Basal radial glia (bRG, also known as oRG) are the main neural progenitor cell subtype implicated in brain evolution. Previous studies showed that the number of bRG morphotypes is higher in species (e.g. primates) with the larger neocortex, an evolutionary newer region of the cerebral cortex, than in species with smaller neocortex (e.g. mice). Therefore, it can be hypothesized that morphotype diversification of bRG is a feature gained in species with expanded neocortex during evolution. To examine this hypothesis, the present study analyzed the morphotypes of bRG in the evolutionary older region of the cerebral cortex, the hippocampus. Comparative morphotype analysis in different mammalian species revealed that the bRG morphotype in the developing dentate gyrus (DG) is highly diversified. This diversity is observed even in mice, at a similar level as in the primate neocortex. These results suggest that the morphotype diversification of bRG is an ancestral feature that the most recent common ancestor of placental animals might have possessed and was lost in the neocortex of some species (e.g. mice) during their evolution.