Doing with less: hominin brain atrophy.

In contrast to hominin encephalization, the final Pleistocene and Holocene reduction in cranial volume has attracted very little attention and remains unexplained. Here it is examined in the light of current neuroscientific and archaeological understanding, and it is shown that the most parsimonious explanation is via the domestication hypothesis of recent humans. Accordingly, rapid atrophy of the brain is partly explained by the culturally based process of sexual selection, first detectable in late robust Homo sapiens perhaps 40,000 years ago. Furthermore it is suggested that this deleterious process of neotenization and brain atrophy was compensated for by the concurrent development of exograms, i.e. means of storing memory outside the brain. Consequently most of human memory and cultural information is now stored external to the brain, which has altered that organ significantly and facilitated a cultural complexity that would be impossible to maintain by biological memory alone. The escalating use of exograms, neotenization and reduction in cranial volume all appear to co-occur with numerous other changes to the human genome.