Scientists have uncovered new information about the Denisovans, a mysterious group of human relatives that interbred with ancient humans in Asia.
The Denisovans had very large and unusual teeth, unlike those of humans or Neanderthals, reports an international team from Germany, Canada and Russia.
New DNA evidence suggests they likely ranged across much of Asia for tens of thousands of years and weren't just a small, isolated population, the researchers wrote in a paper published today in the journal Proceedings of the National Academy of Sciences.
The mysterious relatives of Neanderthals and modern humans were first recognized in 2010, based on DNA in the finger bone of a young girl found in Denisova cave in Siberia's Altai Mountains.
Analysis of that genome showed that Denisovans interbred with modern humans – about five per cent of the DNA of native Papua New Guineans and Australians and 0.2 per cent of the DNA of Asians and Native Americans is Denisovan.
Now, scientists have analyzed two molars found in Denisova cave and confirmed that they belong to two adult male Denisovans.
University of Toronto researcher Bence Viola, a co-author of the paper, was at the cave when a fragment of the first tooth was discovered.
"I thought, 'Oh, that actually looks very human-like,'" he recalled in an interview with CBC News.
But when the rest of the tooth was found, he began to have doubts. "I thought, 'This is too big. This doesn't look human-like at all.'"
A colleague who was an expert on Neanderthals thought it might belong to a cave bear.
But DNA analysis showed the tooth actually belonged to a Denisovan.
"In its size, it's comparable to hominins that lived two or three million years ago…but the age of it shows that it's very recent," said Viola, who analyzed the physical characteristics of the teeth.
While individuals have different tooth sizes, finding two teeth this large from two different, unrelated individuals suggests "the whole group probably had very large and weird teeth."
They also likely had a very large and robust jaw to support such long tooth roots. But aside from that, we know nothing about what they looked like.
60,000 years in Siberia
Differences in the DNA in the two teeth, along with the layers of the cave in which they were found, suggest that the two men lived about 60,000 years apart. The more recent would have lived around 50,000 to 70,000 years ago, while the earlier would have lived up to 130,000 years ago.
The Denisovans also show as much genetic diversity as Neanderthals that lived as far away from one another as Spain and Siberia, said Svante Paabo, an evolutionary geneticist at Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, in an interview with CBCNews.ca.
"It just sort of in general would indicate they have a long history where they had substantial numbers of individuals in the population," added Paabo, another co-author of the paper.
If that's the case, why have we never found any other Denisovan remains outside the Denisova cave?
Viola suspects other Denisovan remains have already been uncovered in China – they just haven't been recognized as Denisovan yet.
"I'm really convinced," he said. "The genetic data shows that these guys were spread over large parts of Asia, so we must have them."