A new discovery of the oldest known human DNA, aged 400,000 years, is expected to create more questions about how homo sapiens evolved than it answers, the New York Times reported.

The DNA is four times older than the previous record-holder and was originally thought to come from a Neanderthal. The discovery, a thighbone was later determined to be from an obscure lineage of humans known as Denisovans.

This DNA revelation has caused the researchers, who published their work Wednesday in the journal Nature, more questions than answers.

The only other Denisovian remains were found in Siberia some 4,000 miles east of where the latest bones were found. The previous finds were estimated to be 80,000 years old, less than a quarter the age of the 400,000-year-old thighbone.

"Right now, we've basically generated a big question mark," study co-author Matthias Meyer, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, told the NYT.

The discovery has caused scientists to entertain the idea of rethinking the course of human evolution. For example, it is now apparently possible that human species yet to even be discovered interbred along the course of history, sharing their DNA.

Scientists found the ancient thighbone in the Spanish cave "the pit of bones" and used a newer method to extract DNA. Meyer said he and the research team were expecting the DNA, found after drilling into the bone, to be of an early Neanderthal. They found the two did not match, but that the DNA actually matched the Denisovian sample found in Siberia.

"Everybody had a hard time believing it at first," Dr. Meyer said. "So we generated more and more data to nail it down."

Next for the researchers will be to further explore theories of human species interbreeding.

"We need all the data we can get to build the whole story of human evolution," Chris Stringer, from London's Natural History Museum, told BBC News. "We can't just build it from stone tools, we can't just build it from the fossils. Having the DNA gives us a whole new way of looking at it."