Every person alive today carries mitochondrial DNA traceable back to a single woman who lived in Africa roughly 150,000 years ago. This film follows geneticist Spencer Wells as he uses that genetic marker, passed unchanged from mother to child and mutating at a predictable rate, to retrace the migration routes that carried her descendants out of Africa and across the planet. Narrated by Danny Glover, the film combines interviews with population geneticists, footage shot in the regions the migrations passed through, from the Rift Valley to Australia and the Americas, and dramatized reenactments of small bands of early humans crossing deserts, coastlines, and land bridges. The genetic evidence gets matched against archaeological finds and climate data to explain why humans left Africa at all and how they reached every continent within tens of thousands of years. The film's case rests on a striking claim: skin color, facial features, and other visible differences between populations are recent and superficial, layered onto a shared and very recent common ancestry.