Interesting questions on foot evolution?

Palaeopodiatry :would like to thank Robert for sharing his work with us.

Dr.Robert Kidd. BA (Hons) PhD, Associate Professor,Human Anatomy School of Biomedical and Health ScienceUniversity of Western Sydney

Little Foot and big thoughts—a re-evaluation of the Stw573 foot from
Sterkfontein, South Africa.
R. Kidd, C. Oxnard ,HOMO—Journal of Comparative Human Biology 55 (2005) 189–212

The part of the fossil assemblage Stw573 consisting of some medial foot bones was initially reported by Clarke & Tobias (Science 269 (2002) 521). They found it to have both ape- and human-like qualities, being human-like proximally and ape-like distally. We have undertaken a re-examination of this pedal assemblage using a multivariate analysis; We report an essentiallyape-like morphology proximally and a human-like morphology distally; the talus and navicular were found to be ape-like and the medial cuneiform human-like.

Introduction

That the human foot evolved from some variety of primitive hominoid ancestral stock is not held in doubt. In doing so it has changed in many features, some large scale and obvious, others more subtle. Three large scale modifications stand apart. These may be summarised as follows: (1) a change in the size and proportions of the pedal skeletal segments with the tarsus becoming much greater in humans; (2) the presence of a divergent first ray and opposable first digit in all apes but not in humans; and (3) the presence of the related longitudinal and transverse arch structures in humans but their absence in all other hominoids. These major features, plus a myriad of minor ones distinguish the human foot from that of apes. There is little doubt that the human foot evolved in a mosaic manner, with certain critical features attaining a human form before others. To understand their sequence, a number of questions need to be answered. For instance, did the lateral pedal column, broadly describing the lateral longitudinal arch, become human-like before or after the medial longitudinal arch? Similarly, did the first toe achieve a state of apposition before or after arch formation? And at what stage did the segmental
proportions of the evolving foot become human-like? Some clue may be found by scrutiny of the fossil record, though this is frustratingly sparse with no complete prehuman specimens being available. The most complete is that from East Africa, the OH8 foot from Olduvai, but even this is missing certain vital parts, namely portions of the calcaneus, the metatarsal heads and all digits.

In a previous study (Kidd et al 1994, 1996) the four hindmost tarsal elements of this foot were studied morphometrically and unequivocal evidence for mixed functional affinities was found, the medial side being ape-like and the lateral column (with due caveat for the incomplete calcaneus) being human-like. The medial cuneiform was not reported. Clarke and Tobias (1995) also report on a fossil foot assemblage, that known as Stw573, consisting of the talus, navicular, medial cuneiform and first ray fragment from Sterkfontein Cave, Member 2 and attributed to the species Australopithecus africanus. They also observed mixed affinities, both human- and ape-like features, though in a manner differing from our findings with OH8; they describe the talus to be essentially human-like, the navicular to be of mixed ape and human morphology, and the medial cuneiform to be largely ape-like. Their findings are clearly in conflict with those of the OH8 study, and they do not seem to be reconcilable with either the OH8 findings of Kidd et al (1996), nor the model of human pedal evolution predicted in part from the OH8 study (Kidd 1995, 1998, 1999). The basis of the findings of Clarke and Tobias is not quantitative; they do not report a metrical analysis. It is appropriate, therefore, to undertake a morphometric analysis of the Stw573 fossil assemblage to establish the functional affinities of the tarsal elements and to see if the quite different findings of the two assemblages may be reconciled. In addition, in order to present a complete and unambiguous picture, the OH8 foot is revisited with an analysis of the medial cuneiform to complete the analyses presented in Kidd et al (1996).