Twenty years ago, paleoanthropologists confidently asserted that there were just two species at the origin of modern humans: Homo sapiens, our direct ancestors, and the Neanderthals. Both had evolved from the earlier Homo erectus. When Homo sapiens began to migrate out of Africa on a large scale around 70,000 years ago, they met Neanderthals in Europe and western Asia, and the two species lived peacefully together and even interbred. Over time, the Neanderthals died out, though we still don't know precisely why, and that left Homo sapiens as the single surviving human species on the planet.

Up to the early years of the twenty-first century, this was generally accepted as an accurate account of the origin of the human species. Then, in 2008, bone fragments were discovered in Denisova Cave in a remote area of the Altai Mountains in Siberia. These weren't the first findings from this cave, but in 2010, these small pieces of bone were subjected to cutting-edge DNA analysis, which produced a truly startling finding. These bone fragments appeared to be the remains of a previously unknown human species that was neither Homo sapiens nor Neanderthal. The discovery led to intense debate in the scientific community and brought about a fundamental change in how we believe early humans developed. This is the surprising and still-emerging story of the ancient people known as the Denisovans.