African ape carpal morphology is not too specialized to represent the ancestral hominin carpus.

How and why hominins evolved key behaviors, such as obligate bipedalism and sophisticated tool production, are key questions in paleoanthropology. Crucial to this inquiry are accurate reconstructions of the morphology and behavior that characterized the panin-hominin last common ancestor (LCA). Because all extant hominines besides Homo sapiens knuckle-walk, this rare hand posture has received special attention as a possible behavior in the LCA's locomotor repertoire. Arguments concerning knuckle-walking ancestry in hominins typically examine functional morphology to explore possible adaptations to knuckle-walking or leverage biological principles to evaluate the likelihood of certain evolutionary scenarios. Because wrist biomechanics in knuckle walking differ from those of other behaviors, carpals are frequently investigated for possible knuckle-walking specializations. Here, we quantified carpal morphology using spherical harmonics analysis to reevaluate purported knuckle-walking adaptations among an anthropoid sample with variable locomotor behaviors. We calculated Euclidean distances between behavioral pairs to investigate whether the carpals in knuckle-walking taxa appear specialized relative to those of other taxa. Results show that nearly all hypothesized knuckle-walking adaptations are broadly present in anthropoids, and several are specially shared with Asian apes. Some African ape carpal features are shared uniquely with humans and may signal knuckle-walking ancestry. These results overall suggest that (1) the carpals of African apes are not specialized for knuckle walking; (2) to evolve knuckle walking, only few morphological changes to ancestral hominid carpals are necessary; and (3) an African ape-like carpus is not too specialized to be a viable ancestral form from which the hominin carpus evolved.