The genome of the Denisovans, an extinct early hominid species, includes DNA that seems to have come from yet another unknown group, a U.S. scientist says.

Snippets of the mystery DNA were found in the genome of a Denisovan, an extinct hominid known to exist from a finger bone and two teeth found in a Siberian cave.

While a lack of any other fossils means nobody knows what Denisovans looked like, geneticists have managed to accurately sequence their entire genome.

The mysterious earlier DNA, found by David Reich of Harvard Medical School, could be evidence of an entirely new species of hominid as yet unknown to science, scientists said.

Reich presented his findings at a Royal Society meeting on ancient DNA in London Monday, NewScientist.com reported.

The genome of Denisovans shows they were cousins of the Neanderthals, but they were not all that similar, Reich said.

"Denisovans appear more distinct from modern humans than Neanderthals," he said, noting about 1 percent of the Denisovan genome looks much older than the rest of it.

It is likely the Denisovans interbred with an unidentified species, and picked up some of their DNA, Reich said.

"Denisovans harbor ancestry from an unknown archaic population, unrelated to Neanderthals," he said.