Recent descriptions of relatively sophisticated stone tools from California's Channel Islands also add strength to a costal path.
And although early studies arrived at some pretty errant dates, the technology has been refined and now, Bamforth notes, "it really works." But because the technology has only come into wide use in the past several years, many sites discovered and described earlier did not have the benefit of OLS dating.Such an old habitation predates the widespread toolmaking tradition known as Clovis, which spread across the continent some 12,800 to 13,100 years ago and was once thought to mark the first wave of settlers in the Americas.The find is "unequivocal proof for pre-Clovis occupation of America," said Steven Forman, of the Department of Earth and Environmental Sciences at the University of Illinois at Chicago.Given the previous finds in Wisconsin, Chile and other sites, John Shea, an associate professor of anthropology at Stony Brook University in New York State, notes that "it's been pretty clear" that humans were living in the Americas long before the Clovis tradition emerged.Likewise, Bamforth was not surprised by the discovery of the new evidence.And when biological material is scant or absent, making radiocarbon dating impossible, scientists can face greater challenges in establishing just how old objects really are—even though, as Bamforth says, it is becoming increasingly obvious that "people have to have been here way longer than radiocarbon dating could suggest." Nevertheless, pinning down a precise date is difficult."Artifacts move around in the ground all the time," Bamforth said.But, he noted, the researchers behind the new work "have shown in great detail that the site is intact," adding that he was impressed with "how carefully they were able to document the age." The team found "uniform particle size distribution" in the clay around the fragments, suggesting that it had not been disturbed when—or since—the rock pieces were dropped, Nordt explained during Wednesday's briefing.Because the researchers did not find enough biological material in the nearby dirt to perform radiocarbon dating, they used optically stimulated luminescence (OLS), which measures the amount of radiation trapped in sediment grains when they were last exposed to sunlight.Uprooting the Clovis-first model Extracting and describing these thousands of small stone tools has been slow going.The research team has been working in the pre-Clovis layers of the site since 2006, uncovering the artifacts, which were scattered in a layer of clay just 20 centimeters thick.