The evolution of larger brains in the last 3 million years played an important role in our ability as a species to think, problem-solve, and develop culture. But the genetic changes behind the expansion that made us human have been elusive. In a pair of papers publishing May 31 in Cell, two teams of researchers identify a gene family, NOTCH2NL, that appears to play an important role in human-specific cortex development and may have been a driving force in the evolution of our large brains. NOTCH2NL genes delay the differentiation of cortical stem cells into neurons, resulting in the production of more neurons across the course of development. The genes are found exclusively in humans, are heavily expressed in neural stem cells of the human cerebral cortex, and are located on a part of the genome implicated in neurodevelopmental disorders.