Frontal brain expansion during development using MRI and endocasts: relation to microcephaly and Homo floresiensis.

A major hall of hominid brain evolution is an expansion of the frontal lobes. To determine if a similar trajectory occurs during modern human development, the MRI scans of 118 living infants, children, and adolescents were reviewed and three specific measurements obtained: frontal width (FW), maximal cerebral width (MW), and maximal cerebral length (ML). The infantile brain is uniformly wide but relatively short, with near equal FW and MW. The juvenile brain exhibits a wider MW than FW, while FW of the adolescent brain expands to nearly equal MW, concurrent with an increase in ML. The preferential frontal lobe expansion during modern human development parallels that observed during the evolution of Homo. In 17 microcephalic individuals, only 6 (35%) exhibited preferential frontal lobe hypoplasia, presumably a reflection of multiple etiologies that adversely affect differing brain regions. Compared to 79 modern human adult endocasts and 12 modern microcephalic endocasts, LB1 (Homo floresiensis) clustered more consistently with the microcephalic sample than with the normocephalic sample.