The pattern of endocranial ontogenetic shape changes in humans

Abstract

Humans show a unique pattern of brain growth that differentiates us from all other primates. In this study, we use virtual endocasts to provide a detailed description of shape changes during human postnatal ontogeny with geometric morphometric methods. Using CT scans of 108 dried human crania ranging in age from newborns to adults and several hundred landmarks and semi-landmarks, we find that the endocranial ontogenetic trajectory is curvilinear with two bends, separating three distinct phases of shape change. We test to what extent endocranial shape change is driven by size increase and whether the curved ontogenetic trajectory can be explained by a simple model of modular development of the endocranial base and the endocranial vault. The hypothesis that endocranial shape change is driven exclusively by brain growth is not supported; we find changes in endocranial shape after adult size has been attained and that the transition from high rates to low rates of size increase does not correspond to one of the shape trajectory bends. The ontogenetic trajectory of the endocranial vault analyzed separately is nearly linear; the trajectory of the endocranial base, in contrast, is curved. The endocranial vault therefore acts as one developmental module during human postnatal ontogeny. Our data suggest that the cranial base comprises several submodules that follow their own temporally and/or spatially disjunct growth trajectories.

Larger mammals have longer faces because of size-related constraints on skull form

“…Size and shape were averaged within species. Size was transformed using the natural logarithm and appended to the matrix of Procrustes shape coordinates to perform a principal component analysis (PCA) in the Procrustes form space [35][36][37] . PC1 is therefore dominated by size and summarises the pooled within group allometry, which represents the 'common allometric component' (p. 684) (ref.…”

Brain shape convergence in the adaptive radiation of New World monkeys

“…For each specimen X-ray computed tomography or micro-computed tomography scans were acquired or downloaded from public repositories (21). Three-dimensional virtual endocasts were generated from these data following a threshold-based 2D segmentation procedure (21,35). From the segmented images, a surface 3D model-the virtual endocast-was generated, and the endocranial volume (ECV) was measured as the volume enclosed by this surface.…”

Neurocranial evolution in modern humans: the case of Jebel Irhoud 1

“…The enlargement of the parietal areas is a discrete and well-expressed trait of the endocranium of H. sapiens (Bruner et al, 2003(Bruner et al, , 2011Bruner, 2004;Neubauer et al, 2009Neubauer et al, , 2010Gunz et al, 2010). Considering the role of the parietal areas in the perception and management of the relationships between inner and outer world (Bruner, 2010a), a two-part model of the evolution of modern humans might also explain some of the differences between early (Middle Stone Age/Middle Paleolithic) and late (Later Stone Age/ Upper Paleolithic) modern human cultures (Klein, 2000(Klein, , 2008Foley and Lahr, 2003;Wynn and Coolidge, 2003;Coolidge and Wynn, 2005).…”