NORTHCUTT,, R.G.: What Do We Want to Know About Vertebrate Brain Evolution?

Four major questions can be asked about how vertebrate brains evolve: 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 the major changes in brain parts over time and have documented them extensively. They have also made progress in understanding the functional significance of these changes, but such progress has been somewhat limited to sensory circuits, rather than integrative or motor circuits, because of the relative ease of manipulating relevant stimuli. Although neuropaleontology continues to provide important insights into when changes have occurred, this approach is generally limited to recognizing changes in overall brain size and, to a lesser extent, changes in the few neural features that can be interpreted from the surface of an endocranial cast. In recent years, most new information regarding when neural changes occurred has been based on cladistic 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 to understanding 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, in large part because it requires a broad interdisciplinary approach that involves both behavior and ecology.