A picture is worth a thousand words. Or maybe three thousand.
Coronal sections obtained from a variety of vertebrates stained for DAPI (blue) and the nuclear transcription factor Tbr2 (red). Tbr2+ cells are present in the developing forebrain of most vertebrates, including reptiles. Notably, Tbr2+ cells are collected into an SVZ structure in the developing chick brain. These data suggest that the solution of generating neurons through Tbr2+ intermediate progenitor cells appeared early in vertebrate evolution.