That a direct offspring of the two ancient humans was found among the first few fossil genomes recovered from the cave suggests, Pääbo says, "that when these groups met, they actually mixed quite freely with each other". "But I never thought we would be so lucky as to find an actual offspring of the two groups".
Palaeogeneticists Viviane Slon and Svante Paabo at Germany's Max Planck Institute for Evolutionary Anthropology carried out genome analysis on a bone taken from the Denisova cave in the Altai Mountains of Russian Federation.
Virtually most modern non-African humans retain traces of Neanderthal DNA and various Asian populations have Denisovan DNA, validating that Homo sapiens, Denisovans and Neanderthals interbred. Even though we have seen evidence of the two hominins interbreeding, she is the only known specimen we have who was a first-generation Neanderthal-Denisovan descendant.
Scientists also believe Denisovans and Homo Sapiens bred together - which explains the difference between humans.
The fingertip was the first of a string of notable archaeological discoveries to be found within the cave.
The Denisova 11 bone fragment was excavated in 2012 from the East Gallery of Denisova Cave.
"An interesting aspect of the Denisova 11 genome is that it allows us to learn things about two populations - the Neanderthals from the mother's side, and the Denisovans from the father's side", said co-author Dr. Fabrizio Mafessoni, from the Department of Evolutionary Genetics at the Max Planck Institute for Evolutionary Anthropology. Until now, scientists had discovered only four Denisovans; the fifth turned out to be a first-generation hybrid.
"The fragment is part of a long bone, and we can estimate that this individual was at least 13 years old".
However, it's also unusual that - if interbreeding were indeed common - the two species never merged and remained totally genetically distinct over hundreds of thousands of years of co-existing. The most recent evaluation compared Denny's DNA to genomes of a Denisovan and a Neanderthal. Denisova's paternal relatives, on the other hand, stuck to the region surrounding the cave-Denisova 3, the hominin whose pinky toe first led scientists to the species, lived in the area a few thousand years after Denisova 11.
"There's always been some groups. who think that Denisovan people were just the eastern branch of Neanderthals, that they weren't a distinct species", he said.
It is possible they simply did not have much opportunity to mate because they lived in small groups spread out across a vast landscape, Paabo suggested.
Among their genetic legacy to some modern humans is a variant of the gene EPAS1 that makes it easier for the body to access oxygen by regulating the production of haemoglobin, according to a 2014 study. Neanderthals lived in Europe and Asia, while fossils of Denisovans are known only from the cave where the fragment was found.
Both groups disappeared some 40,000 years ago. Further analysis of the fragment's nuclear DNA (inherited from both parents), however, yielded equal amounts of Denisovan DNA.
Previously it was thought that modern humans are descended from a single population of ancestors.