A glimpse of humans’ long-lost relatives
Tooth found in Siberian cave yields new genetic clues about mysterious ‘Denisovans’
Like a distant cousin you only know from stories, the Denisovans are just a hazy, hinted-at presence on the human family tree.
We know they lived tens of thousands of years ago in a cave in Siberia’s craggy, ice-ensconced Altai Mountains. But we can’t picture them. We don’t know what they looked like, what kind of communities they built, how tall they grew, how they hunted or how they treated their dead.
In fact, since these long-lost relatives were discovered five years ago, the sum total of evidence of their existence could fit inside a plastic sandwich bag: one tooth and one fragment of a child’s pinky finger, both uncovered from the remote Siberian cave.
But if those bits of bone don’t offer many clues about the Denisovans’ individual lives, scientists have been able to reconstruct their life as a species from fragments of DNA preserved within them. And last week, the life story of the Denisovans got both more confusing and more interesting.
According to a new analysis of a huge, recently discovered fossilized molar, published Monday in the journal Proceedings of the National Academy of Sciences, the Denisovans lived for roughly 60,000 years in Asia alongside both Neanderthals, their close cousins, and more distantly related Homo sapiens — us. Their complicated genetic legacy suggests that they interbred with both species, and possibly with another hominid group that has yet to be discovered.
“The world at that time must have been far more complex than previously thought,” Susanna Sawyer, an author of the study, told National Geographic. “Who knows what other hominids lived and what effects they had on us?”
The new tooth, like the other two items in scientists’ small collection of remnants, comes from Denisova Cave, a remote spot roughly 320 kilometres north of the Kazakhstan border and closer to Beijing than to Moscow.
Speaking to the New York Times in 2012, paleoanthropologist John Hawks said Denisova Cave had the high, arched roof of a Gothic cathedral and the quiet, eerie feeling of hallowed ground. Walking there was like walking in the footsteps of his ancestors.
The cave averages a temperature just about freezing all year long, meaning that its contents are extraordinarily well preserved. In addition to the Denisovan remains, scientists have found Neanderthal fossils and tools used by modern humans (that is, anatomically modern humans living 50,000 years ago, not current-day Russians).
Denisova Cave is the only place in the world where all three species are known to have lived, but researchers believe there must be many more Denisovan remains out there. Perhaps we’ve already found them, the study authors suggested to National Geographic: In China, researchers are analyzing ancient teeth with some striking similarities to the Denisovan remnants.
Though it’s difficult to extrapolate much from a tooth, this new one’s unusual size affirms one of the few things we do know about the Denisovans: they were large. When Bence Viola, an anthropologist at the University of Toronto, first saw the molar in 2010, he “thought it must belong to a cave bear,” he told Science magazine.
But far more information about the Denisovans is hidden in the molar’s interior, in preserved bits of what’s called “mitochondrial DNA,” a portion of genetic material that is passed down solely from the mother and acts as a “molecular clock” that indicates how long it’s been since two specimens had an ancestor in common.
In 2010, scientists sequenced the entire Denisovan genome using material from the pinky bone fragment, which came from a young girl. They found that Denisovans split off from Neanderthals about 400,000 years ago, and had given some of their genetic material to modern-day indigenous Australians and Melanesians. Another paper published later that year found that Southeast Asians had roughly 1 per cent Denisovan DNA, according to the New York Times.
Since then, researchers have been on the lookout for more clues about the Denisovans’ existence — the species is, as Science put it, being “a genome in search of a fossil record.”
The new tooth, known as “Denisova 8,” was just that clue. Though it yielded only a modest amount of DNA, it was enough to compare to the two other specimens to find the most recent common ancestor of the three. Once that was established, the common ancestor served as a baseline for determining the age of the three fragments. Denisovans that showed fewer mutations from the common ancestor’s genetic line were older; ones with more mutations would be younger.
Surprisingly, “Denisova 8” was 110,000 years old, roughly 60,000 years older than the other two. That indicates that the Denisovans survived in Siberia for tens of thousands of years, or left and then returned.
They must have been resourceful to last so long, Viola told the New York Times: “It’s not a very pleasant environment.”
The Denisovans were more geneti- cally diverse than their Neanderthal relatives, a remarkable feat for a group of three who came from a single Siberian cave. And they showed signs of interbreeding throughout their long tenure in Asia.
Indeed, some of their DNA seems to come not from Neanderthals or humans but from some other, unknown hominid.
Todd Disotell, a molecular anthropologist at New York University who was not involved in the new study, said it proved how much the world inhabited by our ancient ancestors “was a lot like Middle-earth.” “There you’ve got elves and dwarves and hobbits and orcs,” he told the New York Times, describing the host of human-like creatures that jointly inhabit J.R.R. Tolkien’s fictional world. On the real Earth, “we had a ton of hominins that are closely related to us.”
That anthropologists are even speculating about the existence of a species that’s never been seen is a testament to how DNA analysis has changed the field. Before it became possible to sequence ancient genomes, species were defined only after careful accumulation and documentation of specimens. They were distinguished from one another by what they looked like and how they acted.
Now, scientists can declare a new species without ever seeing a whole individual. They can even theorize about the possibility of species for which no fossils have been found.
“If you would have told me five years ago I would be talking about species we don’t have any fossils for,” Disotell said, “I would have thought you were crazy.”