Published online 22 February 2007 | Nature | doi:10.1038/news070219-10

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Dating study suggests it wasn't the makers of the Clovis culture.

The Clovis culture is characterized by sophisticated stone weapons. Center for the Study of the First Americans, Texas A&M University

For decades many archaeologists have believed that the first Americans belonged to what is called the Clovis culture — hunter-gatherers who lived in parts of North America roughly 13,000 calendar years ago.

A new study counters this notion by showing that the Clovis culture is nearly 500 years younger than previously thought, and may have lasted for as little as 200 years. There is evidence of other cultures in the Americas well before this new date.

The Clovis culture is characterized by sophisticated stone weapons, first found in Clovis, New Mexico. They would have been used to hunt mammals, including mammoths and mastodons.

The 'Clovis-first' model posits that the original Americans crossed a land bridge linking Siberia and Alaska during the last ice age, and headed south down the eastern side of the Rockies through a gap in the two ice sheets that covered Canada.

When they got beyond the ice, they dispersed rapidly, reaching the southern tip of South America roughly a thousand years later, and carrying the Clovis culture as far as central America.

This picture has been challenged in recent years, most notably by the discovery of archaeological remains in Chile and Wisconsin that have been dated to over 14,000 years ago. Mainstream thinking has shifted away from the Clovis-first model, says Vance Holliday, a geoarchaeologist at the University of Arizona in Tucson: "But the debate is still out there."

Updating the dating

Michael Waters of Texas A&M University in College Station and geochemist Thomas Stafford Jr of Stafford Research Laboratories in Boulder, Colorado, have now re-evaluated the age of Clovis artefacts, many of which were dated in the 1960s and 70s using carbon-dating techniques that are now obsolete.

The duo re-dated the artefacts using a technique called accelerator mass spectrometry. Like standard radiocarbon dating, this method measures the ratio of different forms of carbon in a sample. But it requires much less material and delivers more precise results.

Clovis technology turns out to be younger than previously thought — 13,100 years old, rather than 13,600 — and to have lasted only 200 to 350 years.

That's not long enough for people to reach the southern tip of the Americas, says Waters. Models suggest that the 14,000-kilometre journey would take 600-1,000 years.

Instead, Waters believes that the Clovis culture spread through a pre-existing population. "The old Clovis-first model would have had the Clovis culture marching down into unpopulated South America, all the way to the southern tip," he says. "But our data are showing that these two populations are contemporaneous." The findings are reported this week in Science.1

Fast movers?

The re-analysis is a landmark finding, says archaeologist Donald Grayson of the University of Washington in Seattle. But, he adds, Waters and Stafford's arguments would be stronger if they had analysed material from more sites — many were excluded out of concern for the quality of the artefacts.

"The bigger your sample, the wider the spread of dates is going to become," says Grayson. "As long as their sample is only 11 sites, it stands to reason that the time period is more constrained than it could be if we had more sites."

The exclusion of the Aubrey site in Texas — believed to be one of the oldest Clovis sites — is particularly worrying, says C. Vance Haynes of the University of Arizona, Tucson, who has studied Clovis culture for more than 40 years. Waters and Stafford excluded that site because the samples may have been contaminated.

Haynes agrees with this decision, but points out that excluding the site might have affected the results.

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Geological analyses of Clovis sites support the idea that the culture was relatively short-lived, says Haynes. But he still thinks that people, driven by curiosity and an abundance of game, would have taken much less than 600 years to infiltrate the Americas.

"This is exploration of a new world by a fairly sophisticated group," says Haynes. "I don't believe other people who say it takes hundred of years for this culture to spread."

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