudley Morton's (1935) reconstruction of a ‘hypothetical prehuman foot’, and (b) Clarke & Tobias's (1995) reconstruction of the Stw 573 specimen ‘Little Foot’. (a) Redrawn from Morton (1935); (b) reprinted with permission from RJ Clarke and PV Tobias, Science 269: 521–524 (1995). Copyright 1995 AAAS.

Fig. 3

Hominin phylogeny proposed by Senut et al. (2001). Note that Senut refers some of the Australopithecus afarensis collection (the smaller-bodied individuals) to the taxon A. antiquus (Ferguson 1984). See text for explanation.

Fig. 4

Three possible scenarios for the evolution of body proportions in the early hominins. These are selected from a large number of possible phylogenies and are presented for illustration only. Note that of the three presented phylogenies only the middle one does not imply homeoplasy. Also, all of these hypothetical scenarios imply that the more ape-like body proportions of Australopithecus africanus and Homo habilis are dervied in relation to the intermediate proportions of A. afarensis. If this ultimately proves to be the case, it would suggest that both H. habilis and A. africanus were engaged in locomotor behaviours that would have selected for these more ape-like proportions. Black = ape-like humerofemoral proportions, coarse hatching = human-like humerofemoral proportions, fine hatching = intermediate humerofemoral proportions. See text for explanation. Note that these scenarios do not include all known hominin taxa.

Fig. 5

One possible scenario for the evolution of the hominin foot. Note that this scenario posits that Homo habilis is more similar in its foot morphology to Australopithecus africanus than it is to A. afarensis. The only difference between the A. africanus foot and the H. habilis foot is that the talonavicular complex has changed from an ape-like morphology to a human-like morphology (Harcourt-Smith, 2002). This pattern is reminiscent of the similarity in the humerofemoral proportions in these hominins.