All we have of them is a "pinkie" finger bone and a tooth, found in Denisova Cave in southern Siberia.

But DNA extracted from the fossilised finger has revealed the owner, who lived about 30,000 years ago, was a female member of a previously unknown group of ancient people who were neither Neanderthals nor modern humans.

After the tooth of another member of these extinct human relatives was discovered, they were named the Denisovans.

In a technical tour de force, an international team of scientists extracted nuclear DNA from the finger bone and worked out a draft sequence of the full genetic code, or genome, of the ancient female, a young girl.

When they compared her genome with that of Neanderthals and modern humans, they made the surprise find that some of her relatives may have interbred with the ancestors of Melanesians living today in Papua New Guinea.

The researchers, who were led by Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Germany, said this connection suggested that the Denisovans may have been widespread in Asia in the past.

In May Professor Paabo's team showed there was a little bit of Neanderthal in most people alive today, with some of our ancestors having interbred with the thick-browed cavemen.

A team member, Richard Green, of the University of California in Santa Cruz, said the finds meant recent human history was more complicated than had been thought. "We now see these very intertwined story lines with more players and more interactions than we knew of before."

The finger bone was excavated from the cave by Russian archaeologists in 2008.

A study in March of the owner's mitochondrial DNA, which is passed down from mother to child, raised the possibility it came from a new type of prehistoric human, then nicknamed X-woman.

The team then sequenced the whole genome and their results are published in the journal Nature. "It is almost miraculous how well preserved the DNA is," Professor Green said.

It shows the Denisovans were a sister group to the Neanderthals. He said this suggests that some ancestral humans who left Africa between 300,000 and 400,000 years ago split into the Neanderthals and the Denisovans.

When modern humans left Africa about 70,000 to 80,000 years ago, they encountered Neanderthals and some sexual liaisons occurred, most likely in the Middle East. About 1 per cent to 4 per cent of the genetic code of non-Africans today comes from Neanderthals as a result.

Later on, it seems, one group of modern humans came across the Denisovans as well, a meeting that left traces of Denisovan DNA in the people who eventually settled in Melanesia.