200,000 years ago we took our first steps on the African savanna. Today there are 7 billion of us living across planet Earth.How did our ancestors beat the odds and spread from continent to continent? What was the secret to their success?This is a global detective story, featuring new fossil finds and the latest genetic research. It's a story that revolves around a shocking revelation. In prehistoric times, we met and mated with other types of human – like Neanderthals, Denisovans and Homo erectus. This mixing of genes helped us survive - and ultimately thrive.Scientists are beginning to realize that ours is not a pedigree species, but a patchwork. We are all hybrids.