The radial wrist as a morphological and functional unit in extant African apes and humans

Abstract

Previous research has identified multiple aspects of carpal (i.e., wrist bones) morphology in Homo sapiens (humans) and some fossil hominin species that may reflect adaptations to the habitual use and manufacture of stone tools, particularly among bones from the radial side of the wrist (i.e., the, trapezium, trapezoid, scaphoid, and capitate). Using three-dimensional (3D) surface models of radial-side carpals and 3D geometric morphometrics (3DGM), this study aims to quantify the shape variation of this anatomical region among extant African apes and humans. Extending on previous studies that have typically quantified carpal morphology by studying each bone individually and in isolation from each other, this study marks the first time where these four radial-side carpals are quantitatively analyzed together as articulated units. Based on previous descriptions as well as qualitative and quantitative analyses, Pan troglodytes (chimpanzees), Pan paniscus (bonobos), and Gorilla gorilla gorilla (western lowland gorillas) wrists were predicted to display features that facilitate radioulnar stability and proximodistally-directed loading, which would be more efficient for locomotor behaviours. Conversely, human wrists were predicted to display features that facilitate proximodistal stability and radioulnarly-directed loading, which would be more efficient for manual dextrous behaviours. Results showed that the combined shapes of these carpal bones, as well as their sizes and orientations relative to each other, vary considerably between the African ape and human samples. The African ape wrists showed, as predicted, a complex of features that provides biomechanical advantages for withstanding and distributing forces directed proximodistally during knuckle walking and other locomotor behaviours. In contrast, the human wrists displayed a complex of features that appears to provide biomechanical advantages for withstanding and distributing large forces directed radioulnarly during human-like power and precision grips.