Adaptive evolution of microcephaly genes.

The human brain is dramatically enlarged compared to our closest relatives. Brain expansion is closely linked to the evolution of complex behaviour and cognition in hominins. Identifying the genetic basis of brain expansion provides one route to understanding what is, and what is not, unique about our species. As part of this approach, researchers have turned to neurodevelopmental disorders for candidate mechanisms. Microcephaly, a disorder characterised by a major and specific underdevelopment of brain size, is a well‐studied example. Genes associated with microcephaly evolved adaptively across both primates and nonprimate mammals, and selection on at least two genes, ASPM and CDK5RAP2, is associated with variation in brain size in anthropoid primates, and potentially other mammalian clades. While these results support predictions of a model of brain expansion that focuses on cell fate switches during neurogenesis, the causative mechanisms linking selection on microcephaly genes to the evolution brain size remain unclear.