Another humanoid species co-existed with early humans and Neanderthals
A single finger bone found in this Siberian cave led to an amazing discovery. Early humans and Neanderthals co-existed with another humanoid species called Denisovans. And many present-day humans carry genes that prove our ancestors had children with Denisovans, too.
The new species is named after the cave where the 30,000 year-old finger bone was found.
Researchers had been searching for Neanderthal bones in the area, and were surprised to discover what they initially thought was a fossil from an early human’s little finger.
This adult tooth is one of only two known physical remains of the Denisovan humans. Image courtesy David Reich, Nature
To find out more, they shipped the bone off to the Max Planck Institute in Germany, where evolutionary biologist Svante Pääbo had already sequenced several Neanderthal genomes. Pääbo’s tests gave a shocking result: The genome sequence they got from the bone showed that it was neither human nor Neanderthal.
And yet it was undeniably a human relative, who had clearly lived among humans and Neanderthals thousands of years ago in the caves of Siberia. After careful analysis, a team of genomics experts figured out where the Denisovans fit into the puzzle of human ancestry. Most likely they are descended from a common ancestor shared with Neanderthals.
When early humans left Africa about 300 or 400 thousand years ago, the spread out across Europe and Asia. Those who went west to Europe became the heavy-browed, squat Neanderthals. And those who went East became Denisovans.
This micro-CT scan shows the bone fragment of a fingertip from a young girl Denisovan. Credit: Bence Viola, Max Planck Institute for Evolutionary Anthropology in Leipzig.
Genomics expert Richard Edward Green worked on analyzing the Denisovan DNA. In an email to io9, he explained:
The genome of the Denisovans is more diverged from modern humans than any two humans are from each other. It’s almost exactly as diverged as the Neanderthal genome was. That’s one of the reasons that we think the Denisovans and the Neanderthals are descendants of a single migration event into Eurasia.
Another wave of early human migration spilled from Africa about 70 to 80 thousand years ago. These travelers encountered both Neanderthals and Denisovans, eventually settling down and forming families with them. As a result, many Europeans have Neanderthal DNA; and, as the researchers report today in Nature, some Melanesian people from Papua have Denisovan DNA.
The picture of early human life that emerges is a lot messier than what we believed even just twenty years ago, when many anthropologists believed humans diverged from Neanderthals and the two species never interbred again. Now it seems that humans had many cousin species – at least two that we know of – and that we separated from them only to rejoin them later, forming families and creating lineages that persist to the present day.
So what makes the Denisovans and Neanderthals separate species from early humans anyway, given that all three groups co-habitated and had children together? Green said:
Answering that question – How much DNA divergence is necessary to call something a new species? – is a very difficult one. We know there was admixture between early modern humans and a population related to the Denisovans. We can see this in the genomes of individuals from Papua New Guinea, as described in the paper. Thus, from this perspective they were similar enough to successfully mate with our ancestors. The sad, frustrating truth, though is that there is no simple answer to how much divergence must be present to call something a different species or sub-species or variety or whatever.
Regardless of whether the Denisovans were another species, or just distant cousins, they are proof that humans have not always been alone among the primates. Within the last 50 thousand years, we shared the planet with other intelligent hominids who weren’t quite human.
If we want to know what humanity might look like 50 thousand years from now, after we’ve colonized space and spent millennia evolving in dramatically different environments, we should look back to the Denisovans’ humble cave in Siberia. There, three very different types of human beings met after a long time apart. And formed a community together.
Nature News: A distant cousin raises questions about human origins.
The ice-age world is starting to look cosmopolitan. While Neanderthals held sway in Europe and modern humans were beginning to populate the globe, another ancient human relative lived in Asia, according to a genome sequence recovered from a finger bone in a cave in southern Siberia.
A comparative analysis of the genome with those of modern humans suggests that a trace of this poorly understood strand of hominin lineage survives today, but only in the genes of some Papuans and Pacific islanders.
Named after the cave that yielded the 30,000–50,000-year-old bone, the Denisova nuclear genome follows publication of the same individual’s mitochondrial genome in March1. From that sequence, Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues could tell little, except that the individual, now known to be female, was part of a population long diverged from humans and Neanderthals.
Her approximately 3-billion-letter nuclear genome, reported in this issue of Nature, now provides a more telling glimpse into this mysterious group. It also raises previously unimagined questions about its history and relationship to Neanderthals and humans. “The whole story is incredible. It’s like a surprising Christmas present,” says Carles Lalueza Fox, a palaeogeneticist at Pompeu Fabra University in Barcelona, Spain, who was not involved in the research.
When the ancient genome was compared to a spectrum of modern human populations, a striking relationship emerged. Unlike most groups, Melanesians — inhabitants of Papua New Guinea and islands northeast of Australia — seem to have inherited as much as one-twentieth of their DNA from Denisovan roots. This suggests that after the ancestors of today’s Papuans split from other human populations and migrated east, they interbred with Denisovans, but precisely when, where and to what extent is unclear.
More answers could come from a closer look at Denisovan, human and even Neanderthal DNA. So far, conclusions about interbreeding have been drawn from a relatively small number of human genomes using conservative DNA-analysis methods, says David Reich, a geneticist at Harvard Medical School in Boston, Massachusetts, who led the Denisova analysis. “There may have been many more interactions,” he says. Pääbo says it may be possible to determine roughly when humans interbred with Denisovans by examining the length of DNA segments lurking in various human genomes, with shorter segments corresponding to more shuffling of genes and a longer elapsed time.
A molar discovered in the same cave also yielded mitochondrial DNA resembling that of the finger bone. But the Denisovans were probably more widespread, says Pääbo. Some fossils from China, for example, resemble neither Neanderthals nor modern humans — nor Homo erectus, an earlier human ancestor. Pääbo wonders whether they could be more closely related to Denisovans. His Russian collaborators plan to search for more complete Denisovan fossils that could be matched to others from China.
Chris Stringer, a palaeoanthropologist at London’s Natural History Museum, agrees that Asian fossils, such as the 200,000-year-old Dali skull from central China, could have links to the Denisovans. But he says that firm conclusions about such relationships will have to await the discovery of more complete Denisovan fossils.
Preserved DNA from other Asian fossils would also provide a clearer picture of the Denisovans, which Pääbo, to sidestep controversy, has opted not to call a new species or subspecies of hominin. The challenge will be to make sense of such discoveries and put them in the context of ancient human history, says Lalueza Fox. Palaeoanthropologists are just beginning to scrutinize the Neanderthal genome published earlier this year for clues to ancient human history. With the Denisova genome, “they will need to deal with another surprise”, he says.
Via Nature News