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Summary
During the Mesozoic (∼250 million to 65 million years ago), two distantly related groups of reptiles—the cynodont (or mammal-like) reptiles and the coelurosaurian theropod dinosaurs—gave rise to mammals and birds, respectively. Both mammals and birds evolved brains some 10 times as large, relative to a given body weight, as those of their ancestors (1). In both groups, these brains contributed to the evolution of the ability to control body temperature (endothermy) and complex social interactions, including parental care and a reliance on learning that even involves tool use (2, 3). The size of most parts of the brain increased in birds and mammals, but the cerebral hemispheres and cerebellum, both of which are involved in sensory and motor integration, underwent particularly spectacular development (see the figure). Although mammals and birds evolved from distantly related groups of reptiles, the higher integrative centers and circuitry of their cerebral hemispheres are very similar, and comparative neurobiologists continue to vigorously debate whether these centers evolved from the same ancestral neural centers (4, 5) or from different ones (6–8). Speculation about the evolutionary steps leading to large and complex mammalian and avian brains is equally contentious and unresolved, in part because of the rarity of fossil skulls and, until recently, the need to destroy such skulls in order to expose the endocasts (casts molded by the cranial cavity). Typically, endocasts are the only record of the brain's outward appearance in a transitional form, because brains themselves are rarely fossilized.
