Conservation and Diversification of Pallial Cell Types across Vertebrates: An Evo-Devo Perspective.

As the highest center of sensory processing, initiation, and modulation of behavior, the pallium has seen prominent changes during the course of vertebrate evolution, culminating in the emergence of the mammalian isocortex. The processes underlying this remarkable evolution have been a matter of debate for several centuries. Recent studies using modern techniques in a host of vertebrate species are beginning to reveal mechanistic principles underlying pallial evolution from the developmental, connectome, transcriptome and cell type levels. We attempt here to trace and reconstruct the evolution of pallium from an evo-devo perspective, focusing on two phylogenetic extremes in vertebrates - cyclostomes and mammals, while considering data from intercalated species. We conclude that two fundamental processes of evolutionary change - conservation and diversification of cell types, driven by functional demands, are the primary forces dictating the emergence of the diversity of pallial structures and imbibing them with the ability to mediate and control the exceptional variety of motor behaviors across vertebrates.