Radiocarbon dating had previously shown the Clovis period to range from 11,500 to 10,900 radiocarbon years ago (about 13,300 to 12,800 calendar years ago), giving the culture several hundred years to reach South America.
Radiocarbon years and calendar years don't always match, because the atmospheric abundance of carbon 14—which is absorbed by all living things and on which radiocarbon dating is based—has varied over time.
For their study, the researchers reevaluated materials from all known Clovis sites.
The data the researchers collected narrowed the Clovis time frame to between 11,050 and 10,800 radiocarbon years ago. This translates to roughly 13,100 to 12,900 calendar years ago—a duration of 200 years.
A number of archaeological sites in South America have yielded the same dates.
"The Clovis-first model says it would have taken anywhere from 700 to 1,000 years for people to reach the southern tip of South America," Waters said. "It seems highly unlikely that the Clovis people could have flown down there in 200 years.
"This indicates pretty strongly that there were people living in both hemispheres at the same time."
But if humans lived in the Americas before the Clovis arrived, who were these other people and where did they come from?
"I think we're moving toward understanding that the peopling of the Americas was not a singular event like the Clovis-first model would have us believe," Waters said.
Instead it "was a process with people probably arriving at different times and taking different routes and potentially coming from different places."
(Related photos: the search for the first Americans.)
John Johnson is an archaeologist and ethnohistorian at the Santa Barbara Museum of Natural History in California.
He said the new study and other archaeological research suggest that there were people in place from coast to coast prior to what is known as the Clovis horizon.
"It implies that people here adapted the Clovis technology [of spearpoints], that people didn't arrive full-blown with this technology," he said.
Tom Dillehay, the chair of the anthropology department at Tennessee's Vanderbilt University, suggested that it's time to lay the Clovis-first model to rest.
"Now we can perhaps begin to ask new kinds of research questions," Dillehay said, "about the timing of this existing population, about migrations and movements, and what's going on in North versus South America."
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