A previously unknown kind of human—the Denisovans—likely roamed Asia for thousands of years, probably interbreeding occasionally with humans like you and me, according to a new genetic study.
In fact, living Pacific islanders in Papua New Guinea may be distant descendants of these prehistoric pairings, according to new analysis of DNA from a girl's 40,000-year-old pinkie bone, found in Siberian Russia's Denisova cave.
This "new twist" in human evolution adds substantial new evidence that different types of humans—so-called modern humans and Neanderthals, modern humans and Denisovans, and perhaps even Denisovans and Neanderthals—mated and bore offspring, experts say.
"We don't think the Denisovans went to Papua New Guinea," located at the northwestern edge of the Pacific region called Melanesia, explained study co-author Bence Viola, an anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
"We think the Denisovan population inhabited most of eastern Eurasia in the same way that Neanderthals inhabited most of western Eurasia," Viola said. "Our idea is that the ancestors of Melanesians met the Denisovans in Southeast Asia and interbred, and the ancestors of Melanesians then moved on to Papua New Guinea."
Interbreeding Common Among Various Types of Humans?
Taken together with a May DNA study that found Neanderthals also interbred with modern human ancestors, the Denisovan finding suggests there was much more interbreeding among different human types than previously thought, Stanford University geneticist Brenna Henn said.
"In the actual archaeological record, people have been talking about this for a long time. ... But before six months ago, there was no genetic evidence for any admixture between archaic humans and modern humans," said Henn, who co-authored an article accompanying the study in tomorrow's issue of the journal Nature.
"Then these two papers come out, and I won't say they've turned the field on its head, but they certainly support a view that has not been well recognized for years" by geneticists, said Henn, who wasn't part of the study.
Brian Richmond, a paleoanthropologist at George Washington University, said he expects the new study to spark much interest and excitement.
"Nothing is more intriguing than learning new twists about our origins," said Richmond, who also didn't participate in the Denisovan-genetics research. "And this is another new twist."
Fossil Finger Points to New Human Type
The centerpiece of the DNA study is a Denisovan fossil finger bone discovered in 2008. The fossil is thought to be from a young girl—dubbed X-woman—who was between 5 and 7 years old when she died.
For a previous Nature study, released in March 2010, the team had collected and sequenced mitochondrial DNA, or mtDNA, from X-woman's finger. But mtDNA—inherited only from mothers—contains far less information about a person's genetic makeup than DNA found in the nucleus of a cell, or nuclear DNA (see a quick genetics overview).
In the new study the team reports successfully extracting and sequencing nuclear DNA from the bone.
Then, using DNA-comparison techniques, the scientists were able to determine that Denisovans were distinct from both modern humans and Neanderthals, yet closely related to the latter.
The team estimates Denisovans split from the parent group of Neanderthals about 350,000 years ago.
(Related: "Oldest Skeleton of Human Ancestor Found.")
New Humans Had Huge Teeth
Along with the finger bone, archaeologists from the Russian Academy of Sciences, who excavated the site, discovered a single tooth that belonged to a Denisovan adult.
The tooth, a molar, is bigger than any modern human tooth and is even bigger than the biggest Neanderthal tooth. This could suggest Denisovans were "comparable in size to Neanderthals, maybe a little bit bigger," said George Washington University's Richmond.
Richmond cautioned, however, that tooth size isn't always a good indicator of body size. A hominin "can have big teeth and not be a giant," he said.
Denisovans a New Human Species?
The team has been careful not to call Denisovans a new species, opting instead to label them as a Neanderthal "sister group."
If modern humans and Denisovan humans were separate species, their hybrid children probably wouldn't have been able to reproduce. But the hybrids apparently were able to have babies, otherwise the Denisovan DNA couldn't have been passed down to today's Papua New Guineans. Therefore, study co-author Viola reasoned, Denisovans and modern humans probably weren't separate species.
Scientifically, though, it matters little whether Denisovan is ultimately recognized as a new species, said Terry Brown, a geneticist at the University of Manchester in the U.K., who wasn't involved in the study.
"This whole species thing is a red herring, something that makes a nice headline but does not in my view contribute much to the scientific debate," Brown said in an email.
"We really don't know how to equate differences in genome sequences with the species concept," he said. "You could have two genuine species, whose members cannot interbreed, but whose genomes are very similar.
"So really the nuclear DNA does not help us decide if Denisovans are a new species, though the evidence for interbreeding with modern humans suggests they are not."
Given the mounting evidence that modern humans interbred with Neanderthals and now Denisovans, some evolutionary biologists have even suggested dropping the species designation for Neanderthals and modern humans.
As scientists "produce evidence that Denisovans interbred with modern humans (as did Neanderthals) then the implication is that modern humans, Denisovans and Neanderthals are all subspecies of Homo sapiens," he said.
It's indisputable, though, that each of these groups was genetically distinct, said George Washington University's Richmond.
"Whether you call them subspecies or species, it is clear that modern humans, Neanderthals, and now Denisovans were separated for hundreds of thousands of years, and only later did some of them meet and interbreed."