Comparative Anatomical Network Analysis of Chimpanzee and Human Craniocerebral Topology

We examine which structural components are fundamental in the craniocerebral spatial arrangement of chimpanzees (Pan troglodytes, Blumenbach 1776) and humans, contrast underlying phenotypic patterns of spatial variation and try to comprehend how these structural components and phenotypic patterns might have influenced the evolution of brain morphology in the two lineages.

A chimpanzee craniocerebral network (126 nodes and 539 edges) is introduced and compared with a prior human model by means of network analysis. Network analysis considers both local and global parameters of systems and can assess spatial constraints due to physical interactions.

The chimpanzee craniocerebral system exhibits (1) a structurally relevant inferior-medial region which might be suggested to constrain morphological evolution; (2) efficient diffusion of information, an essential aspect of morphological variability in terms of plasticity; and (3) the concurrence of a longitudinal and a vertical modular partition, indicative of the cranial constraints imposed on cerebral topology.

The craniocerebral systems of both species display many similarities, which probably trace back to their Last Common Ancestor (LCA), 7-10 million years ago. Nevertheless, some differences are also found. The ethmoid bone's contribution to craniocerebral integration, specifically, is exceptional in humans but modest in chimpanzees, possibly due to differences in the growth process of the face. Another difference lies in their modularity: in chimpanzees, the frontal bone is lumped together with other elements of the calvaria and with the brain, whereas in humans, it participates in the facial block, probably owing to the relatively anterior position of the frontal bone in this species.