Patterns of craniofacial integration in extant Homo, Pan, and Gorilla

Brain size increased greatly during Pleistocene human evolution, while overall facial and dentognathic size decreased markedly. This mosaic pattern is due to either selective forces that acted uniquely on each functional unit in a modularized, developmentally uncoupled craniofacial complex, or alternatively, selection that acted primarily on one unit, with the other responding passively as part of a coevolved set of ontogenetically and evolutionarily integrated structures. Using conditional independence modeling on homologous linear measurements of the height, breadth, and depth of the cranium in Pan (n = 95), Gorilla (n = 102), and recent Homo (n = 120), we reject the null hypothesis of equal levels of overall cranial integration. While all three groups share the pattern of greater neurocranial integration with distinct separation between the face and neurocranium (modularization), family differences do exist. The apes are more integrated in their entire crania, but display a particularly strong pattern of integration within the facial complex related to prognathism. Modern humans display virtually no facial integration, a pattern which is likely related to their markedly decreased facial projection. Modern humans also differ from their great ape counterparts in being more integrated within the breadth dimension of the cranial vault, likely tied to the increase in brain size and eventual globularity seen in human evolution. That the modern human integration pattern differs from the ancestral African great ape pattern along the inverse neurocranial-facial trend seen in human evolution indicates that this shift in the pattern of integration is evolutionarily significant, and may help to clarify aspects of the current debate over defining modern humans.