Department of Anatomical Sciences, Stony Brook University
Thursday 8:30-8:45, Grand Ballroom II
African-ape-like characters of the early hominin distal radius have been argued to support the knuckle-walking ancestor hypothesis. Features including a distally projecting dorsal ridge and dorsally-oriented scaphoid notch are thought to effect rapid close-packing of the radius and proximal carpal row, which limits radiocarpal extension. That functional model is tested using a combination of computed-tomography-based carpal kinematic analysis in cadavers and morphometrics based on point-digitized landmarks of dry bones. The kinematics indicate that radioscaphoid rotation reflects the overall extension of the radiocarpal complex, and CT imaging demonstrates that the dorsal lip of the scaphoid notch on the radius demarcates maximum extension. Chimpanzees are only slightly less mobile at the radioscaphoid joint than orangutans, but palmigrade-capable monkeys exhibit considerably greater mobility and match published values for humans. Morphometrically, only Pan and Hylobates differ significantly amongst apes in the relative projection of the scaphoid notch’s dorsal lip (Pan > Hylobates), while all apes differ significantly from humans (apes > humans), which are most similar to the monkeys in this metric. Therefore, radiocarpal morphology restricts mobility in apes vis-à-vis humans and monkeys, but this is probably an artifact of anatomy that accommodates reaction forces arising during climbing and suspensory hand postures rather than having evolved specifically as a knuckle-walking adaptation. As such, the ape-like radiocarpus of early hominins indicates limited proximal-carpal extension, but not necessarily plesiomorphic retention from a knuckle-walking ancestor. Epiphyseal reorientation (and consequently higher radioscaphoid mobility) in later hominins is probably due to infrequent arboreality and/or selection for enhanced manipulative capabilities.
This work was supported by grants from the National Science Foundation (BCS-622515), The Wenner-Gren Foundation for Anthropological Research, Sigma Xi, and Arizona State University.