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

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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.) 

Dr. Meyer and his team at the Planck Institute have developed new techniques for retrieving and sequencing highly degraded ancient DNA, which they first tested on a cave bear bone found at the Sima de los Huesos site. After that success, they gathered a few grains of bone powder from a hominin thigh bone from the cave. They extracted its DNA and sequenced the mitochondrial genome (mtDNA), a small part of the genome that is passed down along the maternal line and occurs in many copies per cell. The researchers then compared this ancient mtDNA with Neanderthals, Denisovans, present-day humans, and apes.

From gaps in the DNA sequences, they calculated that this hominin lived between 125,000 and 640,000 years ago, most likely around 400,000 years ago. After considering the bone's age and Neanderthal-like features, the researchers concluded that the Sima hominins were probably descended from the same group that gave rise to both Neandertals and Denisovans. An alternate theory could be that a small group migrated from Siberia to Spain, bringing the Denisova-like mtDNA into the Sima hominins or their ancestors.

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