self-renewing multipotent cells that primarily generate radial glial progenitor cells, which in turn form the neurons and glia of the nervous system of all animals during embryonic development. Some neural progenitor stem cells persist in extremely limited areas of the brain of adult vertebrates and continue to produce neurons throughout life. Differences in the size of the central nervous system are among the most important differences between species, and hence mutations in the genes that regulate the size of the neural stem cell compartment are among the most important drivers of vertebrate evolution.