Theories about when the last Neanderthals walked the Earth may have to be revised, according to a study that suggests they became extinct in their last refuge in Spain much earlier than previously thought.

Previous dating of bone fossils found at Neanderthal sites in the region put the youngest at about 35,000 years.

But researchers from Australia and Europe re-examined the bones using an improved method to filter out contamination and concluded that the remains are about 50,000 years old.

If true, the study, casts doubt on the idea that modern humans and Neanderthals co-existed—and possibly even interbred—for millennia, because humans aren't believed to have settled in the region until 42,000 years ago. "

The results of our study suggest that there are major problems with the dating of the last Neanderthals in modern-day Spain," said Thomas Higham, deputy director of the Radiocarbon Accelerator Unit at Oxford University in England. "It is unlikely that Neanderthals survived any later in this area than they did elsewhere in mainland Europe."

The study, which was published Monday by the journal Proceedings of the National Academy of Sciences, doesn't completely exclude the possibility that Neanderthals survived until 35,000 years ago. The problem is that the warm climate on the Iberian peninsula quickly degrades a key protein used in so-called radiocarbon dating.

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