The evolution of isocortex.

There are several reasons why we lack detailed and comprehensive theories of how isocortex evolved in the various lines of mammalian evolution. Although current methods allow cortical areas to be defined with a high degree of assurance, few taxa have been studied in detail, and even the most-studied taxa are incompletely understood. In addition, concepts persist from early studies, based on limited data, that confound current theories, and some theories of isocortical evolution have been based on questionable premises. Nevertheless, some conclusions are clearly supportable. Early mammals had small brains with proportionately little isocortex. Mammals with larger brains and proportionately more isocortex evolved in several lines of descent. All mammals appear to have roughly 20 cortical areas, 'the organs of the brain', in common as retentions from an early ancestor, with primary and secondary sensory fields occupying much of cortex. Some of these cortical areas have been greatly modified in some taxa to become significantly expanded in size, highly laminated structurally, or both. Numbers of areas have increased independently in several branches of mammalian evolution, and the functioning of large brains may be enhanced by having more subdivisions. Finally, over many generations new areas may emerge from old by the formation of functionally distinct modules within areas, followed by the fusion of modules to ultimately form separate ones.