25 March 2010

Another human cousin?

The mitochondrial genome of an unknown hominin has been decodedDmitry Safin, Compulenta

Researchers from the Institute of Evolutionary Anthropology named after Max Planck (Germany), the University of Montana (USA) and the Institute of Archaeology and Ethnography SB RAS decoded the mitochondrial genome of an unknown hominin.

Mitochondrial DNA was isolated from a fragment of the phalanx of the finger of the hand, discovered in 2008 during excavations in the Denisova cave in the Altai Territory. This cave is known as a source of Paleolithic artifacts related to the Mousterian culture and Levallois technique; for example, a polished stone bracelet was also found in the layer where the bone fragment was located (see the article in the journal "Archeology, Ethnography and Anthropology of Eurasia").

The age of the bone sample, which seemed quite ordinary, was estimated at 30-48 thousand years. "We didn't pay much attention to him," says archaeologist Mikhail Shunkov, who worked at the excavations. Soon, Russian scientists transferred this fragment of the phalanx to their colleagues from Germany, who performed the decoding of the genome.

The researchers compared the resulting genome of 16569 base pairs with the mitochondrial genomes of 54 modern humans, six Neanderthals and a man whose remains were about 30 thousand years old during excavations in the village of Kostenki. As it turned out, the new sequence of mitochondrial DNA stands out from the rest: when comparing Neanderthals and humans, differences were revealed in an average of 202 nucleotides, while comparing the sample from Denisova Cave with humans gave an average of 385 differences.

Processing of the collected data shows that the branch of the evolutionary tree on which a possible new species of hominins is located separated from the branch leading to us about one million years ago – long before the paths of modern man and Neanderthal diverged (see Figure below). Consequently, this hominin, if the authors were not mistaken in their calculations, had to leave Africa after Homo erectus, whose migration began 1.9 million years ago, but before the immediate predecessors of the Neanderthals Homo heidelbergensis, who moved 300-500 thousand years ago.

Evolutionary links between modern humans, Neanderthals and the Denisova Cave hominin
(illustration from the journal Nature).

"The results exceeded all our expectations," admits study participant Svante Paabo. "It seems to me a fantastic success." A colleague of the authors, Eske Willerslev, a biologist from the University of Copenhagen (Denmark), recalls, however, that on the basis of mitochondrial DNA alone, which is inherited from the maternal line, it is impossible to uniquely identify new species. To clearly determine the status of a hominin, nuclear DNA is required.

Scientists agree with this and are already engaged in decoding the complete genome. This sequence should become the "oldest" of the known genomes of the genus Homo, much older than the reconstruction of the genome of the ancient Greenlander presented last month.

If the scientists' assumptions are confirmed, the new species of hominins will receive an official name. In this case, he will become the first extinct human relative identified by DNA analysis.

The full version of the report (Johannes Krause et al., The complete mitochondrial DNA genome of an unknown hominin from southern Siberia) is published in the journal Nature.

Prepared based on Nature News (Rex Dalton, Fossil finger points to new human species).

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