February 22, 2006

Carbon dating dashes old theories

New advances in radiocarbon dating are threatening to upend old theories about when modern humans colonized Europe from Africa, and how rapidly they advanced. The research casts new light on significant patterns of human migration into Central and Western Europe in the crucial period from 50,000 to 35,000 years ago, scientists say. It suggests that the dispersal of anatomically modern Homo sapiens into Europe was more rapid than previously thought.

That, in turn, would mean that their coexistence with Neanderthals was briefer and their introduction of cave art, symbolic artifacts and personal ornamentation much earlier.

“Evidently the native Neanderthal populations of Europe succumbed much more rapidly to competition from the expanding biologically modern populations than previous estimates have generally assumed,” Paul Mellars, an archaeologist at the University of Cambridge in England, wrote in the journal Nature that was published Wednesday.

While other scientists have for several years been pondering the implications of the revised radiocarbon dating for archaeological research throughout the world, Mellars’s description of the new techniques and their significance is the first comprehensive review of the subject in a major journal.

The most pronounced discrepancies between radiocarbon and actual ages coincide with the fateful epoch when modern people first made themselves at home in Europe.

For years, it had been thought that modern humans from Africa began arriving in Western Europe at least 40,000 years ago, and so could have competed and mingled with the local population for at least 12,000 years.

The revised dating of fossils and artifacts leaves much less time when the two could have been in close contact.

Mellars concludes from the revised chronology that the overlap between Neanderthals and new arrivals must be shortened to about 6,000 years in Central and Northern Europe, perhaps only 1,000 to 2,000 years in regions like western France.

Katerina Harvati, a paleontologist at the Max-Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said the advances “can potentially lead to a breakthrough in our understanding of this critical time period in European prehistory.”

Harvati agreed that the new chronology suggested “an earlier appearance of early modern human complex behaviors and an earlier Neanderthal extinction and also suggests a shorter coexistence interval of the two species.”

Radiocarbon dating, introduced shortly after World War II, has been widely used in measuring time in prehistory, back to the method’s effective limit of 50,000 years ago.

It assumes that the proportion of radioactively unstable carbon 14 to stable carbon 12 has remained virtually constant in Earth’s atmosphere through this time period. It works by measuring the rate of decay of carbon 14 in once- living materials, such as plant and animal remains.

Although scientists once estimated the dating uncertainty to be no more than several hundred years, they came to suspect two potential sources of greater error.

One source was contamination of test samples by intrusions of more recent carbon. The other was fluctuations in the proportions of carbon 14 to carbon 12, which scientists came to recognize as a consequence to variations in cosmic radiation reaching the upper atmosphere.

Recent research at the University of Oxford, Mellars said, has led to a more effective filtration process to reduce contamination in test samples.

Other investigations of deep-sea sediments off Venezuela and ice-core records from Greenland yielded evidence of carbon variation problems, which turned out to be especially pronounced between 30,000 and 40,000 years ago. Accordingly, radiocarbon dates were recalibrated.

The revised dates, for example, show that a standard radiocarbon reading of 40,000 years translated into a calendar age of 43,000.

Even more consequential, a date of 35,000 years is revised to an actual age of 40,500, Mellars reported.

If correct, the new chronology means that fossil and archaeological evidence, especially in the crucial 30,000-to- 40,000-year period, is much older than once estimated.

Modern people may have arrived in Europe slightly earlier, but the extinction of the Neanderthals, previously thought to have occurred around 30,000 years ago, is now subject to greater revision because the standard dating yielded the most serious underestimates of true ages.

The degree of age discrepancies is also illustrated by the revised date for the splendid wall art in Chauvet cave, in southern France.

The charcoal used to produce the Chauvet drawings was originally dated around 31,000 to 32,000 years ago. A team of scientists reported in 2004 in the journal Science a revised date closer to 36,000 years ago.

In previous estimates, the modern human dispersal through Europe took place 43,000 to 36,000 years ago.

The 7,000-year period implies an overall dispersal rate of about 0.3 kilometers a year – less than two-tenths of a mile. Starting somewhat earlier, the faster dispersal over 5,000 years is now clocked at a rate of 0.4 kilometers a year.

Scholars note that this is similar to the dispersal rate of early agricultural communities entering Europe between 10,000 and 6,000 years ago.

Mellars cautioned that even the revised dating based on new research must be viewed as provisional, concluding, “The full implications of these studies for the interpretation of the human archaeological and evolutionary record will need to be kept under active and vigilant review.”

New advances in radiocarbon dating are threatening to upend old theories about when modern humans colonized Europe from Africa, and how rapidly they advanced.

The research casts new light on significant patterns of human migration into Central and Western Europe in the crucial period from 50,000 to 35,000 years ago, scientists say. It suggests that the dispersal of anatomically modern Homo sapiens into Europe was more rapid than previously thought.