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Why scientists are baffled by a half-million-year-old human thigh bone (+video)

Scientists sequenced 400,000-year-old mitochondrial DNA, exploding the previous record for oldest DNA and introducing new questions into European and Asian history.

By Correspondent / December 4, 2013

Skeleton of a Homo heidelbergensis from Sima de los Huesos, a cave site in Northern Spain.

Javier Trueba/Madrid Scientific Films/Max Planck Institute

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With help from a new technique, a team of geneticists in Germany have mapped the mitochondrial DNA (mtDNA) from a 400,000-year-old thigh bone belonging to one of our ancient cousins. The bone is much too old to be Homo sapiens sapiens, but it also doesn't appear to be Homo sapiens neanderthalensis, our Neanderthal cousins. The previous record for the oldest fragment of DNA was set in 2006, when a team of French and Belgian researchers mapped the mtDNA of a 100,000-year-old Neanderthal tooth.

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Scientists have discovered the oldest known genetic material ever to be recovered from an early human. They extracted mitochondrial DNA from the femur of a 400,000-year-old human ancestor in a cave known as the Pit of Bones in the Sierra de Atapuerca in northern Spain.

The German researchers found correlations between the ancient thigh bone and a 40,000-year-old finger bone found in the Denisova Cave in southern Siberia. That bone's DNA was distinctive enough that the team designated a new branch on the human family tree that they called the "Denisovans." But now the genetic connection between the Siberian bone and the newly-sequenced Spanish bone are prompting new questions.

“The fact that the mtDNA of the Sima de los Huesos hominin shares a common ancestor with Denisovan rather than Neanderthal mtDNAs is unexpected since its skeletal remains carry Neanderthal-derived features”, says Matthias Meyer, lead author of a paper in the current issue of Science, in a press release. 

The thigh bone was found in Sima de los Huesos, "the pit of bones," a cave in northern Spain. The cave has been largely sealed off for much of the past half-million years, preserving the bones so well that their mtDNA is nearly intact. Until now, the only way to retrieve ancient DNA was from frozen material recovered from the permafrost.

Sima de los Huesos is well named: it has produced the world’s largest collection of hominin fossils more than 100,000 years old. It includes at least 28 different skeletons. Those without Neanderthal features have been classified as Homo heidelbergensis, a vaguely defined species sometimes used as a catchall for ambiguous Homo specimens.

(Note that scientists now use "hominin" instead of "hominid," the previous term for our proto-human ancestors, since genetic studies of our ape cousins have revealed that gorillas, chimps, orangutans, and humans are all technically hominids – all part of the Hominidae family.) 

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