How Does evolution build a complex brain?

Leah Krubitzer, Center for Neuroscience and Department of Psychology, University of California, Davis.

We know that mammals differ in brain size, and that mammals with large brains generally have more complex sensory, perceptual, and cognitive behaviors than mammals with small brains. Comparative studies on the organization and connections of the neocortex, like those done in our laboratory, indicate that size differences alone cannot explain this high degree of behavioral variability observed across mammals. Rather, it is the number of functional areas and their connections that are responsible for these remarkable differences in complex behaviors. This is particularly true for the neocortex. Fossil records together with comparative studies indicate that the first mammals had small brains with a very little neocortex, and only a few cortical areas. Thus, small brains with few parts were the forerunners of the large, complexly organized brains. The driving question is: How are large brains with multiple, functional parts generated? What is the evolutionary (inherited, genetic) contribution and what role does activity from peripheral receptor arrays play in the generation of cortical organization. We can answer these questions by using a comparative approach to examine the products of the evolutionary process, and in turn, make inferences about the process itself. We can also study the development of the neocortex, since phenotypic variability is generated, at least in part, by modifications to developmental regimes. Our comparative studies have allowed us to determine the similarities in cortical organization across species, and to propose homologous features of the neocortex. These types of studies have also allowed us to identify ubiquitous modifications made to extant brains and determine constraints imposed on evolving nervous systems. To test theories generated from comparative studies, we can manipulate developing nervous systems to mimic natural modifications that have been made during evolution. We do this in an effort to produce a neocortex that is organized in a manner consistent with the type of organization which occur naturally. This type of approach allows us to unravel the genetic and activity dependent contribution to neocortical organization, and to appreciate how these two mechanisms act in concert to produce the types of brains observed in extant mammals, including humans.

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