Today, some six billion people are spread across the planet. But there was a time when the human species numbered only a few thousand and the world was a single continent-Africa. Then a small group left their African homeland on a journey into an unknown, hostile world. The story of how these extraordinary explorers and their descendants went on to conquer the Earth can now be told through the science of genetics. Dr. Spencer Wells has been deciphering this epic story from evidence all people carry within them-in DNA inherited from those ancient travelers. Wells travels to every continent in search of the people whose DNA holds humanity's secret history, including Namibian Bushmen, Chukchi reindeer herders of the Russian Arctic, Native Americans, and Australian aborigines.