Covariation between cranial vault thickness and endocast shape in present-day humans and Neanderthals

While cranial vault thickness (CVT) is often used to discuss the taxonomic assignment of fossil specimens within the human lineage, the evolutionary processes accounting for its variation remain elusive. Numerous hypotheses have been proposed to interpret CVT arrangements, whether they relate to systemic, metabolic, biomechanical, allometric factors and/or to brain evolution. As they reach similar brain volumes via different developmental trajectories, comparing the CVT of present-day humans and Neanderthals allow us to evaluate whether brain development is likely to affect CVT. Here we document the CVT arrangements among these two groups and we test if differences are likely to be explained by variation in brain shape. We have used a sample of computed-tomographic scans consisting of 75 present-day humans and 6 Neanderthal adult individuals. Their respective endocranial shapes were quantified using landmarks and semilandmarks-based geometric morphometrics. The CVT was calculated at different semilandmarks positions and the cubic root of the endocranial volume was used for standardizing size in each measurement. Finally, the covariation between the shape of the endocast and CVT measurements was subsequently explored via two-block partial least squares analysis. Our results demonstrate that Neanderthals tend to display a thicker cranial vault, but this variation is still comprised within the range of present-day humans. The distinctive globular shape of present-day humans is characterized by thinner parietal walls. These results suggest that the differences in brain development in these two groups partly explain their respective CVT arrangements.