National Geographic : 2013 Jul
missing ancestor 97 of the total DNA in a specimen, most of which comes from soil bacteria and other contaminants. None of the Neanderthal fossils Pääbo and his colleagues had ever tested contained even 5 per- cent endogenous DNA, and most had less than one percent. To their amazement, the DNA in the finger bone was some 70 percent endogenous. Apparently, the cold cave had preserved it well. Given so much DNA, the scientists easily ascertained that there was no sign of a male Y chromosome in the specimen. The fingertip had belonged to a little girl who had died in or near Denisova cave tens of thousands of years be- fore. The scientists had no idea, at first, what she looked like—just that she was radically different from anything else they had ever seen. For a while they thought they might have her toe too. In the summer of 2010 a human toe bone had emerged, along with the enormous tooth, from Layer 11. In Leipzig a graduate stu- dent named Susanna Sawyer analyzed its DNA. At the symposium in 2011 she presented her results for the first time. To everyone’s shock, the toe bone had turned out to be Neanderthal, deepening the mystery of the place. The green stone bracelet found earlier in Lay- er 11 had almost surely been made by modern humans. The toe bone was Neanderthal. And the finger bone was something else entirely. One cave, three kinds of human being. “Denisova is magical,” said Pääbo. “It’s the one spot on Earth that we know of where Neanderthals, Deniso- vans, and modern humans all lived.” All week, during breaks in the conference, he kept return- ing alone to the cave. It was as if he thought he might find clues by standing where the little girl may have stood and touching the cool stone walls she too may have touched. p ääbo grew up in Stockholm with his single mother, a chemist, and on cer- tain days with his father, a biochem- ist named Sune Bergström, who had another, legitimate family and would later win a Nobel Prize. Pääbo’s own first passion was Egyptology, but he switched to molecular biol- ogy, then fused the two interests in 1984 with his work on mummy DNA. Once anchored in the study of the past, he never let go. He is 58 now, tall and lanky, with large ears, a long, narrow head, and pronounced eyebrows that arch up and down animatedly when he’s excited—about Denisova, for instance. How had all three kinds of human ended up there? How were Neanderthals and Denisovans related to each other and to the sole kind of hu- man that inhabits the planet today? Did their ancestors have sex with ours? Pääbo had a his- tory with that kind of question. The Neanderthal DNA he had made headlines with in 1997 was utterly different from that of any person now alive on Earth. It seemed to suggest that Neanderthals had been a separate species from us that had gone extinct—suspi- ciously soon after our ancestors first migrated out of Africa into the Neanderthals’ range in western Asia and Europe. But that DNA, like Krause’s first extract from the Denisovan finger, was mtDNA: It came from the mitochondria, the energy-producing organelles inside the cell, and not from the cell nucleus, where the vast bulk of our genome resides. Mitochondrial DNA in- cludes only 37 genes, and it’s inherited only from the mother. It’s a limited record of a population’s history, like a single page torn from a book. By the time of the Denisova symposium, Pääbo and his colleagues had published first drafts of the entire Neanderthal and Deniso- van genomes. Reading so many more pages al- lowed Pääbo and his colleagues, including David Reich at Harvard University and Montgomery Slatkin at the University of California, Berkeley, to discover that human genomes today actually contain a small but significant amount of Nean- derthal code—on average about 2.5 percent. The Neanderthals still may have been swept into ex- tinction by the strange, high-browed new people who followed them out of Africa, but not before some commingling that left a little Neanderthal in most of us, 50,000 years later. Only one group of modern humans escaped that influence: Afri- cans, because the commingling happened out- side that continent. Although the Denisovans’ genome showed neanDerThal. The pinkie was someThing else.