Evolutionary morphology of the haplorhine hamate

Primates adopt a variety of hand postures during an impressive diversity of locomotor and manipulative behaviors. Morphological research has found that elements of the hand skeleton, such as the hamate, hold key information for inferring hand use and locomotor kinematics in extinct species. However, debate remains surrounding how and when specialized locomotor hand postures evolved. This debate persists due to uncertainty concerning which taxa possess derived character states for given traits and whether those states evolved via selective response to functional demands or are the result of other biological processes, such as developmental constraint or phylogeny. Phylogenetic comparative methods provide one way to disentangle the role of phylogeny in shaping skeletal morphology and thus can provide clarity concerning which traits arose via other means, such as directed selection. Here, we quantify several functionally relevant hamate traits and employ phylogenetic methods (bayou, OUwie) to infer if and how selection has shaped these traits in particular taxa or clades. We find that selection caused the hamulus to reduce in tarsiers and to lengthen in cebids and hominoids, potentially related to increased demand for grip capability during behaviors like tool use or-in hominoids-below-branch suspension. Evidence from traits associated with suspension suggests that this behavior evolved independently in hylobatids and hominids. Shifts toward shorter hamate body lengths in Gorilla and hominins cannot be explained by phylogeny alone. Variability among hominines eschews simple interpretation, but this result suggests that if knuckle-walking evolved once in hominines, then the hamate subsequently lengthened in Pan.