A reconstructed Neanderthal skeleton, right, with a modern human skeleton.
Australian scientists have re-dated a collection of Neanderthal bones from southern Europe and found they are about 10,000 years older than first thought.
The new evidence means many of the region's Neanderthal specimens may be much older than previous studies suggest, and casts doubt on the date archaeologists believe the human subspecies became extinct.
Since the early 1990s several research groups have claimed to have radio-carbon dated some of the youngest Neanderthal bones, several just 32,000 years old, in a region known as Iberia, modern Spain and Portugal.
But the leader of the new research, Rachel Wood, said many archaeologists were not convinced of the specimens' ages.
"Their dates are very controversial," said Dr Wood, a radio-carbon dating expert at the Australian National University.
The warm temperatures of Spain and Portugal were not ideal for preserving bones, which meant they could have been easily contaminated, she said.
"If you add 1 per cent modern carbon to a sample that is 50 000 years old, you'll get a measured date of 37, 000 years, so it is very sensitive to contamination," Dr Wood said.
In their bid to establish an accurate chronology of Neanderthals in Iberia, the team screened specimens - the bones of animals eaten by, or buried around, Neanderthals - excavated from 11 sites across the region for their protein content. Radio-carbon dating measures a sample's amount of carbon 14 isotope, which decays at a constant rate over time. Bone protein is less likely to be contaminated than bone mineral.
Of the 11 sites analysed, only two produced usable samples for dating.
"At one site there are a lot of Neanderthal bones, but those bones don't have any protein and they've been covered in PVA glue so they can be conserved, but it means we can't date them," she said.
Before the team dated the samples, they were prepared using a new purification method, called ultrafiltration dating.
"The ultrafilter removes the smallest contaminants from the mixture of protein.
When the bones were re-analysed, Dr Wood and her international colleagues found they dated beyond the 50,000 year limit of radio-carbon dating.
"That means they are at least 50,000 years old," said Dr Wood, whose findings are published in the journal Proceedings of the National Academy of Sciences.
The sites had previously been dated to less than 42,000 years old.
The team concluded that "this collection of Neanderthals should no longer be cited as providing evidence" that the species lived in southern Iberia later than 42,000 years ago.
"There is a gap now where we don't have any reliable dates from about 45,000 years to about 38,000 years ago," she said.
"We don't know if there were Neanderthals there or modern humans. Or they both could have been there."