An unknown type of human, nicknamed "X-Woman," coexisted with Neanderthals and our own species between 30,000 to 50,000 years ago, according to a new study that suggests at least four, and possibly more, different forms of humans existed in Asia after Homo sapiens migrated out of Africa.

The as-of-yet-unnamed new human species, documented in the journal Nature, represents the first time that a hominid has been described not from the structure of its fossilized bones, but from the sequence of its DNA.

Researchers focused on mitochondrial DNA (mtDNA), genes passed down from mothers to their children -- hence the X-Woman nickname.

Her mtDNA shows that X-Woman shared a common ancestor with Neanderthals and modern humans one million years ago, so X-Woman and her species likely migrated out of Africa 500,000 years before the ancestors of Neanderthals left Africa.

Modern humans are thought to have made the journey much more recently, at just 50,000 years ago.

"So whoever carried this mtDNA out of Africa was a creature that was not on our radar screen before," co-author Svante Paabo told Discovery News.

Paabo, who is director of the Department of Genetics at the Max Planck Institute for Evolutionary Anthropology, and his colleagues made the discovery after extracting and sequencing mtDNA from a single pinky finger bone. The bone, probably from a child, was found at Denisova Cave in the Altai Mountains of southern Siberia.

The genetic sequence was then compared with those for 54 present-day modern humans, a Late Pleistocene early modern human from Russia, six complete Neanderthal mtDNAs, one bonobo and one chimpanzee. None of them matched with the new sequence, but they revealed that the individual was a human that carried twice as many genetic differences as Neanderthals do with our species.

Since Neanderthals and modern humans were also living less than around 62 miles away in Siberia at the time, Paabo said, "At least three different forms of humans may have coexisted 30,000 to 40,000 years ago," making human history "a lot more complex and interesting" than previously thought for this period.

Yet another human species, the recently identified Homo floresiensis, aka "Hobbit human," lived just 17,000 years ago in Indonesia.

Based on archaeological finds from Denisova Cave, the researchers suspect X-Woman and her species, along with the Neanderthals and modern humans, hunted large game, such as woolly mammoths and woolly rhinos.

Conditions were often cold then in Siberia, as they are now, so everyone probably wore heavy, protective clothing. Ornaments dating to the period, such as bracelets, were also found in the cave.

Because the different humans appear to have lived within close proximity of each other, this "increases the potential for interaction," including inbreeding, Paabo said during a press conference yesterday in London with colleague Johannes Krause.

The apparently peaceful coexistence may not have lasted long, however, since only our species survived into modern times. As a result, Paabo said the extinction of the other human groups may have been "early genocide" or due to environmental factors or competition for resources.

In a separate commentary also appearing in this issue of Nature, Terence Brown, a professor of life sciences at the University of Manchester, wrote, "The relationship between the Denisova sample and Neanderthals and modern humans will become clearer when nuclear DNA is obtained."

Paabo, Krause and their colleagues are already planning such work, which they believe could take months.

Brown concluded, "The demonstration that a bone fragment can provide evidence for an unknown hominin will surely prompt more studies of this kind and, possibly, increase the crowd of ancestors that early modern humans met when they travelled into Eurasia."