One small change during human evolution sparked a cascade of revolutionary effects. We humans are one of the great apes, which are typically ‘male philopatric’ – meaning that on reaching maturity, males stay in their territory whereas young females move out. Bonobos are matriarchal in their social organization, but resemble the other great apes in being male philopatric. In the human case, females began reversing this pattern at an early stage. By remaining at home with their relatives, young mothers could call on support from grandmothers and other kin, leading to cooperative childcare which in turn led to profound changes in psychology and social structure.