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When an organism dies, its DNA breaks down into smaller and smaller fragments, while also becoming contaminated with the DNA of other species like soil bacteria.So piecing the fossil DNA together is a bit like putting together a jigsaw puzzle created by a sadist.In 2006, a team of French and Belgian researchers obtained a fragment of Neanderthal DNA dating back 100,000 years, which until now held the record for the oldest human DNA ever found. That discovery shed light on how Neanderthals and humans’ ancestors split from a common ancestor hundreds of thousands of years ago.It also revealed that Neanderthals and humans interbred about 50,000 years ago. “We still are a bit lost here.” A version of this article appears in print on December 5, 2013, on page A10 of the New York edition with the headline: Baffling 400,000-Year-Old Clue to Human Origins.“It got lost in one lineage but made its way in the other,” suggested Jean-Jacques Hublin, a Max Planck paleoanthropologist who was not involved in the research.Beth Shapiro, an expert on ancient DNA at the University of California, Santa Cruz, favors an even more radical possibility: that the humans of Sima de los Huesos belong to yet another branch of humans.“This would not have been possible even a year ago,” said Juan Luis Arsuaga, a paleoanthropologist at Universidad Complutense de Madrid and a co-author of the paper. Meyer then compared it to the DNA of the Denisovans, the ancient human lineage that he and his colleagues had discovered in Siberia in 2010. “Everybody had a hard time believing it at first,” Dr. “So we generated more and more data to nail it down.” The extra research confirmed that the DNA belonged on the Denisovan branch of the human family tree.Finding such ancient human DNA was a major advance, said David Reich, a geneticist at Harvard Medical School who was not involved in the research. Since the 1970s, Spanish scientists have brought out a wealth of fossils from the cave dating back hundreds of thousands of years. Arsuaga, who has found 28 nearly complete skeletons of humans during three decades of excavations. Arsuaga has argued that they belonged to ancestors of Neanderthals, which lived in western Asia and Europe from about 200,000 to 30,000 years ago. Meyer and his colleagues drilled into the femur, they found ancient human DNA inside, just as they had hoped. The new finding is hard to reconcile with the picture of human evolution that has been emerging based on fossils and ancient DNA.
For example, the amount of cratering on the moon, based on currently observed cratering rates, would suggest that the moon is quite old.It is possible, for example, that there are many extinct human populations that scientists have yet to discover. Scientists hope that further studies of extremely ancient human DNA will clarify the mystery.“Right now, we’ve basically generated a big question mark,” said Matthias Meyer, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a co-author of the new study.Their shared ancestors split off from humans’ lineage and left Africa, then split further into the Denisovans and Neanderthals about 300,000 years ago.The evidence suggested that Neanderthals headed west, toward Europe, and that the Denisovans moved east.
“Our expectation was that it would be a very early Neanderthal,” Dr. Denisovans were believed to be limited to East Asia, and they were not thought to look so Neanderthal-like.