Pandora's growing box: Inferring the evolution and development of hominin brains from endocasts.

The brain of modern humans is an evolutionary and developmental outlier: At birth, it has the size of an adult chimpanzee brain and expands by a factor of 2 during the first postnatal year. Large neonatal brain size and rapid initial growth contrast with slow maturation, which extends well into adolescence. When, how, and why this peculiar pattern of brain ontogeny evolved and how it is correlated with structural changes in the brain are key questions of paleoanthropology. Because brains and their ontogenies do not fossilize, indirect evidence from fossil hominin endocasts needs to be combined with evidence from modern humans and our closest living relatives, the great apes. New fossil finds permit a denser sampling of hominin endocranial morphologies along ontogenetic and evolutionary time lines. New brain imaging methods provide the basis for quantifying endocast-brain relationships and tracking endocranial and brain growth and development noninvasively. Combining this evidence with ever-more detailed knowledge about actual and fossil "brain genes," we are now beginning to understand how brain ontogeny and structure were modified during human evolution and what the adaptive significance of these modifications may have been.