Scientists for the first time have found the remains of a person who had parents of different human species.
The 90,000-year-old bone fragments were found in a cave in Siberia, where 10 years ago archaeologists found the remains of a previously unknown human species, dubbed the Denisovans.
But rather than being another Denisovan, DNA testing of one of the fragments showed it was half-Neandarthal.
"My first reaction was disbelief," Viviane Slon of the Max Planck Institute for Evolutionary Anthropology (MPIEA) in Germany told magazine New Scientist.
"I thought they must have screwed up something," said Svante Pääbo, also of the MPIEA, told journal Nature.
When initial testing showed the even split of Neandarthal and Denisovan genes, they suspected they'd found the remains of a human descended from both Denisovans and Neandarthals - just like most modern humans have a little Neandarthal DNA in them.
But when they looked closer, they found the bones' mitochondrial DNA - the half inherited from the mother - was all Neandarthal. The rest came from a Denisovan.
That meant these remains weren't just any mix - they were a first-generation hybrid, and belonged to a 13-year-old girl.
"We'd almost caught these people in the act," said Dr Pääbo.
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Denisovans are believed to have colonised central and eastern Asia, while Neandarthals went west after leaving Africa, into Europe.
So far the Denisova Cave in Russia is the only place remains of pure Denisovans have been found, but traces of their DNA remain in modern humans, particularly in Aboriginal populations found in Australia and Papua New Guinea, and southeast Asia.