Early developmental patterning sets the stage for brain evolution

The evolution of brain structure and function has long fascinated biologists. This fascination was initially prompted by the observation that allometric relationships exist between the size of the brain—or brain region—and body size across a wide range of vertebrates (1). Besides random drift, two main ideas have been advanced to explain how brains evolve, and both have found considerable support. From a purely adaptationist point of view, selection on a specific set of behavior patterns or sensory specializations is thought to result in “mosaic” changes in only the brain regions that mediate these processes (2). In contrast, selection on any single brain region would cause the brain to change as a whole unit owing to developmental constraints and processes that regulate the formation and growth of a range of brain regions overall (3). Beyond the “just so” stories that often characterize the interpretation of the causes and origins of brain diversity (4), two problems have vexed this line of research. First, it is not at all obvious how an increase in (relative) size would give rise to functional differences (e.g., increased cognitive abilities, novel sensory specializations, or behavioral complexity). Although a larger number of neurons and/or synapses might well result in greater processing power and/or speed, there is no clear relationship between such measures and behavioral or cognitive outcomes. Second, our understanding of the developmental mechanisms that give rise to the observed variation in brain structure is still very limited, and most studies have suggested that neurogenesis later in development generates diversity, which might result in the differential expansion of various brain areas (3). In this context it is also important to keep in mind that differences in brain structure and function can be as much a consequence of genetic and/or developmental control as they can be the result …