New finding casts doubt on theory that ancestors of modern humans interbred with Neanderthals
Tuesday 05 February 2013
A new finding has cast doubt on the theory that ancestors of modern humans interbred with Neanderthals over thousands of years.
Scientists have re-dated fossil bones from two sites in southern Spain and discovered they are much older than previously thought.
According to the new evidence, it is unlikely Neanderthals and modern humans ever lived together in the region. Researchers now think the Neanderthals had long gone before the arrival of the first Homo sapiens.
Since the 1990s experts have believed the last Neanderthals sought refuge in the Spanish peninsular and finally died out around 30,000 years ago.
That would have provided easily enough time for the Neanderthals to mix their DNA with that of modern humans, which are believed to have colonised Spain more than 10,000 years earlier.
But the new research using an improved dating method indicates that the Neanderthal occupation of Spain only lasted until around 45,000 to 50,000 years ago.
Interbreeding has been suggested as the reason why traces of Neanderthal DNA can be found in people living today, especially Europeans.
However the issue has divided experts. Some believe the genetic link is due to Neanderthals and modern humans having a common ancestor which may have lived in North Africa.
Neanderthals and modern humans are distantly related sub-species of ancient human. Both are thought to have emigrated to Eurasia from Africa, but at different times.
When modern humans branched out they replaced other human species such as Neanderthals and Homo erectus that had gone before them.
Scientists carrying out the new research tested several animal bones found alongside ancient stone tools and bearing cut marks and other signs of human interference.
Previously, radiocarbon dating had supported the idea that the bones were evidence of late surviving Neanderthals.
Lead researcher Dr Rachel Wood, from Oxford University, said: "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.
"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."
Radiocarbon dating uses the decay rate of a carbon isotope to estimate the age of organic material. It is said to be unreliable for dates older than around 50,000 years, partly because of false readings caused by accumulated contaminants.
The new work, reported in the journal Proceedings of the National Academy of Sciences, employed a process called ultrafiltration to purify bone samples and ensure they were free from contamination.
One bone from a wild goat, found at a similar depth to Neanderthal fossils, was previously dated to around 33,300 years ago. The "cleaner" dating process showed that in fact it was more than 46,700 years old.
Co-author Dr Jesus Jorda, from the Universidad Nacional de Educacion a Distancia in Madrid, said: "Although it is still controversial to change the theory in force, the new concept which presents new data indicating that Neanderthals and H. sapiens did not co-exist in Iberia, is becoming accepted."
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