Ancient DNA that was discovered in a skeleton is casting doubt on previous theories of ancient humans, evolution and migration.
Skeletal remains of homo heidelbergensis found in the Sima de los Huesos ("Pit of Bones") cave in northern Spain is creating more questions than answers after analysis of its genetic material.
"Right now, we've basically generated a big question mark," study researcher Matthias Meyer of the Max Planck Institute for Evolutionary Anthropology in Germany told The New York Times.
Some scientists believe the primitive species lived approximately 400,000 years ago during the Middle Pleistocene time period. The previous oldest human fossil found was roughly 100,000 years old.
"We had been operating for a while under the assumption that the oldest DNA we're going to get is about 100,000 years," Todd Disotell, an anthropology professor at New York University, told Yahoo. "We might take a shot at some older samples that we just never would have bothered with in the past."
The bones were found in a cave that was discovered more than 30 years ago and has yielded the remains of 28 ancient humans.
The remains were thought to be the relatives of Neanderthals, a species of ancient hominin, but after a new genetic sequencing technique was used, that theory has been cast into doubt.
The results, published Dec. 4 in the journal Nature, stated the researchers used the new method to isolate DNA from a thigh bone. The DNA isolated from the recovered fossils indicates that there could have been many more species of ancient humans than previously thought.
"It's extremely hard to make sense of," Meyer explained. "We still are a bit lost here."
One idea that has been raised is that the remains found in the cave were the evolutionary ancestors of both the Neanderthals and Denisovians, another ancient human species.
The new findings comes a few weeks after a November conference regarding ancient DNA at the Royal Society that stated ancient humans interbred with several different ancient human species including Homo erectus, according to Nature News.
Researchers also think that Homo erectus, whose fossils could date back a few million years, hold the genetic link between ancient human species and modern humans.