One bone, which came from a wild goat, was found in Zafarraya Cave in a similar layer as Neanderthal fossils. The bone was previously estimated as 33,300 years in age. However, using an ultrafiltration technique that cleansed the bone of modern carbon impurities that can give inaccurate younger dates, they found the bone was more than 46,700 years old.
"Our work suggests that at present, it is unlikely that Neanderthals survived any later in this area than they did elsewhere in mainland Europe," said researcher Thomas Higham at the University of Oxford in England.
The most surprising thing "was the enormous difference that the ultrafiltration dating made to the chronologies of the sites we looked at," Wood said. "At other sites in Europe, we have seen that this improved method of dating bone makes a difference, making old bones older. However, we do not normally see such consistently large differences. This is probably because the preservation of the organic materials - bone and charcoal - that are normally radiocarbon dated is really poor in warm climates like southern Spain."
Analysis of the remaining samples revealed they were at least 10,000 years older than previously estimated. Instead, they were close to or more than 50,000 years old, the upper limit for radiocarbon dating.
When Neanderthals died out "Our results cast doubt on a hypothesis that has been broadly accepted since the early 1990s - that the last place for surviving Neanderthals was in the southern Iberian Peninsula," Wood said. "Much of the evidence that has supported this idea is based on a series of radiocarbon dates, which cluster at around 35,000 years ago. Our results call all of these results into question."
These findings suggest modern humans and Neanderthals might not have interacted in this area. In northern Iberia, about 150 miles (250 kilometers) north of Jarama VI, past research suggested modern humans were only present starting about 42,000 years ago. These new findings hint that modern humans and Neanderthals did not coexist for millennia as before thought, and did not live side-by-side. (10 Mysteries of the First Humans)
"The results of our study suggest that there are major problems with the dating of the last Neanderthals in modern-day Spain," Higham said. "We now have to look very cautiously at the model of late Neanderthal survival in southern Iberia and focus our efforts on more rigorous dating programs."
One site, Cueva Antón in Spain, did seem as young as previously thought. However, it remains uncertain whether the artifacts there are linked with Neanderthals - they may belong to modern humans.
The researchers caution they are not definitely saying that there were no Neanderthals in southern Iberia after 42,000 years ago. "What we have is a gap where we have no reliable radiocarbon dates. There might have been Neanderthals or modern humans or both or neither," Wood said. Also, "there are several circumstances which could have obscured later interbreeding events in Europe, so it is not possible to say, for example, that at one time there was not more Neanderthal DNA in Europeans."
The scientists detailed their findings online Feb. 4 in the journal Proceedings of the National Academy of Sciences.
This article originally appeared on LiveScience.com.
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