Comparative Endocranial Development in Primates
Nadia Scott, 2017, Fellow Writing-Up
By comparing species-specific developmental patterns, we can approach the question of how development shapes adult morphology. We focus here on shape change trajectories of the primate endocranium, an integrated system that arises as the result of a complex interplay between bone, meninx and the expanding brain. Previously, we have shown that the pattern of endocranial development in modern humans deviates from that of chimpanzees (Neubauer et al., 2010) and Neanderthals during the first year of life (Gunz et al., 2010; 2012), but subsequently reverts to an ontogenetic pathway shared by all three groups. To explore whether this ontogenetic pattern is shared among extant hominoid species, we characterised and compared shape changes of the endocranium from infancy (erupted deciduous dentition) to adulthood in a cross-sectional sample of modern humans (n=87), chimpanzees (n=59), gorillas (n=67), orangutans (n=75) and gibbons (n=21). On virtual endocasts generated by segmenting computed tomographic scans of dried crania, we measured 29 three-dimensional endocranial landmarks as well as several hundred semilandmarks on curves and the endocranial surface. Following sliding of semilandmarks, Procrustes superimposition was used to standardize location, orientation and scale. To account for the non-linearity of the developmental trajectories, we determined the similarities of ontogenetic patterns between species by interchanging their ontogenetic trajectories. The results of our developmental simulations indicate that, from the eruption of complete deciduous dentition, the patterns of endocranial development are similar among hominoids, but differ in the amount of shape change produced. To determine to what degree endocranial development is independent of facial development, we performed partial least squares analyses on our great ape data, which allows us to examine the interdependence of two or more developmental modules. Using the face and the endocranium as separate modules, our results indicate that evolutionary integration is similar between species, indicating that development is highly conserved between species. Developmental integration, however, differs with ontogeny, transitioning from a weakly integrated period during infancy to a highly integrated period from the eruption of the deciduous dentition onwards. Our results indicate that early changes to the face are not translated to the endocranium, but that later changes occur in tandem between the two developmental modules.