"The data that they provide is certainly of the nature to arrive at the conclusions that they do," comments Stephan Schuster of The Pennsylvania State University, who worked on the recent sequencing of Archbishop Desmond Tutu's nuclear genome as well as the nuclear genome sequencing of a woolly mammoth. "All the detected sequence differences clearly indicate that this is a novel variant of a [hominin]."
Paleoanthropologist Ian Tattersall of the American Museum of Natural History in New York City noted that the finding should not necessarily come as a surprise. "We know the fossil record is far from complete, but what we have already shows that the [hominin] evolutionary bush is quite luxuriantly branching," he remarks. "One more branch is not something that ought to give us indigestion."
The association of the mystery hominin with those Middle and Upper Paleolithic artifacts is peculiar though, because elsewhere in Eurasia they have only turned up with Neandertal and modern human remains. Krause notes that it is possible that the pinky bone originated in an older, deeper layer of the cave sediments and over time got mixed in with the overlying artifacts. Thus far, however, there is no evidence for extensive perturbation. Another possibility, he says, is that the finger bone is that of an early modern human who carried an ancient mtDNA as a result of interbreeding between his or her ancestors and this previously unknown hominin group.
But other experts are not so sure about the team's interpretation of their data. "I don't know—and nobody else does—how many base-pair changes make a new species," says Erik Trinkaus of Washington University in Saint Louis, an authority on Neandertals and early modern humans. "I would like to have more than the number of mtDNA base pair differences to go on."
"The result doesn't mean that they've found a new species, and I don't believe it requires a separate pre-Neandertal migration out of Africa," argues John Hawks of the University of Wisconsin–Madison, whose research focuses on human genetic evolution. "Those explanations are both compatible with the result, but I don’t think the data require them yet." Hawks notes that the history of an mtDNA sequence—which is just a tiny fraction of a person's total DNA—does not necessarily reflect the history of a species.
A comparably distinctive nuclear genome sequence would significantly strengthen the claim that the Denisova mtDNA represents a previously unknown type of hominin. To that end, Krause and Pääbo are launching a Denisova genome project to obtain a full nuclear genome sequence from the bone that yielded the novel mtDNA. Comparisons of this genome with the full genome sequence they have obtained for the Neandertal as well as with the genomes of people living today could yield insights into the genetic changes that defined H. sapiens. "At the end we get more information about the big question [of] what makes humans humans," Krause reflects.
Meanwhile, paleoanthropologists are eager for more fossils to confirm the DNA-based claim. With luck, continued excavation at Denisova cave this summer will turn up additional remains—and put a face on this long-lost relative.