By comparing genetic variation among humans from around the globe, geneticists can calculate a family tree of everyone’s relationships to one another, and by calibrating that tree, estimate when everyone last shared a common ancestor. Hundreds of such studies using data from thousands of people concur that all living humans can trace their roots to a common ancestral population that lived in Africa about 300,000 to 200,000 years ago, and that a subset of humans dispersed out of Africa starting about 100,000 to 80,000 years ago. In other words, until very recently, all human beings were Africans. These studies also reveal that all living humans are descended from an alarmingly small number of ancestors. According to one calculation, everyone alive today descends from a population of fewer than 14,000 breeding individuals from sub-Saharan Africa, and the initial population that gave rise to all non-Africans was probably fewer than 3,000 people. Our recent divergence from a small population explains another important fact, one that every human ought to know: we are a genetically homogenous species.