Morphological integration and the evolution of knuckle-walking

The evolution of knuckle-walking has profound implications for our understanding of the emergence of bipedalism. The modern debate surrounding its evolution is concerned with whether or not it is homologous in chimpanzees and gorillas. Here, this problem is approached using the methods of morphological integration to test hypotheses of patterns and magnitudes of integration in the third manual ray and capitate. If knuckle-walking morphologies are highly integrated and evolve in a correlated bundle (i.e., comprising a functional complex), it seems reasonable that they could have been recruited independently relatively easily in gorillas and chimpanzees, thus increasing the likelihood of homoplasy. If, however, there is no evidence for a knuckle-walking complex, then it seems less likely that chimpanzees and gorillas would have evolved knuckle-walking independently. Results indicate that chimpanzees and gorillas are not characterized by high magnitudes of integration or unique patterns of integration that distinguish them from non-knuckle-walking taxa. This does not support the hypothesis of a knuckle-walking complex, nor does it support the contention that knuckle-walking could have been easily evolved independently in chimpanzees and gorillas. Implications for trait analysis and the evolution of bipedalism are discussed, as are recent analyses supporting the independent origins of knuckle-walking.