How did brains evolve?

Abstract

Three reports on mammalian brain evolution^1,2,3 analyse the same comparative data on brain component volumes⁴ but come to partially conflicting conclusions. Clark et al.³ conclude from their analysis of volumetric brain proportions (“cerebro-types”) that cerebellum size is invariant across mammalian taxonomic groups, the neocortex and cerebellum do not co-vary in size (in contradiction to ref. 1), and cerebrotype-based measures identify directional changes in brain architecture. Here I provide evidence that calls each of these conclusions into question. The failure of the cerebrotype measure to identify species differences in brain architecture that are independent of gross brain size undermines the proposal by Clark et al. that it could be useful for detecting evolutionary patterns and phylogenetic relationships.