New analysis of a bone fragment from a Siberian cave has suggested that two very distinct groups of humans did in fact interbreed - and a few times.
"We knew from previous studies that Neanderthals and Denisovans must have occasionally had children together", said co-lead author Dr. Viviane Slon, a researcher in the Department of Evolutionary Genetics at the Max Planck Institute for Evolutionary Anthropology. But I never thought we would find an actual offspring of the groups'.
The team, led by palaeogeneticists Viviane Slon and Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, conducted the genome analysis on the bone fragment uncovered in the Denisova Cave in Russian Federation.
Most modern non-African humans have traces of Neanderthal DNA and some Asian populations have Denisovan DNA, showing that homo sapiens, Neanderthals and Denisovans interbred.
Scientists have long suspected that groups of Neanderthals and Denisovans, the closest extinct relatives of modern humans, intermingled and had children together.
Scientists know quite a bit about Neanderthals, who were not, as commonly portrayed, shorter or stupider than modern humans.
Kelley Harris, a population geneticist at the University of Washington, tells Nature that sexual encounters between Neanderthals and Denisovans may have been common, especially since the number of pure Denisovan bones that have been uncovered are few and far in between.
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Genetic tests also revealed that the Denisovan father had at least one Neanderthal ancestor further back in his family tree.
But this is the first time a first-generation offspring from the pairing has been discovered.
The bone fragments, from which DNA was extracted, are likely to have come from the arm or leg of a 13-year-old female, according to researchers.
Scientists confirmed Denny came from two separate hominins - and not two hybrids - by looking at where the genomes between Neanderthals and Denisovans differ. Scientists took DNA from a small fragment of the girl's bone and were able to map her complete genome from it. It proves that Neanderthals migrated eastward for tens of thousands of years before he died, so they had a chance to meet with the denisovans and early man.
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.
"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.
Broader interbreeding may have gained momentum when modern humans emerged from Africa roughly 70,000 years ago.
Neanderthals and Denisovans might have intermingled even more but for the fact that the former settled mostly in Europe, and the latter in central and East Asia, the researchers speculated. Neanderthals disappear from the fossil record 40,000 years ago. What's more, when the genome of the newly identified Denisovans was compared to that of modern humans, it turned out that some populations alive today - for example Melanesians and Aboriginal people - still carry some of these ancient human genes.