Human Carpal Evolution and its Implications for Hominin Locomotor Ancestry

Abstract

The ancestral locomotor repertoire from which hominin bipedalism emerged remains a subject of frequent debate, with multiple competing hypotheses maintaining support among active researchers. This study contributes to this ongoing debate by reconstructing the morphological and locomotor evolution of hominoids based on patterns of morphometric variation in the capitate, hamate, lunate, and triquetrum among extant and fossil anthropoids, including humans and fossil hominins. Aspects of carpal morphology found to consistently covary with locomotor behavior are used to reconstruct adaptive transitions within the anthropoid clade associated with locomotor behavior. The prevalence of different locomotor behaviors at the origin of nested clades within Hominoidea, including the one we share with Pan, are also estimated.While there is some inconsistency of morphological covariation with function and phylogeny among carpal elements, results are consistent in supporting frequent parallelism during hominoid locomotor evolution. This support is strongest as it applies to suspensory behaviors, with the last common ancestors of both apes and great apes predicted to have remained quite generalized relative to the clades’ extant representatives. The last common ancestor of humans and chimpanzees/bonobos is also estimated to have lacked adaptations in association with knuckle‐walking, providing further support for the hominin clade having descended from a relatively generalized ancestral morphotype that is not well modeled by any of the surviving hominoid lineages.