National Geographic : 2013 Jul
missing ancestor 101 about a third as much as there is between any two living humans. The differences were sprinkled across the genomes, which ruled out inbreeding: If the girl’s parents had simply been closely related, they would have had huge chunks of exactly matched DNA. The pattern indicated instead that the Denisovan popula- tion represented by the fossil had never been large enough to have developed much genetic diversity. Worse, it seemed to have suffered a drastic decline sometime before 125,000 years ago—the little girl in the cave may have been among the last of her kind. Meanwhile the ancestral population of mod- ern humans was expanding. Myriad fossils, libraries full of books, and the DNA of seven billion people are available to document our subsequent population history. Pääbo’s team discovered a completely different one inside a single bone chip. The thought tickles him. “It’s incredibly cool that there is no one walking around today with a population history like that,” Pääbo told me, his eyebrows shooting up. And yet the Denisovans also have something to say about our own kind. With virtually every letter of the Denisovan genetic code in hand, Pääbo and his colleagues were able to take aim at one of the profoundest mysteries: In our own genomes, what is it that makes us us? What de- fining changes in the genetic code took place after we separated from our most recent an- cestor? Looking at the places where all living humans share a novel genetic signature but the Denisovan genome retains a primitive, more apelike pattern, the researchers came up with a surprisingly short list. Pääbo has called it the “genetic recipe for being a modern human.” The list includes just 25 changes that would alter the function of a particular protein. Intriguingly, five of these proteins are known to affect brain function and development of the nervous system. Among them are two genes where mutations have been implicated in au- tism and another that’s involved in language and speech. Just what those genes actually do to make us think, act, or talk differently than Deni- sovans, or any other creature that has walked the Earth, remains to be seen. The lasting contribu- tion of studying Denisovan DNA, Pääbo says, “will be in finding what is exclusively human.” But what of the little girl herself ? The tiny bit of bone that is all we ever had of her—or at least the half that went to Leipzig—is gone now. In pulling DNA from it, Johannes Krause and Qiaomei Fu eventually used it all up. The little girl has been reduced to a “library” of DNA frag- ments that can be exactly copied again and again forever. In the scientific paper discussing the his- tory of her population, Pääbo and his colleagues did mention, almost in passing, a few facts about her that they had gleaned from that library: She probably had dark hair, dark eyes, and dark skin. It isn’t much, but at least it sketches in broad strokes what she looked like. Just so we know whom to thank. j The bone A replica shows the size and position— on Pääbo’s pinkie—of the bone chip that allowed his lab to discover the Denisovans through their DNA. The chip belonged to an eight-year-old girl. recorD? who were The Denisovans really?