A 400,000-year-old thigh bone from an early European human is causing confusion among genetic anthropologists who say its genetic material is related to another mysterious species believed to have lived only in East Asia.

The femur was pulled from Spain's Sima de los Huesos, or "pit of bones," a cold, damp tunnel 90 feet below the surface of the earth in the Sierra de Atapuerco in northern Spain. The pit is said to contain the fossilized remains of 28 individuals.

In a paper published Wednesday in the journal Nature, researchers said they believed the relic belonged to an extinct species of hominin known as Homo heidelbergensis, a direct ancestor of Neanderthals.

However, after analyzing mitochondrial DNA extracted from the bone, researchers found that the Spanish hominin was not as closely related to Neanderthals as they believed. Instead, they found that it shared a common ancestor with a little-known "sister group" of Neanderthals known as Denisovans.

"This really raises more questions than it answers really," said senior author Svante Paabo, a biologist at the Max Planck Institute for Evolutionary Anthropology in Germany. "It's a big surprise."

The study's authors were at a loss to explain the link between an early European hominin and another population in Asia, but said they hoped that analysis of nuclear DNA would help resolve the mystery.

DNA taken from the nucleus of bone cells will offer abundantly more information than the DNA taken from mitochondria, tiny structures outside the nucleus that produce energy for the cell.

"It's possible that we're simply so far back in the history of these populations that we are close to the population that was ancestral to all these individuals," Paabo said in a Nature podcast. "Another alternative is that this ancestral group actually interbred with something much older, something like Homo erectus, and obtained its mitochondrial DNA from them."

Modern-day humans, or Homo sapiens, are not the descendants of Neanderthals or Denisovans, although they did live as contemporaries at one time and interbred, according to scientists. At some point about 30,000 years ago, all other species of the Homo genus became extinct, leaving only us.

Fossil remains of European Neanderthals have allowed scientists to describe their appearance quite accurately. This is not the case with Denisovans, however, as evidence of their existence is based only on a fragment of bone from a child's finger, and two teeth. These artifacts wee discovered in 2010 in Denisova Cave in the Altai Mountains of Siberia.

"We know quite well what Neanderthals look like. They were quite more robust than modern humans ... more muscular and probably more adapted to living in a harsh northerly climate," Paabo said. "What's fascinating about the Denisovans is we know next to nothing about how they looked. We have their genome and we have two teeth, and those teeth are huge. ... The only thing we can say is they must have been very big, or at least have big mouths."