Primate brain evolution: Integrating comparative, neurophysiological, and ethological data

Undoubtedly the most distinctive trait of the Primates, wherein this order contrasts with all other mammalian orders in its evolutionary history, is the tendency towards the development of a brain which is large in proportion to the total body weight, and which is particularly characterized by a relatively extensive and often richly convoluted cerebral cortex (p. 228).(1) While this statement is generally true, primate brains vary in size nearly one thousand-fold, from a mass of 1.8 g in the tiny mouse lemur to 1,300 g in modern humans. Many attempts have been made to understand both the distinctiveness of primate brains and the variation observed within the order: How did such variation evolve and why, and what are its cognitive implications? Following Jerison's(2) masterly review thirty years ago, comparative studies have highlighted suggestive correlations of brain size. However, the meaning and validity of these correlations have been vigorously debated. It has become clear that progress depends on taking great care in the use of comparative methods and in finding multiple converging strands of comparative evidence as opposed to making speculative interpretations of single correlations. In particular, recent work demonstrates the value of examining how evolutionary changes at different anatomical levels interrelate.