Evidence for independent brain and neurocranial reorganization during hominin evolution
Human brains differ substantially from those of great apes, and equally important differences exist between their braincases. However, it remains unclear to which extent evolutionary changes in brain structure are related to changes in braincase structure. To study this question, we use combined computed tomography (CT) and MRI head data of humans and chimpanzees and quantify the spatial correlations between brain sulci and cranial sutures. We show that the human brain–braincase relationships are unique compared to chimpanzees and other great apes and that structural rearrangements in the brain and in the braincase emerged independently during human evolution. These data serve as an important frame of reference to identify and quantify evolutionary changes in brain and braincase structures in fossil hominin endocasts.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.