Geneticist Spencer Wells traces every living human back to a small population of ancestors in Africa, using DNA samples collected from men on six continents. He travels to the Kalahari Desert to sample Bushmen whose Y-chromosome markers point toward humanity's deepest roots, then follows the genetic trail through Ethiopia, India, Central Asia, Australia, and the Americas, tracking the mutations that mark each branch of the migration. Wells meets Chukchi reindeer herders in Siberia and Native American communities whose ancestry the genetic data connects across the Bering land bridge, and he lays out the evidence linking Aboriginal Australians to one of the earliest departures from Africa, some 50,000 years ago. Interviews with population geneticists explain how a single mutation, passed from father to son for thousands of generations, can double as a timestamp and a map. The film's case is direct: the differences between human populations are recent and superficial, and the Y-chromosome record underneath them tells one continuous story of a single small group leaving Africa and filling the planet.