Adaptations to knuckle-walking and digitigrady: A three-dimensional kinematic and morphometric analysis of the anthropoid wrist

The African apes (Pan and Gorilla) purportedly exhibit an adaptive complex of the wrist that results in a rigid distal forelimb to facilitate knuckle-walking. Documentation of traits associated with such a complex in fossil hominins has been marshaled to support the knuckle-walking ancestor hypothesis, but these arguments rely on understanding the biomechanical and adaptive significance of wrist morphology. Although finger positioning in knuckle-walkers is peculiar, digitigrade baboons (Papio) use a similar posture of the hand and wrist, and such ‘vertical manus’ postures render the forelimb an efficient lever during terrestrial locomotion. For this dissertation, comparative kinematic and morphometric analyses test the hypothesis that the African ape carpus is adapted to the use of a vertical manus. It was predicted that African ape radiocarpal and midcarpal joint complexes function to produce a more rigid wrist, and that analogous mechanisms also differentiate Papio from species capable of extended-wrist palmigrady.;Computed-tomography-based carpal kinematic analysis indicates that chimpanzees and baboons differ from the palmigrade-capable taxa in exhibiting (1) lower global ranges of wrist extension resulting from variably reduced mobility at each joint in the articular chain; and (2) a midcarpus that effects a rapid engagement of the proximal and distal carpal rows, which ‘lock up’ midway through hand dorsiflexion. Pan also exhibits triquetrohamate kinematics suggesting a midcarpal screw clamp that dynamically stabilizes a pliable proximal row, and os centrale mechanics are consistent with the hypothesis that scaphoid-centrale fusion limits scaphocapitate mobility and contributes a solid radial-side block to facilitate screw-clamping.;Despite the kinematic results, morphometrics fail to support adaptive hypotheses for many proposed components of the knuckle-walking complex. Notably, distal projection of the radiocarpal dorsal ridge (identified in Australopithecus) does not distinguish the African apes or baboons from their respective outgroups. Abbreviated capitate lunatocapitate articular paths do characterize vertical manus taxa, suggesting that this character (plus scaphoid-centrale fusion in African apes) may reflect midcarpal rigidity. These features appear in Australopithecus and other early hominins; however, evolutionary interpretation is complicated by reports of a mobile midcarpus in Ardipithecus ramidus. If correct, then either Ardipithecus was autapomorphic or later hominins secondarily evolved limited midcarpal mobility.