A new type of prehistoric human has been discovered via DNA from a child's pinky finger found in a central Asian cave, a new study says.
"We had no inkling that this thing existed, and suddenly it's there. That in itself is a remarkable discovery," said Terry Brown, a geneticist at the University of Manchester in the U.K. and co-author of a news article released alongside the study Wednesday by the journal Nature.
If confirmed by further genetic testing, the discovery—dubbed X-woman—will mark the first time that a new human species has been identified solely on the basis of DNA (quick genetics overview).
The 40,000-year-old specimen isn't good for much else—it's far too fragmentary to contain clues to the creature's skeletal structure, musculature, brainpower, or appearance, researchers say.
(Related: "Oldest Skeleton of Human Ancestor Found.")
New Human Species: Out of Africa, Quietly
The new-human discovery implies that there was a wave of human migration out of Africa, the birthplace of humanity, that was completely unknown to science.
"We think Homo erectus"—an upright-walking but small-brained early human, or hominid—"was the first [hominid] to leave Africa two million years ago," Brown explained. After that the record went blank until about 500,000 years ago, until now.
"This hominid seems to have left about a million years ago, so it fills in a bit of a gap," he said.
The fossilized pinky bone was discovered in a cave called Denisova in the Altay Mountains of southern Siberia, Russia.
Though the pinky's owner has been dubbed X-woman, scientists think the pinky belonged to a child between 5 and 7 years old, but experts can't tell if it was a boy or a girl.
Analysis of DNA extracted from the fossil reveals it is significantly different from the DNA of Neanderthals or of modern humans.
So far only mitochondrial DNA, or mtDNA, has been extracted. Inherited from the mother, mtDNA contains much less information than nuclear DNA, which contains most of a body's genetic information.
What mtDNA lacks in storage capacity, however, it makes up in volume. There are two copies of nuclear DNA per cell but several thousand copies of mtDNA.
For this reason, the mtDNA of the child was much easier to read, or sequence, explained Richard Green, an ancient-DNA expert at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
A study of the child's mtDNA suggests the hominid belonged to a species that last shared a common ancestor with Neanderthals and modern humans about a million years ago. (See a photo of the first model of a Neanderthal based in part on ancient DNA evidence.)
"We don't know if the three species lived at the same place at the same time," Manchester's Brown said. "One million years is a long time, and populations of these hominids were not huge, so they could have been in different parts of Europe and Asia."
Truly a New Human Species?
X-woman appears to be a new type of human, but is it truly a new species?
Among the criteria used to determine whether different animals are distinct species are inability to interbreed, genetic dissimilarity, and anatomical variation.
It's impossible to determine whether any of these criteria apply to the Denisova child based solely on mtDNA.
For this reason, study co-leaders Johannes Krause and Svante Pääbo, also of the Max Planck Institute, are planning to harvest nuclear DNA from the fossil for analysis—a painstaking process. Until then they're refusing to call X-woman a new species.
Green, who did not participate in the research, calls the team's caution "appropriate."
Because so little bone was actually discovered, scientists have no idea what the child looked like. (See "Face of Ancient Human Drawn From Hair's DNA.")
It may be possible to one day to reconstruct the Denisova child's features from its DNA, but that's a long way in the future, Green said.