Researchers have identified what may be the first known Neanderthal/human hybrid remains. A paper published yesterday in the journal PLOS One details features of skeletal fragments from northern Italy that indicate evidence of gradual interbreeding between the two species .

Neanderthals were a different type of early human (Homo neanderthalensis), and archaeological evidence suggests that they coexisted with anatomically modern humans (Homo sapiens) in parts of Europe for thousands of years before disappearing around 30,000 years ago- phased out by anatomically modern human conquest, interbreeding, or both.

Genetic analysis has indicated that any modern human descended from Europeans and Asians has between 1 and 4 percent Neanderthal DNA, but until now there has been only ambiguous fossil evidence of Neanderthal/human hybrids.

The PLOS One study examines a fossil jawbone that was recovered in 1957 from Riparo Mezzena, a rocky region in the Monti Lessini mountains of northern Italy. According to Discovery News, the jawbone is up to 40,000 years old.

The researchers analyzed the fossil's mitochondrial DNA (mtDNA), which is passed down intact from mother to child, and confirmed that it belongs to a late Neanderthal. However, imaging analysis of the jaw seemed to indicate that its shape was somewhere between that that of anatomically modern human and classical Neanderthal specimens.

"From the morphology of the lower jaw, the face of the Mezzena individual would have looked somehow intermediate between classic Neanderthals, who had a rather receding lower jaw (no chin), and the modern humans, who present a projecting lower jaw with a strongly developed chin," said study co-author Silvana Condemi, an anthropologist at the University of Ai-Marseille, to Discovery News.

More evidence is necessary to confirm their conclusion, but based on the mtDNA and morphological analysis, the researchers believe that the fossil came from an individual who was a Neanderthal/human hybrid, descended from a female Neanderthal who paired with a male Homo sapiens.

"In our view, this change in morphology among late Neanderthals reopens the debate on the "more modern like" morphology of late Neanderthals and can lend support to the hypothesis of a certain degree of continuity with [anatomically modern humans] or a possible interbreeding with them," they wrote in PLOS ONE.

According to archaeological evidence, anatomically modern humans moved into the Riparo Mezzena area in northern Italy around 41,000 years ago. At that point, Neanderthals already had a long-standing culture in the region.

The researchers suggested to Discovery News that there was a territorial struggle between the two groups in the Italian peninsula over the next millennium, and that Neanderthal/human hybrids resulted from the rape of Neanderthal females.

The interbreeding was likely gradual, not a one-off event. Fossil evidence shows that Neanderthal culture continued for some time after modern humans arrived, so the researchers believe that modern human DNA gradually infiltrated the Neanderthal gene pool as modern humans encroached on Neanderthal territory.

Some experts doubt the Neanderthal/human hybrid hypothesis, based on evidence from Spain that Neanderthals in the Iberian Peninsula died out thousands of years before modern humans arrived there. They say that any genetic or morphological links between modern humans and Neanderthals result from having a common ancestor in the distant past.

Still, Condemi's team leans on the side of the Neanderthal/human interbreeding hypothesis, at least in the Italian peninsula. The story of where they ended and we began continues to evolve, and it's likely that much more evidence will arise before anyone gets any closer to resurrecting a Neanderthal human.

The complete study is available for free on PLOS One.