Fossilised finger points to previously unknown group of human relatives

'Denisovans' shared Asia with Neanderthals and modern humans 30,000 years ago, DNA analysis of the finger shows

A fossilised little finger discovered in a cave in the mountains of southern Siberia belonged to a young girl from an unknown group of archaic humans, scientists say.

The missing human relatives are thought to have inhabited much of Asia as recently as 30,000 years ago, and so shared the land with early modern humans and Neanderthals.

The finding paints a complex picture of human history in which our early ancestors left Africa 70,000 years ago to rub shoulders with other distant relatives in addition to the stocky, barrel-chested Neanderthals.

The new ancestors have been named "Denisovans" after the Denisova cave in the Altai mountains of southern Siberia where the finger bone was unearthed in 2008. Field workers excavating the site have found various stone tools and bones that suggest the cave was occupied by early humans for 125,000 years.

A large molar tooth, measuring around 1.5cm on each side and found at the site in 2000, also belongs to a Denisovan individual. The adult tooth was too large to belong to a modern human or Neanderthal, but similar to molars seen in the more primitive humans Homo habilis and Homo erectus.

Molar tooth from the Denisova cave
Molar tooth found in the Denisova cave. Photograph: David Reich et al/Nature Photograph: David Reich et al/Nature

Researchers led by Svante Pääbo at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, ran genetic tests on the fossilised finger and found that the Denisovans shared a common ancestor with Neanderthals.

The greatest surprise came when the team compared DNA from Denisovans and modern humans. This revealed that the Denisovans had genetic material in common with modern populations in Papua New Guinea, due to interbreeding with ancestors of the Melanesians. Previously, Pääbo's group has reported evidence for interbreeding between Neanderthals and the ancestors of non-Africans alive today.

"What is interesting is that at the time when Neanderthals existed in western Eurasia, there was this other group with a distinct history that was presumably widespread in east Asia," Pääbo told the Guardian. "We are now starting to get a more comprehensive picture of them. We want to know, who were the archaic people that were there and that modern humans met when they came out of Africa?"

Pääbo decided not to name the group a new human species to avoid academic fights over whether they represent a separate species or not. "Even for Neanderthals, where we have more remains than from any other group, palaeontologists can still not agree on whether they are a species or a subspecies. It is a sterile academic discussion because there will never be a resolution and I don't want to get into that," he said.

The link between the Denisovans and modern Melanesians was completely unexpected and shows the Denisovans must have lived far outside Siberia, Pääbo added. "This tells us that modern humans had babies not only with Neanderthals, but with the Denisovans too, and those kids became incorporated in human ancestral groups and contributed to us today. That is fascinating. There are two archaic groups that live on in us today and probably more," Pääbo said. The study is reported in the journal Nature.

In March, the same researchers extracted DNA from mitochondria in the finger bone. Mitochondria are the power plants of cells and contain DNA that is passed only down the female line. These tests gave the first hint that the finger came from an unknown group of humans.

Richard Green, a co-author at the University of California in Santa Cruz, said: "The story now gets a bit more complicated. Instead of the clean story we used to have of modern humans migrating out of Africa and replacing Neanderthals, we now see these very intertwined story lines with more players and more interactions than we knew of before."

Palaeontologists expect further excavations at the Denisova cave to uncover more fossilised remnants of the Denisovans. Some fossils may already be lurking unnoticed in museum collections around the world.

"There are a lot of fossils around that are enigmatic. No one really knows what they are. It might well be that many of them are Denisovans, but the only way to know would be either to extract DNA from them and show they're related or excavate in the Denisova cave and find more bones so we can compare them with other fossils. The Denisovans may not be as unknown as we think," Pääbo said.


Ian Sample, science correspondent

The GuardianTramp

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