Denisovans, mysterious human relatives, looked just like us in at least 1 way

They may have had huge, brutish-looking teeth, but Denisovans — ancient cousins of humans and Neanderthals — had slender, delicate fingertips like ours, a new study shows.

After rediscovering a missing piece of one of only five Denisovan fossils ever discovered, researchers from France, Russia and Canada digitally reconnected the two halves from the tip of the pinky finger from a Denisovan girl. 

The reconstruction showed it was indistinguishable from a human fingertip and different from the knobbed or clubbed fingertip bones of Neanderthals, they reported Wednesday in the journal Science Advances.

The finding is a small piece of the puzzle in figuring out who the mysterious Denisovans were, given that only five small fossils of the hominin species have ever been found.

"Until now, all we knew was how their teeth looked," said Bence Viola, a University of Toronto anthropologist who co-authored the paper. "All these little pieces allow us to build a picture of who the Denisovans were."

The new finding suggests researchers looking for Denisovan fossils might have to be more open-minded about what to look for, and that some fossils previously identified as modern humans may, in fact, be Denisovans, said Eva-Maria Geigl, an anthropologist at the Institut Jacques Monod at the Centre national de la recherche scientifique and the University of Paris who also co-authored the report.

More closely related to Neanderthals

Denisovans were first identified in 2010 as a new species of hominin using DNA extracted from the bottom half of the finger bone. Genetic analysis showed Denisovans were more closely related to Neanderthals than modern humans, but Denisovans interbred widely with modern humans, and some human groups have more than five per cent Denisovan DNA.

However, scientists know little about what they looked like, other than the fact they had massive molars similar to those of a more ancient hominin, Homo erectus.

Viola was a postdoctoral researcher at the lab Max Planck Institute for Evolutionary Anthropology in Leipzig that analyzed the DNA from the bone from the base of the fingertip. What the researchers didn't know at the time was that after the bone was excavated from Denisova Cave in Siberia in 2008, it was cut in half.

They only learned a couple of years ago that the other half had been sent to a different lab in the U.S. for analysis.

That half was subsequently photographed and measured by Geigl, who noticed that it had been cut. After she analyzed it genetically, she realized that it had exactly the same DNA as the bone that identified the Denisovans as a new species, and therefore must be part of the same bone.

But because the DNA analysis had already been published by the German researchers, and she was asked to return the bone to the U.S., she set the project aside.

2 halves reunited

Some years later, she said, she spoke to the Viola and the German researchers about the bone, which they didn't know about.

"It was a shock to us that somebody had the other half," Viola said in a phone interview from Kyrgyzstan, where he was doing field work.

Geigl offered to show the bone to them and suggested they find some way to publish the information.

"It's pity that nobody knows … that I'm the only one who has the pictures of this bone in my computer," she recalled telling them."

Viola was keen to see the photos, but quite surprised when he received them.

"This looks very different than what I expected," he recalled, writing in response.

Based on their huge teeth and a small fragment of skull, he had the impression Denisovans were even heftier and therefore less human-like than Neanderthals: "All of those are very large and robust. They come from huge individuals."

Geigl said the fact the bone looked more human than Neanderthal wasn't really a surprise for her. 

"Evolution is not something linear. Some features evolve faster than others."

The new finding suggests the knobs on the ends of Neanderthals' fingertips evolved relatively late.

Viola combined the images of the two halves to reconstruct the entire finger. That allowed the researchers to see that based on the growth of the bone, the girl it belonged to was likely a couple of years older than previously thought — about 14 or 15.

Matthew Tocheri, who holds a Canada Research Chair in Human Origins at Lakehead University in Thunder Bay, Ont., and was not involved in the study, said he thought the results were "kind of neat."

Like Geigl, he thought the similarity to humans was not that surprising. He noted modern humans, Neanderthals and Denisovans are all closely related, and, in fact, many Neanderthal and human features are indistinguishable. The fingertips just happen to be one difference.

Similarly, many Denisovans characteristics will likely be very human-like, and we don't yet know their "distinguishing" features, he said.

Tocheri said it was nice to have a bone that could be compared in modern humans, Denisovans and Neanderthals. But he added, "That's one bone out of over 200 elements to the skeleton. So we've got another 199 or so to go."