The Organization of Neocortex in Early Mammals
Based on fossil evidence, a more complete understanding of the branching of the tree of mammalian evolution, and the results of comparative studies of the organization of the neocortex of the present-day mammals, it has been possible to reconstruct the likely organization of the neocortex of early mammals in some detail. The common ancestor of all extant mammals was small and had a small brain with little neocortex. This neocortex included five somatosensory areas, four visual areas, and a primary auditory area. There were likely two to three areas of prefrontal cortex, two to three areas of cingulate cortex, two retrosplenial areas, and possibly subdivisions of perirhinal cortex. Overall, early mammals had roughly 20 cortical areas. The primary motor area, M1, and the premotor areas of placental mammals, as well as the corpus callosum connecting the two cerebral hemispheres, did not appear until placental mammals emerged.
