Artist's interpretation of the human ancestors who lived near Sima de los Huesos.

A fossil discovered in a Spanish cave has given researchers the oldest human DNA found to date. According to The New York Times, the DNA comes from a femur bone and is believed to be around 400,000 years old. But while it's helping to shed light on early human evolution, it's actually making matters more complicated. The fossil's anatomy reportedly made researchers believe at first that it came from an early Neanderthal, but the DNA appears to come from a separate branch of humans called Denisovans. Even more puzzling, until now Denisovan DNA has reportedly only been found in Siberia, at a site 4,000 miles away from this new discovery.

"Now we have to rethink the whole story."

The discovery is being published today in Nature by researchers led from the Max Planck Institute for Evolutionary Anthropology in Germany. According to the Times, Denisovans were previously believed to have lived only in East Asia. They were also not believed to carry such a resemblance to Neanderthals. "Now we have to rethink the whole story," Juan Luis Arsuaga, a co-author of the paper, tells the Times. One possibility is that those living near the Spanish cave, Sima de los Huesos, were neither Denisovans nor Neanderthals, but actually an ancestor of both of them. The researchers are also considering whether such DNA may have initially been present in Neanderthals but disappeared later on in their evolution.

Until now, the oldest found DNA was 100,000 years old. "This would not have been possible even a year ago," Arsuaga tells the Times. DNA reaching so far back has previously only been found in permafrost, but USA Today reports that the cave was able to properly preserve it in a new environment. From there, the researchers were able to use an updated version of a DNA extraction technique that had been developed at the Max Planck Institute in 1997 to recover it, reports the Times. Though the fossil's DNA is opening up more questions, there's still more left to study: the femur bone was just one small part of the 28 skeletons found at Sima de los Huesos.