Interspecific diversity in the neuronal composition of the mammalian cortex arises from heterochrony in neurogenesis

Mammals share a laminar cerebral cortex, with excitatory neuron subtypes organized in distinct layers. Although this framework is conserved, subtype balance varies markedly between species due to largely unknown mechanisms. Here, we show that species-specific neuronal composition arises from non-uniform scaling of the temporal dynamics of neurogenesis. Comparative histology of eight mammalian species reveals a significant, rat-specific expansion of the deep layer in the somatosensory cortex. This feature of the rat cortex results from a specific extension of the early neurogenetic phase of deep-layer neuron production before transitioning to the upper layer, as confirmed by neuronal birthdating and single-cell transcriptomics. The duration of deep-layer neuron production is regulated by a genetic program controlling neural progenitor cell aging, including canonical Wnt signaling. Comparative single-cell transcriptomics revealed that cortical progenitor cells in rats exhibit significantly elevated Wnt ligand expression. Therefore, while sequential cortical neurogenesis is conserved, its progression is non-uniformly scaled between species. Precise heterochronic fine-tuning allows evolutionary refinement of cellular configuration without drastic remodeling of the conserved corticogenesis program.