Tibetans are able to live at high altitude thanks to a special gene they inherited from a mysterious now-extinct branch of the human family scientists reported on Wednesday. The ancestors of today's Tibetans acquired a key variant of a gene regulating oxygen in the blood when they mated with a species of human called the Denisovans they said. Contemporaries of the Neanderthals -- and like them possibly wiped out by anatomically modern man Homo sapiens -- the Denisovans first came to light only four years ago. Their existence was determined through a piece of finger bone and two molars unearthed