Harry Jerison argued in his Evolution of the Brain and Intelligence that the initial enlargement of the brain of ancestral mammals resulted from refinement of the sensory processes of audition and olfaction, improving adaptation to life in nocturnal niches (Jerison, 1973). Every relationship and transition hypothesized in this statement is an intriguing subject for investigation. Twenty-five years later, challenged by Jerison's vision, we (and he) have learned a great deal about the variation in the structure of the brains of extant mammals and of those mammals represented in the fossil record (Jerison, 1991). Concurrently, the explosion of knowledge in neuroscience has profoundly changed our view of the organization, localization and development of functional systems in the brain. We can now make much better sense of variation in neural structure in terms of behavioral function. In particular, appreciation of how functions can be distributed over multiple brain components is coming to replace our initial single structure-single function models. Furthermore, we can now extend Jerison's initial evolutionary question to ask how brain changes come about developmentally.

In this review, we first examine the extent to which specific structures in the brain may be the targets of selection, or whether change occurs at the level of interconnected functional systems, other organizational units, or at the most global level of brain size. After we have described where variation occurs in the brains of extant mammals, and at which level or levels this variation occurs, our job as developmental neurobiologists is to describe how development produces these differences.