Multiple Origins of Neocortex: Contributions of the Dorsal Ventricular Ridge

The uniqueness of mammalian neocortex may ultimately only be clarified with improved understanding of the evolutionary origins of cortical structure and cortical functions. Comparative studies of the organization of the nonmammalian and mammalian telencephalon may provide valuable clues for understanding the evolution of neocortex. In the nonmammalian telencephalon, there are neuronal populations which correspond to cell groups in the neocortex of mammals in terms of connections, single unit-responses, and functions. Some of these populations lying within the dorsal ventricular ridge, however, are organized in a non-laminar, rather than laminar fashion. These observations suggest that the emergence of basic “cortical” circuit and laminar organization are distinct evolutionary events that can be differentiated and studied independently in order to understand each of their respective contributions to the cognitive functions of the neocortex. Moreover, in contrast to an argument that many cortical visual areas are derived from a single area by gene duplication (Allman, 1977, in press), the origins of neocortex can be separable into at least the precursors of non-laminar and laminar regions, and thus multiple evolutionary origins of neocortex are proposed.