The story of human evolution has taken remarkable turns in the last few years. What once seemed like a neat and tidy story of one human relative flowing into the next is shaping up to be the story of multiple migrations, several relatives, and cross-species interbreeding.

A new paper published today in Nature adds another fold to this story. New evidence from a Russian cave reveals a first generation cross breeding between Neanderthals and their Asian cousins, the Denisovan, found in the teeny bone fragments of a prehistoric girl.

The Denisovan is perhaps the most mysterious known human ancestor. The fossil evidence is small—based entirely off a couple teeth and a finger tip. But those fragments were filled with DNA, which told a previously undiscovered story of an unknown relative that split off from Neanderthals at some point, migrating into east Asia and down into Oceana before disappearing entirely.

But the reconstruction of most of that story has come from understanding the DNA and seeing where Denisovan genetics show up. Just as many Europeans have a hint of Neanderthal DNA, some native Himalayans, Filipino tribes, and other indigenous South Asian groups have up to five percent Denisovan DNA.

Max Planck Institute

Why It's Important

The Denisovan hominid was first detected at the Denisova Cave in Siberia. Previously, both Homo sapien and Neanderthal bones had been found there, and the cave had the perfect conditions to hunt for DNA samples in the emerging field of paleogenetics.

"I never thought we would be so lucky as to find an actual offspring of the two groups."

Svante Pääbo, a Max Planck Institute researcher, tested the fingerbone and discovered that its DNA was like nothing before—in fact, what was written off as likely a Neanderthal bone chip was a whole new, previously undiscovered species. But we still have no idea what they look like.

The new fragment, Denisova 11, is a chip off of a larger bone. The chip itself is so small, scientists aren't sure exactly what bone the chip came from. The original 2008 sample was a small fragment of a finger bone. They also represent different times in the Denisovan species lives. This particular fragment came from 7,000 years before previous the genetic evidence.

The new specimen also seems to come from a first generation hybrid between the two species. The mother was Neanderthal, the father Denisovan. There's a small chance both parents were from a hybrid Neanderthal / Denisovan population, accounting for her roughly half-and-half DNA mix, but it's considered less likely, especially as her mitochondrial DNA—which is only passed genetically between females, or went from your mother's mother's mother to you—is all Neanderthal, which would require another 50/50 split.

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“We knew from previous studies that Neandertals and Denisovans must have

occasionally had children together," co-lead author Viviane Slon, researcher at the MPI-EVA, said in a press release. “But I never thought we would be so lucky as to find

an actual offspring of the two groups.”

However, the father's lineage wasn't entirely Denisovan—hundreds of generations back, there was a little bit of Neanderthal ancestry as well, from a group only distantly related to her mother.

From Past to Present

Replica of Denisovan bone fragment discovered at the Denisova Cave in 2008. Thilo Parg Wikimedia Commons

Modern humans, Denisovans, and Neanderthals all come from a common ancestor—but Denisovans are more closely related to Neanderthals. Homo sapien and homo neanderthalis separated around 750,000 years ago, give or take a few hundred thousand years. The most likely common ancestor was Homo antecessor or Homo heidelbergensis, based mostly on fossil (rather than genetic) evidence.



"Mixing among archaic and modern hominin groups may have been frequent when they met."

From there, within 300 generations of the modern human / Neanderthal split, Neanderthals and Denisovans split, the former chiefly inhabiting Europe, the latter mostly inhabiting East Asia—though a lack of clear fossil evidence makes that hard to nail down completely.

Aside from the genetic tracing and the four-ish fossils identified thus far from Denisova cave, there's a 400,000 year old femur of the Homo heidelbergensis discovered in Spain in 2013. DNA evidence shows that it was closely related to both the Denisovan and the Neanderthal, but more-so to the Denisovan in the mitochondrial DNA, which is exclusively passed through the mother. This also may cast doubt on heidelbergensis as a common ancestor between these species and modern humans, but points to a likely common origin between Denisovans and Neanderthals from heidelbergensis.

A Cave of Bones

Denisova Cave Демин Алексей Барнаул Wikimedia Commons

The Denisova Cave is such an archeological treasure because it is filled with thousands of bones and bone fragments. Humans have occupied that region for tens of thousands of years—and that includes Homo sapiens, Neanderthals, and Denisovans—and they interbred at or near the cave. Other fossils from the cave have turned up Homo sapien/Neanderthal hybrids, while the previously discovered Denisovan bones showed distant Neanderthal ancestry, as well as a mixture with another unknown primitive human species, possibly Homo erectus.

Paleogenetics has unlocked the idea that modern Homo sapiens didn't have a clean evolutionary path, but were instead populations of human branching out of Africa, settling in, and mating with new groups they encountered. The paper even states that the sheer amount of hybrid DNA at sites where different human species overlapped "suggests that mixing among archaic and modern hominin groups may have been frequent when they met."

These bone fragments still aren't enough to build a picture of what these Denisovans look like, but their close kinship with the Neanderthal gives us some idea. It will be further DNA research that finally gives us the answer. This means that scientists will need to look out for a Neaderthal-ish bone or bones and genetically test it. If it's more Denisovan than Neanderthal, that could build a picture of what the population looked like. It could also give us an idea of their movements across the ancient world.

But right now, it remains one of the few species known nearly-exclusively through their genetic imprint rather than their fossil record—so while we know for sure they mated with Neanderthal, we still have a long time to go before we find out more.

But this fragment was just one of 2,000 found on this particular bone hunt, so maybe there are more and better bones waiting for us to study its DNA and unlock their ancient secrets.

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