The evolutionary origin of the mammalian isocortex: insights from molecular developmental biology

The isocortex is a distinctive feature of the mammalian brain, which has no clear counterpart in the cerebral hemispheres of other amniotes. Historically, there have been long-standing controversies regarding possible homologues of this structure in reptiles and birds. In these vertebrate classes, a structure denominated dorsal ventricular ridge develops in the lateral aspect of the hemisphere and, like the mammalian isocortex, receives ascending auditory and visual tectofugal projections. On these grounds, it has been postulated that part of the dorsal ventricular ridge is homologue to part of the mammalian isocortex (i.e. the auditory and the extrastriate visual cortices). Dissenting views have claimed that the dorsal ventricular ridge originates from a topographically different part of the hemisphere than the isocortex, and therefore there is no embryonic similarity between these two structures. Furthermore, recent evidence on the expression patterns of regulatory genes strongly suggests that a large part of the dorsal ventricular ridge arises from a region denominated the intermediate territory or ventral pallium, which in mammals gives rise to parts of the amygdalar complex among other structures. Considering that embryological criteria are in some cases more reliable to determine homology than comparisons of adult states, we are inclined for the developmental approach, which prescribes non-homology between the isocortex and the dorsal ventricular ridge. Additionally, we suggest a scenario for the origin of the isocortex as an expansion of the reptilian dorsal cortex, which is consistent with current evidence