Postnatal interaction of size and shape in the human endocranium and brain structures

The uniqueness of human brain growth and development has been considered promising for its contribution to understanding the origins of the unique human cognitive abilities. Compared with that of chimpanzees, the human endocranium undergoes several characteristic shape changes immediately after birth, which has been termed “endocranial globularization.” However, how the brain structures and surrounding neurocranium interact with each other during early development in the context of brain–neurocranium integration remains to be investigated. We investigated shape and size changes in the human brain and endocranium during postnatal development using magnetic resonance imaging, and analyzed spatial constraints and interactions among subdivisions of the brain influencing endocranial morphology. Our results suggest that, during postnatal development, the relative size changes of supratentorial and infratentorial regions and the cranial base largely constrain brain and endocranial shape. Specifically, a disproportionate increase in the size of the infratentorial region (i.e., cerebellum plus brainstem) relative to the cranial base affects the infratentorial spatial packing constraint in neonates, causing inferoposterior expansion of the posterior cranial fossa and coronal reorientation of the petrous pyramid of the temporal bone without flattening the angle between the two sides of the tentorium cerebelli. The dramatic size increase of the infratentorial region relative to the cranial base immediately after birth is inferred to be characteristic of human development and should be compared with non‐human primates and potentially applied to fossil cranial series to obtain more evolutionary insight into the human cognitive ability.