Radio-ulnar deviation of the primate carpus: an X-ray study.

The lack of contact between the ulna and first row of carpals characterizes the wrist morphology of hominoids as compared to other primates. This distinctive feature--generally interpreted as a significant synapomorphy between humans and other hominoids--was a priori considered to be an indication of an increased capacity for ulnar deviation, allowing a greater diversity of hand movements. This X-ray study aimed to test this hypothesis by comparing the shifting of the carpals throughout radioulnar deviation in eight extant genera endowed with ulno-carpal contact or lacking it. The results show that the amplitude of ulnar deviation is not directly correlated with the presence or absence of an ulno-carpal contact: most ulnar deviation takes place at the antebrachio-carpal joint in those primates that lack an ulno-carpal contact (hominoids), instead of at the mid-carpal joint in primates whose ulna articulates with the triquetral (cercopithecoids, platyrrhines, most strepsirrhines). Analysis of the X-ray suggests that the loss of ulno-carpal contact improved the ability to supinate at the radio-ulnar joints, with correlated unfitness for palmigrade/semidigitigrade walking. This evolutive change, associated with a considerable reduction of the share of body weight carried on the forelimbs, likely cleared the way for either knuckle walking or bipedalism and handiness.