DNA from a female of previously unknown hominid – dubbed the X-woman – lived in southern Siberia some 40,000 years ago and could be a new branch on the human family tree, a finding that would rewrite Homo's exodus from Africa.

In a technical feat, scientists sequenced DNA from the bone fragment of a pinkie finger, possibly from a small female child, found in a cave in the Altai Mountains.

The bone found in Denisova Cave was extricated in 2008 from a soil layer carbon-dated to between 30,000 to 48,000 years ago.

Teased from a cellular component called mitochondria, the genome was compared to the code of our extinct cousins the Neanderthals, Homo sapiens, the bonobo and chimpanzee.

The Siberian hominid, the investigation found, had some 400 genetic differences, which makes it a candidate for being a distinct species of Homo, as the genus for humans and closely related primates is known.

It, us and the Neanderthals all shared a common ancestor who lived around a million years ago, say the investigators.

"It's absolutely amazing... It's some new creature that's not been on our radar screen so far," co-researcher Svante Pääbo told reporters.

The study, published in the weekly journal Nature, is led by Johannes Krause of Germany's Max Planck Institute for Evolutionary Anthropology. Members of the team previously sequenced most of the genome of the Neanderthal.

(SciAm)