Scientists have determined that fossils found in Spain over the last three decades could unravel another mystery about human evolution.

Neanderthal and humans were believed to have developed separately, though in recent years, some scientists have argued that the groups mingled and reproduced. The remains found in the Sima de Huesos (Pit of Bones) in Spain may support that idea.

Neanderthals had large skulls, thicker bones, and large brows and jaws. They were shorter than modern Homo sapiens and are considered humans' closest known extinct relatives -- about 1.5 to 2.1 percent of the DNA of any modern human outside Africa is Neanderthal. But little physical proof supported the connection between the two until now.

Remains were discovered in the Sima de Huesos in 1976, and paleoanthropologists have been excavating the area since 1984. The pit, 100 feet below ground level, has yielded around 7,000 human fossils, corresponding to the bodies of at least 28 humans. University of Alcala paleontology professor and study co-author Ignacio Martinez said the collection includes 17 fragmentary skulls.

The study, published in the journal Science and co-written by Juan-Luis Arsuaga, paleontologist at and director of the Joint Center for Evolution and Human Behavior in Madrid, revealed that the new fossils share characteristics of both Neanderthals and early humans. Their skulls were smaller, human-sized, but still had large jaws that open widely, which show they used their teeth as tools.

According to the Washington Post, not much was discovered with the fossils, giving researchers few clues about the development of the hominids. However, researchers said the transition from Neanderthal to human was not peaceful.

The paper states that Arsuaga and his colleagues believe the Neanderthal lineage did not get all of its characteristics in a set but that they developed over time in "a mosaic pattern of evolution with different anatomical and functional modules evolving at different rates."

The findings from Sima make these the oldest in the Neanderthal lineage and confirm that ancient humans and Neanderthals diverged more than 430,000 years ago.