The hominoid wrist and the role of developmental perspectives on inferring homoplasy

Developmental data can be a useful resource for resolving character homology. Both humans and orangutans have shortened pisiforms despite dramatic differences in forelimb function. Humans accomplish this through the unusual loss of a growth plate, while orangutans retain two separate ossification centers implying parallel evolution by distinct developmental mechanisms. In hominoids, pisiform reduction occurred within the context of ulnar withdrawal and superposition of the pisiform upon the triquetral. Thus, we conducted a comparative multivariate analysis to test the hypothesis that pisiform reduction is correlated with functional changes in the triquetral. Principal components analysis of 38 metrics spanning the wrist confirms greater similarity of the African ape and human carpus relative to orangutans. However, when analysis is limited to 11 metrics from the triquetral, humans and orangutans are separated from African apes. Human and orangutan triquetrals share a medial and distal placement of the pisiform articulation and a projecting pisiform pillar relative to African apes. Interestingly, Ardipithecus provides a combination of human/orangutan and African ape features suggesting that the African ape triquetral is also derived relative to the ancestral condition. Similarities in humans and orangutans may reflect mechanisms facilitating joint stability with the lost of constraints for quadrupedal locomotion, while African apes reflect parallel refinements for knuckle-walking from a palmigrade ancestor. As Proconsul retains ulnar-pisiform contact, it is possible that ulnar withdrawal also evolved in parallel between Asian and African hominoids. We present approaches that combine functional and comparative genomics that can test these hypotheses.