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In a Tooth, DNA From Some Very Old Cousins, the Denisovans
New York Times, November 16, 2015

A tooth fossil discovered in a Siberian cave has yielded DNA from a vanished branch of the human tree, mysterious cousins called the Denisovans, scientists said Monday.

Their analysis pushes back the oldest known evidence for Denisovans by 60,000 years, suggesting that the species was able to thrive in harsh climates for thousands of generations. The results also suggest that the Denisovans may have bred with other ancient hominins, relatives of modern humans whom science has yet to discover.

Todd Disotell, a molecular anthropologist at New York University who was not involved in the new study, said the report added to growing evidence that our species kept company with many near relatives over the past million years. The world, Dr. Disotell said, “was a lot like Middle-earth.”

“There you’ve got elves and dwarves and hobbits and orcs,” he continued. On the real earth, “we had a ton of hominins that are closely related to us.”

The Denisovans are named after the cave where their bones were found in the Altai Mountains in southern Siberia. Every summer, a team of Russian scientists led by Anatoly Derevianko of the Russian Academy of Sciences explores the cave, unearthing thousands of bone fragments.

Before the latest discovery, Denisovans were known only from DNA in another tooth and a finger bone found in the cave in 2008. Analysis had shown them to be at least 50,000 years old.

In 2010, Dr. Derevianko and his colleagues reported that the genetic material in the bone and the tooth belonged to the same lineage of hominins, which they called Denisovans.

With virtually no bones to study, scientists struggle to guess what the Denisovans were like. Their closest relatives were the Neanderthals, those stocky, big-brained hominins who hunted big game in Europe and western Asia 300,000 to 40,000 years ago.

Scientists estimate that Neanderthals and Denisovans diverged on the human family tree 400,000 years ago.

Since their initial discoveries, the Russian researchers have sifted through more bone fragments from Denisova. In 2013, the scientists reported the discovery of a Neanderthal toe bone in the cave with enough DNA to reconstruct the entire genome.

The newest batch of Denisovan DNA comes from a tooth discovered in 2010. Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues described it in the Proceedings of the National Academy of Sciences.

The new tooth, called Denisova 8, yielded only a modest amount of DNA. But the scientists gathered enough to draw some important conclusions.

Denisova 8, it turns out, is much older than the previously discovered remains. The researchers estimated its age at 110,000 years.

Bence Viola, a paleoanthropologist at the University of Toronto and a co-author of the new study, said the Denisovans either lived near the cave at least 110,000 to 50,000 years ago, or came into the region at least twice.

They must have been resourceful to endure in Siberia for so long, Dr. Viola said. “It’s not a very pleasant environment,” he noted.

Dr. Viola and his colleagues got another intriguing clue about the Denisovans when they tallied up the variations in the DNA sample. The Denisovans, they found, had almost as much genetic diversity as modern Europeans even though the remains were found in a single cave. “You actually see more diversity in the Denisovans than you’ve seen in Neanderthals from Spain to the Altais, and that, I think, is pretty astonishing,” Dr. Viola said.

He speculated that the Neanderthals became inbred because ice age glaciers drove them into isolated refuges in southern Europe. The Denisovans, though, were able to move south through large regions of Asia that were not covered by glaciers.

Another clue that Denisovans traveled far south of Siberia is in the DNA of living humans. Chunks of Denisovan DNA are found in Australian aborigines, New Guineans and Polynesians.

Some of the DNA in the Denisova 8 tooth hints at an even older interbreeding. While most of the genetic material in the tooth bears a close kinship with Neanderthals, some of it seems only distantly related to Neanderthal or human DNA.

One possible explanation, Dr. Paabo said in an interview, is that Denisovans interbred with another hominin species that lived in Asia. It is conceivable that this hominin was a species already known from fossil discoveries, such as Homo erectus. But it could also be a related species.

“If you would have told me five years ago I would be talking about species we don’t have any fossils for, I would have thought you were crazy,” Dr. Disotell said.

Copyright 2015 The New York Times Company. Reproduced with permission.
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