Revista Chilena de Historia Natural 83 (4): 469-477, 2010
A developmental approach to homology and brain evolution
Although homology is central to evolutionary interpretations, establishing it has become a
highly disputed issue in some instances. Here I argue for a developmental understanding of evolution, where modifications of the
developmental programs are a key source of evolutionary novelty. Although this perspective is not new, in comparative neurobiology it
has remained controversial. Specifically, the evolutionary origin of the mammalian neocortex has been a particularly debated point. I
propose a perspective that could help reconcile a long standing controversy: either the mammalian neocortex corresponds as a whole
to the dorsal hemisphere of reptiles and birds, or alternatively its lateral aspect corresponds to the lateral cerebral hemisphere and is
partly homologous to the dorsal ventricular ridge (DVR), a brain mass that receives the bulk of sensory input in reptiles and birds.
Genetic and embryonic evidence strongly favor a dorsal origin for the whole neocortex, while the DVR derives from the lateral
hemisphere. Nevertheless, the phylogenetically new elements of both the neocortex and the avian DVR derive largely from intermediate
progenitor cells located in the embryonic subventricular zone (SVZ), a zone of late proliferating activity located deep to the ventral, the
lateral and the dorsal hemisphere. I suggest that, despite originating in different embryonic regions (lateral vs. dorsal hemisphere), the
evolutionary new cellular elements in both the avian brain and in the mammalian neocortex derive from the activation of a similar
genetic pathway, possibly activated by the gene Pax-6, that induces the late proliferation of embryonic neural progenitors. This pathway
can be ancestral to amniotes, reflecting genetic homology. In mammals and birds independently, this precursor proliferative activity
differentiated into an SVZ, recruiting neuronal precursors from different parts of the cerebral hemisphere in each group, to contribute to
brain expansion.
Key words:
dorsal ventricular ridge,
neocortex, pallium, subventricular zone, ventricular zon
