Remarkable discoveries were made recently in the caves of the Siberian mountains. These have produced storming debates in the world of archaeology. In 2008, scientists found a 41,000-year-old pinky finger bone, whose DNA matched neither humans nor Neanderthals. Instead, it belonged to a previously unknown group of hominins which they named Denisovans. Three Denisovan teeth were also found in the cave. Since then, various tests have been performed that led to the discovery of traces of Denisovan DNA in humans as well. The DNA was found mainly in the inhabitants of Asia and Melanesia—suggesting that long ago, humans and Denisovans mated and had children. That was the entire knowledge researchers had on the subject until now.

The new discoveries however paint an even more interesting picture. These findings tell us that Denisovans also interbred with Neanderthals. A bone fragment in the cave according to DNA analysis, belonged to the daughter of a Neanderthal mother and a Denisovan father. Hence the evidence is ground-breaking to say the least.

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“My first reaction was, ‘What did I do wrong?’” says Vivian Slon, a researcher at the Max Planck Institute for Evolutionary Biology.  Ancient DNA is quite fussy. Old genetic material is so degraded and fragmented, it is often easy to get radical but false results. She repeated her experiments extracting DNA six times. “It’s really when we saw this over and over again we realized, in fact, it was mixed Neanderthal and Denisovan ancestry,” she says.

Neanderthals and Denisovans split off from each other some 400,000 years ago. This made them far more distinct than any other groups of modern humans living today. But as evidence suggests, both appeared to have lived in or around the Denisova Cave. In 2010, excavators also found a Neanderthal toe bone in the cave. This new bone fragment, which appeared to have belonged to the daughter of a Neanderthal and a Denisovan suggested the two groups resided in the same place at the same time.

Her mother’s half of the genome however, resembled DNA from a Neanderthal found in Croatia. It did not particularly match DNA from the Neanderthal found in the Denisova cave in 2010. This suggested that Neanderthals migrated west to east multiple times. Her father’s Denisovan half of the genome had a touch of Neanderthal DNA which established that he had Neanderthal ancestor/ancestors as well.

The discovery has stunned scientists, but it also has them questioning whether it is so stunning after all. Svante Pääbo, the director of the Max Planck Institute for Evolutionary Anthropology believes that interbreeding is rare now, but after sequencing just six individuals from the Denisova cave, they have already found a direct hybrid offspring. So, maybe it wasn’t so uncommon back then.

“When you find a needle in a haystack, you have to start wondering if what you’re really looking at is a needlestack,” says John Hawks, an anthropologist at the University of Wisconsin-Madison. “This genome shows that hybrids were nowhere near as rare as people have been assuming. They must have been really common.”

After the discovery of the bone fragment, an illustration was drawn of a girl holding hands with her Neanderthal mother and Denisovan father looking out of the cave. The study’s authors said however that there was no way to know if this peaceful coexistence is an accurate representation. When asked about it, Pääbo said, “I will try to avoid the question by saying how we speculate about back then says much more about our ideas about humans and our fantasies and fascinations than anything about what happened back then.” He then added that when humans interbred with Neanderthals and Denisovans, their children survived and passed on their genes. “It can’t be they were total outcasts,” he says, because their descendants still walk among us today.

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