Understanding vertebrate brain evolution.

Four major questions can be asked about vertebrate brain evolution: 1) What major changes have occurred in neural organization and function? 2) When did these changes occur? 3) By what mechanisms did these changes occur? 4) Why did these changes occur? Comparative neurobiologists have been very successful in recognizing major changes in brain structure. They have also made progress in understanding the functional significance of these changes, although this understanding is primarily limited to sensory centers, rather than integrative or motor centers, because of the relative ease of manipulating the relevant stimuli. Although neuropaleontology continues to provide important insights into when changes occurred, this approach is generally limited to recognizing variation in overall brain size, and sometimes brain regions, as interpreted from the surface of an endocranial cast. In recent years, most new information regarding when neural changes occurred has been based on cladistical analysis of neural features in extant taxa. Historically, neurobiologists have made little progress in understanding how and why brains evolve. The emerging field of evolutionary developmental biology appears to be the most promising approach for revealing how changes in development and its processes produce neural changes, including the emergence of novel features. Why neural changes have occurred is the most difficult question and one that has been the most ignored, in large part because its investigation requires a broad interdisciplinary approach involving both behavior and ecology.