A Never-Ending Search for the Evolutionary Origin of the Neocortex: Rethinking the Homology Concept

Abstract

Dugas-Ford and colleagues studied a number of genes that encode proteins related to various functional features of the cells (such as different ion channels or proteins related to cell homeostasis) or late development and functional maturation. However, not all genes play an equal role in morphological evolution and, therefore, not all genes are equally useful for identifying homologies. Current views on evolution, which incorporate evo-devo ideas, point to the relevance of developmental regulatory genes for explaining morphological evolution [Carroll, 2008]. In contrast to structural and house-keeping genes, the genes that regulate early development often encode proteins (transcription factors or morphogens) that play a crucial role in the formation and patterning of specific body structures. The function of these proteins is so important that their coding DNA sequences are under considerable evolutionary constraint and are, therefore, highly conserved across phyla (key functions of these regulatory proteins are often also conserved; see Gehring [2005]). Since their effects are dosage-dependent, there is also a Scientific literature on the evolutionary origin of the neocortex is quite abundant and continued interest can be explained by our need to understand the origin of human cognitive abilities. In a recently published article, Dugas-Ford et al. [2012] analyze the origin of the neocortex using gene expression data at the cellular level, which they compare in representative mammals, birds and reptiles. They find molecular support for similarity between neurons in specific neocortical layers of mammals and specific neurons in the pallium of birds and reptiles [including neurons of the dorsal ventricular ridge (DVR)], thus reinforcing the cell-type homology hypothesis initially proposed by Karten [1969; 1997]. Any new contribution to our understanding of the organization and evolution of the pallium is welcome. However, in our opinion, the data of Dugas-Ford et al. [2012] are inconclusive for homology purposes, and the search for the evolutionary origin of the neocortex remains open. We explain our reasoning below, and add some thoughts on the contribution of evo-devo (evolutionary developmental biology) to understanding the mechanisms of evolution and the homology concept. Published online: March 21, 2013