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New Discovery on Hominin DNA Suggests Link to Mystery Population

Paleontologists gleaned DNA from a 400,000-year old femur from Spain and discovered an unexpected link between Europe’s hominin inhabitants of the time and a cryptic population, the Denisovans, known to have lived much more recently in southwestern Siberia.

The DNA sequence, representing the oldest hominin sequence yet published, brings paleontologists 300,000 years back in history. However, researchers baffled because most of them thought that the bones would be more closely connected to Neanderthals instead of Denisovans. “That is not what I have expected and that is not what anyone else would have expected,” says Chris Sringer, a palaeanthropologist at London’s Nature History Museum.

These fossils was excavated in 1990s from a deep cave in a well-studied site in northern Spain named Sima de los Huesos (“pit of bones”). In the past, researchers believed that the femur and the remains were attributed either to early forms of Neanderthals or to Homo heidelbergensis, a loosely defined hominin population that gave rise to Neanderthals in Europe and probably humans in Africa. However, a closer link to Neanderthals rather than to Denisovans was revealed by the group led by Svante Pääbo, a molecular geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

However, Denisovans lived in Asia that was thousands of kilometers away and hundreds of thousands years later.

 Hominin-graphic

*Image source:Nature

It is difficult to extract nuclear DNA from femur; the mitochondrial genome tells the history of just an individual’s mother as well as her mother, etc. Therefore, the findings can’t reach an undeniably agreement that the Sima de los Huesos hominins are more closely associated with the Denisovans. This baffles scientists and everyone seems to have their own ideas.

One possible speculation is the theory of losing mitochondrial genome. Previous study suggested that Neanderthals and Denisovans had a common ancestor that lived up to 700,000 years ago. Since the mitochondrial genome can be lost by accidents, for example, if a female doesn’t give birth to a daughter, although her nuclear DNA can be inherited, the mitochondrial genome is totally gone. Thus, maybe both of the Sima de los Huesos hominins and Denisovans carried the mitochondrial sequence seen in the caves but Neanderthals could have simply lost that sequence while lived on in Denisovan women.

There is another theory – interbreeding theory. Not far from the caves, researchers discovered hominin bones that have been attributed to an archaic hominin called Homo antecessor, thought to be European descendant of Homo erectus. It is possible that this species interbred with a population that was ancestral to both Sima de los huesos and Denisovans hominins, introducing the newly decoded mitochondrial lineage to both populations, while Neanderthals didn’t do the same thing. Previous study suggested that Denisovans did interbreed with mysterious population, but it was not clear which the population was.

Pääbo’s team hopes to eke nuclear DNA out of the bones from the Sima de los Huesos hominins and then they can provide more information and might resolve the puzzle.

Source: Nature.

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