Study of the mechanisms that control growth of the cerebral cortex has largely followed by analogy from work in invertebrate systems such as fly and worm. However, the identification of several genes that cause human microcephaly has provided new avenues of investigation into the mechanisms that control cell identity during cerebral cortical development. In vivo studies suggest that many forms of microcephaly result from defects in the control of cell fate: precocious formation of neurons during early developmental stages produces deficiencies in progenitor cells at later stages of neurogenesis, resulting in an overall small cerebral cortex. Also, some of the genes that are mutated in human microcephaly seem to have been targets in the evolution of humans from distant primate ancestors.