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Better bone dates reveal bad news for Neanderthals

February 22, 2006 By Michael Hopkin This article courtesy of Nature News.

Modern humans took over Europe in just 5,000 years.

Advances in the science of radiocarbon dating - a common, but oft-maligned palaeontological tool - have narrowed down the overlap between Europe's earliest modern humans and the Neanderthals that preceded them.

Refinements to the technique, which estimates an artefact's age by sampling the amount of radioactive carbon left over from when it was formed, suggest that Homo sapiens wrested Europe from its prehistoric counterpart even quicker than had been thought.

Previous estimates suggested that at least 7,000 years elapsed between H. sapiens arriving in eastern Europe more than 40,000 years ago, and the disappearance of the last known Neanderthals (H. neanderthalensis) from western France. But newly calculated dates shrink the overlap to 5,000 years.

Radiocarbon dating, also known as carbon dating, is a reliable method for dating artefacts back to around 23,000 years ago. But for items older than this it has tended to underestimate ages by up to several thousand years.

Carbon dating is based on the rate of decay of radioactive carbon-14 atoms found in living matter such as bones. Because carbon-14 decays to a non-radioactive form over time, older samples give off less radiation.

If you'd have talked to me two years ago I would have said that radiocarbon dating was a dead loss.
Paul Mellars,
University of Cambridge
But carbon-14's half-life is 5,730 years. So any sample older than about 30,000 years will have only 3% of its original carbon-14. For such samples, even tiny amounts of contamination can yield wildly inaccurate results.

What's more, the method originally assumed that levels of carbon-14 in the environment have remained constant, meaning that all samples had the same initial level of radioactivity. But this is not true.

Giant hurdle

Palaeontologists studying the colonization of Europe had feared that these complications were insurmountable, and that samples of the vintage in which they were interested could not be dated with confidence.

"Two years ago I would have said that radiocarbon dating in that period was a dead loss," says archaeologist Paul Mellars of the University of Cambridge. "We were all in despair - I thought I'd have to just go off and become a bus driver, or something."

Now, however, as Mellars describes in this week's Nature1, two key advances have put carbon dating back on the map. By 'ultrafiltering' bone samples to get rid of smaller molecules and retain only the larger ones, researchers can prepare far purer samples. And recent analysis of sea sediments from the Cariaco Basin near Venezuela have provided the most accurate record yet of how environmental carbon-14 levels have fluctuated, allowing the technique to be calibrated back to around 50,000 years.

Swift conquest

All this means that modern humans' displacement of the Neanderthals was probably swifter than previously thought. Previous dating had suggested that H. sapiens arrived in Europe 43,000 years ago, and covered the continent by 36,000 years ago. But the refined figures are 46,000 and 41,000 years, says Mellars - just 5,000 years to colonize an entire continent.

Neanderthals are not expected to have lasted long in the face of such an influx. "Modern humans had better weapons, more complicated language, and were better organized," Mellars says. "Once they arrived, Neanderthals didn't survive for long."

He suspects that our bulky cousins, despite being well adapted to cold, were killed off by a "double whammy" of competition with humans and a climatic cold snap that occurred at around the same time. "I would be surprised if the two species coexisted in any one place for more than around 1,000 years," Mellars says.

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  1. Mellars P., et al. Nature, 439 . 931 - 935 (2006).


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