At some point during human evolution, a handful of genetic changes triggered a dramatic threefold expansion of the brain's neocortex, the wrinkly outermost layer of brain tissue responsible for everything from language to self-awareness to abstract thought. Identifying what drove this evolutionary shift is fundamental to understanding what makes us human, but has been particularly challenging for scientists because of ethical prohibitions against studying the developing brains of our closest living relative, the chimpanzee, in the lab.