Quintana-Urzainqui, I.; Oel, P.; Berger, T.; Pan, L.; Papadopoulos, N.; Serka, G.; Verbanac, A.; Boersig, M.; Torres-Sabino, D.; Rollan-Delgado, I. C.; Santangeli, L.; Kaessmann, H.; Arendt, D.

The emergence of predation and associated complex behaviors in jawed vertebrates (gnathostomes) have been major driving forces in brain evolution. To gain insight into the neuronal complexity of the last gnathostome common ancestor, we have studied the development of the shark telencephalon combining single-cell and spatial transcriptomics. Our findings suggest an ancient origin of basal progenitors for pallial indirect neurogenesis, which was so far only described for tetrapods. We also find evidence for migrating GABAergic neurons populating the dorsal pallial field, where we observe incipient layering. Finally, we provide evidence for the existence of Cajal-Retzius cells in the developing shark telencephalon, with conserved molecular profile and tissue localisation. Our data antedates the evolutionary origin of major landmarks of the vertebrate pallium by several hundred million years, to the gnathostome stem.