Abstract
The level of craniofacial diversity of hominids (the group that includes great apes and humans) is much higher than that of their sister group, the hylobatids (also known as gibbons or lesser apes), despite the similar timeline of diversification and a similar level of genetic differentiation between the two clades. To shed light on the evolutionary dynamics shaping these varying levels of craniofacial diversity, we used three-dimensional high-density geometric morphometric approaches and phylogenetic comparative methods. We show that neurocranial diversity exceeds that expected from neutral evolution in the great apes with respect to the gibbons, whereas facial diversity does not. These results indicate that neurocranial diversity across the great apes has been shaped by genus-specific neurocognitive, social or ecological selective pressures that are not observed in the gibbons, whose neurocranial diversity is constrained by stabilizing selection and gene flow. However, facial diversity results from similar evolutionary dynamics across both clades. Within this general pattern of differences and similarities between the great and lesser apes, humans emerge as the only species that consistently shows the highest evolutionary rate across almost all craniofacial regions in both males and females, thus pointing to strong human-specific selective pressures in neurocranial and facial evolution.