Evidence for independent brain and neurocranial reorganization during hominin evolution.

Throughout hominin evolution, the brain of our ancestors underwent a 3-fold increase in size and substantial structural reorganization. However, inferring brain reorganization from fossil hominin neurocrania (=braincases) remains a challenge, above all because comparative data relating brain to neurocranial structures in living humans and great apes are still scarce. Here we use MRI and same-subject spatially aligned computed tomography (CT) and MRI data of humans and chimpanzees to quantify the spatial relationships between these structures, both within and across species. Results indicate that evolutionary changes in brain and neurocranial structures are largely independent of each other. The brains of humans compared to chimpanzees exhibit a characteristic posterior shift of the inferior pre- and postcentral gyri, indicative of reorganization of the frontal opercular region. Changes in human neurocranial structure do not reflect cortical reorganization. Rather, they reflect constraints related to increased encephalization and obligate bipedalism, resulting in relative enlargement of the parietal bones and anterior displacement of the cerebellar fossa. This implies that the relative position and size of neurocranial bones, as well as overall endocranial shape (e.g., globularity), should not be used to make inferences about evolutionary changes in the relative size or reorganization of adjacent cortical regions of fossil hominins.