Comparing endocranial ontogenetic trajectories in extant great and lesser apes

Abstract

Differences in adult morphology arise as a result of modifications to ancestral developmental patterns. Studies of evolutionary changes to brain development in primates can provide important clues about the evolution of human cognition, but are hindered by the lack of preserved neural tissue in the fossil record. As brain structure is mirrored in the endocranium, we have used endocasts (virtual casts of the endocranial cavity) to capture brain shape in a geometric morphometrics analysis. Here, we compare the postnatal developmental changes of the endocasts of modern humans (n=93), chimpanzees (n=61), gorillas (n=64), orangutans (n=72) and hylobatids (n=21) to assess evolutionary changes in the tempo and mode of brain development. We tested whether ontogenetic trajectories based on Procrustes shape coordinates and centroid sizes overlapped in shape‐space and form‐space and differed in their direction. In the subspace of the first three principal components, explaining >;71% of the variance in shape‐space, our results show a conserved pattern of endocranial shape development in hominids. Hominids and hylobatids have similar ontogenetic trajectories in shape‐space, yet have distinct shapes at all stages of development. These findings indicate that differences in ontogenetic shape changes are observable from the first stage of postnatal development, suggesting that divergences from an ancestral pattern of primate development occur at an earlier prenatal stage. This research was supported by the Max Planck Society and by the Natural Sciences and Engineering Research Council of Canada (NS).