Cranial vault thickness, its internal organization, and its relationship with endocranial shape in Neanderthals and...

Compared to the more elongated crania of Neanderthals, modern humans have a rounder, more globular cranial vault. The factors contributing to this globular cranial and endocranial morphology remain poorly understood. Cranial vault thickness (CVT) plays a role in shaping the braincase. It has been proposed that CVT variation in hominins reflects distinct stimuli influencing the cranial vault layers to different degrees. We aim to determine to what extent CVT differences could explain the well-documented endocranial shape differences between modern humans and Neanderthals. Additionally, we quantify the contributions of each cranial vault layer-the inner and outer tables and the diploë-to elucidate the processes driving CVT variation. We analyzed crania from seven Neanderthals and 75 modern humans using a ray casting method to measure cortical, diploic, and overall CVT. We generated morphometric maps of thickness distributions for each cranial vault layer. We then quantified the endocranial shape using geometric morphometrics and analyzed its relationship with CVT using two-block partial least squares analysis and regression models. Our findings reveal that Neanderthals generally have greater CVT and thicker layers of the cranial vault than modern humans. However, their ranges overlap with those of modern humans. The thicker vaults of Neanderthals are primarily driven by the diploic layer, with notable differences in thickness distribution patterns. Additionally, the inner and outer cortical tables exhibit distinct thickness distribution patterns between the two groups. Structural differences between the parietal bones of Neanderthals and modern humans are particularly pronounced. Furthermore, we observed a unique correspondence between CVT and endocranial shape in modern humans, which is not shared by Neanderthals.