Evolution of the neocortex: a perspective from developmental biology

Abstract

In spite of its stereotypic laminar and columnar organization, the cerebral neocortex displays numerous species-specific adaptations of old and acquired new traits that subserve specific functions introduced during 100 million years of mammalian evolution. The human neocortex, a substrate of our unique cognitive abilities, has many distinct traits in addition to a larger surface, including different places of neuronal origin, distinct migratory pathways and acquisition of new cell types that were traditionally studied by comparative anatomists. The contemporary, evo–devo approach uses developmental principles and mechanisms uncovered by experiments in embryos of living species to obtain a glimpse into how the human neocortex may have developed at the cellular and molecular level in extinct common ancestors. The radial unit model of cortical evolution provides insight into how mutation of genes that control the transition from the symmetric to asymmetric mode of cell division in the proliferative ventricular zone subjected to radial constraint during migration can generate neocortical expansion in surface rather than in thickness. The protomap hypothesis of differential enlargement of the existing and introduction of new cytoarchitectonic areas has been tested in mouse embryos by mutation and/or changes of gene expression and transcriptional factors in the neural stem cells of the proliferative ventricular and subventricular zones. Understanding of the species-specific difference in tempo and sequence of cortical development as well as genesis of new cell subtypes, functional columns and synaptic connectivity is essential for design of therapies for trauma, congenital malformations, neurodegenerative disorders and ageing of the human cerebral neocortex.