Tibetans have lived in the thin mountain air for more than 6,000 years thanks to a gene variant they originally inherited from the ancient Denisovans. The thin air has favoured the persistence of one particular version of a gene called EPAS1, which allows these mountain-dwellers to get along just fine despite the shortage of oxygen. But it’s not only high-altitude humans who have traces of ancestral ‘ghosts’ in their DNA, it’s their pets too.