Dr. Paabo and his colleagues isolated a small bundle of DNA from the bone’s mitochondria, the energy-generating structures within our cells. Dr. Paabo and his colleagues were surprised to discover that the Denisova DNA was markedly different from that of either humans or Neanderthals. “It was a great shock to us that it was distinct from those groups,” Dr. Paabo said in an interview.

Dr. Paabo and his colleagues immediately set about to collect all the DNA in the Denisova finger bone. Once they had sequenced its genome, they sent the data to researchers at Harvard Medical School and the Broad Institute in Cambridge, Mass., to compare with other species.

The Massachusetts scientists concluded that the finger bone belonged to a hominin branch that split from the ancestors of Neanderthals roughly 400,000 years ago. Dr. Paabo and his colleagues have named this lineage the Denisovans.

Next, the researchers looked for evidence of interbreeding. Nick Patterson, a Broad Institute geneticist, compared the Denisovan genome to the complete genomes of five people, from South Africa, Nigeria, China, France and Papua New Guinea. To his astonishment, a sizable chunk of the Denisova genome resembled parts of the New Guinea DNA.

“The correct reaction when you get a surprising result is, ‘What am I doing wrong?’ ” said Dr. Patterson. To see if the result was an error, he and his colleagues sequenced the genomes of seven more people, including another individual from New Guinea and one from the neighboring island of Bougainville. But even in the new analysis, the Denisovan DNA still turned up in the New Guinea and Bougainville genomes.

If the Denisovans did indeed have a range spreading from Siberia to South Asia, they must have been a remarkably successful kind of human. And yet, despite having the entire genome of a Denisovan, Dr. Paabo cannot say much yet about what they were like. “By sequencing my complete genome, there’s very little you could predict about what I look like or how I behave,” he said.