A bone belonging to a very unusual teenage girl has once again shaken up our understanding of human history.

Around 50,000 years ago, Eurasia was home to two very distinct groups of humans, separated by thousands of years of evolution – the Neanderthals and the Denisovans, The Independent reports.

Now analysis of a bone fragment from a Siberian cave has revealed the girl it came from had a Neanderthal mother and a Denisovan father.

Scientists have been gradually unpicking the complicated relationships of our distant ancestors, using ancient DNA.

Increasingly, these efforts have suggested few displayed any form of prejudice when selecting partners.

"But I never thought we would be so lucky as to find an actual offspring of the two groups,” says Dr Viviane Slon, one of the researchers behind the discovery at the Max Planck Institute for Evolutionary Anthropology.

Denisova Cave, which is so far the only place physical evidence of the mysterious Denisovans been unearthed, has proved to be a treasure trove of human history.

The bone fragment, “Denisova 11”, that the new study was based on was discovered there by Russian archaeologists in 2012 before being taken to Germany for genetic analysis.

The key message from the new discovery, outlined in the journal Nature, is that not only was interbreeding happening, it was happening a lot.

“To find an actual hybrid of such a mating in a still sparse fossil record must surely indicate that these matings could not have been rare events, at least when the populations met each other, under whatever circumstances,” said Professor Chris Stringer, an expert in human origins based at the Natural History Museum, who was not involved in the research.

Analysis of the genome also revealed that the Denisovan father of the girl, who was 13 at the time of death, had at least one Neandertal ancestor further back in his family tree, meaning there were multiple instances of interbreeding within this one family.