Further comments on the evolutionary origin of the mammalian brain

This paper is an extension of a previous report on the origin of the mammalian neocortex. Two main aspects are elaborated. The first is the evolution of visual projections from the midbrain to the telencephalon, featuring the encephalization of visual functions. Associated to this, the progressive fusion of the two main visual systems (thalamofugal and tectofugal) in the mammalian telencephalon (striate and extrastriate cortex, respectively) is viewed in the context of increasing cortico-cortical connectivity in the evolution of the mammalian brain. In addition, the issue of a presumed homology between mammalian extrastriate cortex and reptilian anterior dorsal ventricular ridge (ADVR) is reviewed in some detail, and it is concluded that extrastriate cortex is a derived character of mammals while ADVR is a derived character of reptiles and birds. It is not likely that ADVR is ancestral to extrastriate cortex. The second aspect under analysis is the origin of the inverted (inside-out) lamination pattern of mammalian neocortex that differs from the outside-in pattern of reptilian cortex. Furthermore, mammals have developed a transient embryonic cell layer (the subplate zone) that serves as a waiting compartment for thalamic and cortico-cortical axons while their prospective target cells end their migration process to reach their final positions. It is postulated that both, inverted lamination and the subplate zone arose in evolution as successive and complementary strategies to maximize synaptic contacts between thalamic afferents and the new cortical cell types (belonging to prospective granular and supragranular layers) that were being originated at that moment.