Genetics of cortico-cerebellar expansion in anthropoid primates: a comparative approach

What adaptive changes in brain structure and function underpin the evolution of increased cognitive performance in humans and our close relatives? Identifying the genetic basis of brain evolution has become a major tool in answering this question. Numerous cases of positive selection, altered gene expression or gene duplication have been identified that may contribute to the evolution of the neocortex, which is widely assumed to play a predominant role in cognitive evolution. However, the neocortex co-evolves with other, functionally inter-dependent, regions of the brain, most notably the cerebellum. The cerebellum is linked to a range of cognitive tasks and expanded rapidly during hominoid evolution, independently of neocortex size. Here we demonstrate that, across primates, genes with known roles in cerebellum development are just as likely to be targeted by selection as genes linked to cortical development. In fact, cerebellum genes are more likely to have evolved adaptively during hominoid evolution, consistent with phenotypic data suggesting an accelerated rate of cerebellar expansion in apes. Finally, we present evidence that selection targeted genes with specific effects on either the neocortex or cerebellum, not both. This suggests cortico-cerebellar co-evolution is maintained by selection acting on independent developmental programs.