Fossil hominins, quadrupedal primates and the origin of human bipedalism: a 3D geometric morphometric analysis of the Primate hamate

Abstract

This note illustrates the value of studying non- human primates, especially quadrupedal primates, in order to investigate the origins of human bipedalism. Two distinct hypotheses postulate that hominins and African great apes share a common ancestor predominantly engaged in special- ized forms of locomotion, i.e., arboreal orthogrady (climbing or arboreal bipedalism) on the one hand and semiterrestrial locomotion (which includes climbing and quadrupedalism) on the other. Both hypotheses are supported by analysis of the wrist morphology of Pliocene hominins, and both have recently been challenged by a third hypothesis based on the study of Ardipithecus ramidus wrist morphology, which has shown general affinities between the latter hominin and qua- drupedal primates. However, all three interpretations rely on rather limited knowledge of the variability of wrist bones in quadrupedal primates. Here, we propose to address the ques- tion of the origins of human bipedalism by means of a three- dimensional analysis of a carpal bone, the hamate, whose morphology appears to vary according to the locomotor behaviour of primates. We compared the original specimens of Pliocene hominins (Australopithecus) with a large sample of non-human primates, including various quadrupedal anthropoids. Our results confirm that, on the one hand, the shape of the hamate in primates varies significantly accord- ing to their locomotor behaviour and, on the other hand, that the hypothesis of the semiterrestrial origin of human biped- alism can be rejected. The affinities between Pliocene homi- nins and most of extant quadrupedal primates indicate that the hands of early hominins partly retained a morphology inherited from a generalist quadrupedal ancestor, which con- curs with the hypothesis recently proposed from the hand bones of Ar. ramidus.