l'extinction de Neandertal n'est pas dûe au changements climatiques

12 September 2007; | doi:10.1038/news070910-7 / http://www.nature.com/news/2007/070910/full/070910-7.html

Neanderthals 'not killed by climate change'

Study suggests demise did not coincide with climate cooling.

Michael Hopkin

Just the two of us: Did Homo sapiens (right) wipe out neanderthals (left) ? NPG

Whatever it was that sealed the fate of the Neanderthals, it looks unlikely to have been climate change. That is the verdict of a new study that used climate records from Venezuela to deduce what happened at the Neanderthals' last stand at the southern tip of Europe.

The research suggests that a switch to a cold, dry climate was probably not the telling factor in the demise of the Neanderthals, because of all the probable dates for their extinction, most do not lie near major cold events in the climate record.

Neanderthals (Homo neanderthalensis) lived in Europe until around 30,000 years ago - not long after Homo sapiens arrived on the scene 40,000 years ago. The Neanderthals are thought to have lasted longest in the region around Gibraltar, off the southern tip of Spain.

"There are different factors that have been invoked to explain the Neanderthal extinction," says Chronis Tzedakis of the University of Leeds, UK, who led the new research. "Clearly the appearance of anatomically modern humans is the prime suspect, but given that the extinction happened during the last glacial period, when climate was changing, what we know is that the climate was extremely unstable at that time."

The main problem with testing the different theories comes from the difficulty in dating accurately the age of Neanderthal fossils and tools to compare their ages with records of past climate.

This is because 'radiocarbon dating' used on Neanderthal remains - in which researchers measure the amount of the radioactively decaying isotope carbon-14 in a sample to determine its age - is not directly related to calendar years. For very old samples, it can be used to tell whether one object is older than another, but not to determine their exact ages.

Like with like

Tzedakis and his colleagues got around this problem by comparing the radiocarbon dates of Neanderthal tools from Gorham's Cave in Gibraltar, with a very accurate set of radiocarbon dates of ocean sediments, in which the lives of tiny sea-creatures record the climate of the time. These well-dated sediments happen to come from Cariaco Basin, Venezuela.

The researchers report in Nature, that of the three main radiocarbon dates given as possible extinction times for the Neanderthals - 32,000 years, 28,000 years and 24,000 years - only the most recent seems to have occurred at the same time as a climate shift. This most recent date is also the most controversial, meaning that it is generally more likely that it was competition with modern humans, rather than the bitter cold, that did for the Neanderthals.

"The take-home message is that we can eliminate catastrophic climate change as a factor for Neanderthal extinction," Tzedakis says.

The same method can be applied to assess the climatic conditions during any 'snapshot' event that is represented by accurately carbon-dated samples, Tzedakis adds. The climate in Venezuela is reflective of the climate in Europe, he adds, because many of Europe's climate shifts involved changes in the Gulf Stream, which influences climate from tropical America to the northern reaches of the Atlantic Ocean. And large climate swings, from warm and wet to cold and dry, tended to occur more or less all at once globally. "Changes from one condition to the other were extremely abrupt - of the order of a few decades," Tzedakis explains.

Reference : Tzedakis, P.C., Hughen, K.A., Cacho, I.& Harvati, K., . Nature 449 , 206 - 208 (2007).