Tens of thousands of years ago, in what is now the Altai Mountains of Russia, a Neanderthal woman had an intimate encounter with a very exotic man – so exotic that he could even be regarded as another species.
He was a Denisovan, another old man who, like the Neanderthals, once walked among modern people but is now extinct.
That old encounter has produced a daughter.
More than 50,000 years later, the discovery of a chip from one of its bones in the Denisova cave in Siberia – and the DNA in it – has surprised scientists and enthusiasts about the old story it tells.
Small amounts of Neanderthal DNA have previously been found in both modern humans and Denisovans (and Denisovan DNA is found in modern humans), suggesting that there is some, although not much, cross between the groups.
But archaic human fossils are so rare that only about five Denisovan fossils and a few hundred Neanderthal fossils have ever been found.
So scientists can not believe their luck with this new discovery.
"It's exciting because we knew from other research that Neanderthals and Denisovans sometimes had to have children together, "said Viviane Slon, the first author of the new study published on Wednesday in the journal Nature." But the fact that we are one of those children are completely unexpected. "
So when Slon first saw the results of the DNA analysis she was carrying out, she thought something had gone wrong in the lab.
"I was surprised", she recalls in a Skype interview from Tanzania, where she is traveling.
When she repeated the experiment and rechecked the result, there was no doubt: the mitochondrial DNA of the bone fragment, inherited only from the mother, was Neanderthal. But the nuclear DNA, inherited from both parents, was half Neanderthal and half Denisovan. The child was a cross between a Neanderthal mother and a Denisovan father.
Slon, a postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology in Germany, shared the results with Bence Viola, an assistant professor of anthropology at the University of Toronto, who manages the catalog of fossils of Denisova Cave.
In the beginning he could not believe it either.
Only about five Denisovan fossils have ever been found, all in the Denisova cave. And even Neanderthal fossils only count in the hundreds, so scientists were very excited when they found the fossil of a human man with a Neanderthal great-great-grandfather.
"The chance to find the one who is the love child of a Neanderthal and Denisovan is even lower, "he said, but he was convinced by Slon's analysis.
Slon and Svante Paabo, director of the Max Planck Institute for Evolutionary Anthropology, asked Viola if he could find out how old the hybrid man was when she died. They wondered if they might have physical problems that would kill a hybrid in childhood or childhood – something that often occurs in hybrids between species.
(There is some debate about whether Neanderthals and Denisovans are separate species, they are genetically more of each other than the two people living today, but closer genetically than any gorilla or chimpanzee subspecies.)
Viola compared the thickness of the bone with that of Neanderthals of different ages and concluded that it probably belonged to an adult woman, but possibly an adolescent girl as young as 13 years old.
He thinks that the discovery is "unambiguous" evidence that different types of old people have been inter-crossed, and probably more often than we thought. But it still leaves unanswered questions.
"What they have met exactly is a question, "he said. I'm very curious if they recognized each other as separate or they just thought: "These are pretty strange guys in the next valley." How did this work culturally? "
The botchip discovered by the Russian technologists at Denisova Cave in 2012 was brown and about as long as the adult's small toe.
It was one of the tens of thousands of bone chips, most of ibex, deer and horses, recovered from the cave using river water and a sieve. It was then screened by researchers at the University of Oxford using a technique that can broadly tell which animal group it was from, based on differences in the protein collagen.
The bot chip was marked as belonging to the human family, so the Oxford student, master Samantha Brown, brought it to Slon and her colleagues in Leipzig, Germany.
A bone that has been through a lot
Half of the bone has since been crushed for radiocarbon dating, which showed that it was more than 50,000 years old. The other half was ground for DNA analysis, Viola said.
"There is practically nothing left of it, which is a bit of a shame. "
He worked from a small, gray plastic replica 3D printed with a micro-CT scan. However, it is so detailed that you can see textures that suggest how the story ended.
"It looks sour, and I think that's because it was consumed by a hyena," Viola said.
While Denisovans may have lived in the Denisova cave, it seems that they have not buried their dead there, so all the bone fragments that were found inside were likely to have been vomited by predators such as cave hyena's.
Two other facts that resulted from the analysis of the DNA of the hybrid woman were:
Her Neanderthal mother was more closely related to Neanderthals in Western Europe than any other Neanderthal man found in Denisova Cave who lived there for some time earlier, suggesting that Neanderthals migrated through Eurasia.
Her Denisovan father had her own Neanderthal ancestors and revealed a new interaction between Neanderthals and Denisovans.
Researchers who were not involved in the research said they were enthusiastic about the results.
Tangled tree of life
"Oh, I thought it was really cool," said Hendrik Poinar, an evolutionary geneticist at McMaster University in Hamilton and director of the old DNA center of the school.
He said that while people like to arrange animals into individual "species" that can not cross, in nature, such groups are not as well defined as people would think, nor is the path of evolution.
Fossils and genetics show us that the tree of life is very confused and "much more careless than we thought," he said.
Fernando Racimo, an assistant professor of population genetics at the University of Copenhagen, who investigates the genome of old modern people, Neanderthals and Denisovans, was also impressed by the discovery.
"It is astonishing that we continue to get great taken in the same cave in the middle of Siberia, "he said.
Richard Green, associate professor of biomolecular engineering at the University of California Santa Cruz, who studies old DNA and the evolutionary history of man, said that he was surprised by the findings that Neanderthals and Denisovans were quite inbreed, implying that crosses were rare.
Poinar, Racimo and Green all thought the study was well done and convincing, and adds to the evidence that the crossing of Neanderthals, Denisovans and modern people is more common than previously thought.
"It is actually repeating the same story over and over, "said Green," that when two human groups live close enough together, they will hybridize. "
The study was funded by the Max Planck Society, the Max Planck Foundation, the European Research Council and the Russian Science Foundation.