Abstract Neuroscientists make inferences about the human brain by studying nonhuman species, an enterprise that depends on assumptions about the nature of evolution. Traditionally, many neuroscientists have supposed that all mammals possess variants of the same brain, differing only in size and degree of elaboration. Under this model, the brains of nonhuman species can be treated as simplified versions or models of the human brain. However, there is evidence that mammalian cerebral organization is much more variable than is commonly acknowledged. The diversity of mammalian brain organization implies that neuroscientists can make better inferences about human brain organization by comparing multiple species chosen based on their evolutionary relationships to humans, than by studying single "model" or "representative" species. The existence of neural diversity also suggests that nonhuman species have evolved cognitive specializations that are absent in humans.