Department of Anatomy, Midwestern University, Downers Grove, IL 60515
Thursday 1:15-1:30, Ballroom C
Reconstructing the evolution of hominin wrist mechanics is critical for understanding the adaptive history of human locomotion and manipulative capabilities. In this study, the complex series of bone rotations that occurs during wrist extension (dorsiflexion) is examined via three-dimensional (3D) computed-tomography-based analysis of carpal kinematics in several anthropoid taxa. In addition, 3D morphometrics are used to identify anatomical correlates of wrist function. In turn, these data are employed to reconstruct wrist mechanics in fossils and infer the probable sequence of functional transformations in hominins and other anthropoids. The results indicate that proximal-carpal-row mobility during extension is restricted in extant apes vis-à-vis palmigrade-capable monkeys due to a volar declination of the distal radius and consequent dorsal ridge, which is present in Ardipithecus ramidus, Australopithecus anamensis, and A. afarensis, but absent in later hominins. More distally, chimpanzees and humans exhibit shared-derived midcarpal kinematics resulting in rapid engagement of the proximal and distal rows, such that the midcarpus is close-packed midway through wrist dorsiflexion. Scaphoid-centrale fusion and an abbreviated lunatocapitate articular arc are key correlates of the Pan-Homo kinematic pattern, and these features characterize all hominins for which wrist anatomy is known—indicating that a rigid wrist in extension is primitive for the lineage. Later hominins primarily owe their increased total dorsiflexion to secondary reorientation of the distal radius, which increased radiocarpal mobility while retaining midcarpal constraints. These secondarily-derived radiocarpal modifications may have resulted from relaxed selection on climbing competency or positive selection on hand mobility (possibly to facilitate tool behaviors).
Funding was provided by the National Science Foundation (BCS-622515), the Wenner-Gren Foundation for Anthropological Research, and Arizona State University.