Some 15,500 years ago early nomadic North Americans had already set up camp near Buttermilk Creek in central Texas's hill country, where they left behind impressive array of stone tools and artifacts.

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.

The area where the tools were found, northwest of Austin, must have been an appealing campsite for millennia, because it bears a record of nearly continuous occupation from 15,500 years ago. The discovery is detailed in a new study, published online March 24 in Science.

When the makers of these tools were using the site (from 15,500 to 13,200 years ago), the region would have been slightly cooler than it is today, probably by an average of about 5 to 6 degrees Celsius—"rather amiable at that time period," Lee Nordt, of Baylor University's Department of Geology and co-author of the new study, said in a press briefing on Wednesday. But the resources in the area were likely plentiful, added Michael Waters, of the Center for the Study of the First Americans at Texas A&M University in College Station and co-author of the new study. With the rich hill country around them, "it's not surprising people came back time and time again."

The people who left the tools and fragments described in the study were likely hunter–gatherers, passing through the site from time to time over thousands of years. "This was a mobile tool kit—something that was easily transported," Waters said.

The prevalence of Clovis style tools—epitomized by fine, fluted (grooved) stone points—across the continent had suggested to many archaeologists for decades that the groups who made these tools must have comprised the first wave of settlement in the Americas. This arrival would have placed the initial migration from northeastern Asia over the Bering Land Bridge and through the Arctic corridor that opened between ice sheets at some 15,000 years ago.

This latest tool evidence, however, suggests that people were already making and discarding stone tools about 15,500 years ago, which would mean that the migration likely occurred even earlier. "You'd have to get to central Texas, and that would probably take a little while," Waters said.

Waters argues that their find of 15,528 artifacts (made from chert, a flint-like rock), which span the 2,400 years before the accepted emergence of Clovis technology 13,100 years ago, is the nail in coffin of the theory that Clovis toolmakers were the first inhabitants of the New World, the so-called Clovis-first model. "This is almost like a baseball bat to the side of the head to the archaeological community to wake up," he said.

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.

Douglas Bamforth, a professor of anthropology at the University of Colorado at Boulder who was not involved in the new research, calls the work "beautiful excavation and beautiful analysis."

The Buttermilk Creek site is not the first evidence of pre-Clovis habitation of the Americas. Several sites, including two in Wisconsin as well as one in Pennsylvania and one in Oregon, had already offered up a handful of stone tools that predated Clovis. These have offered up many fewer artifacts, and the dating of some pieces has drawn scrutiny over the years.

The striking discovery of 14,100- to 14,600-year-old stone tools at a site in Monte Verde, Chile, raised questions about just how quickly the new settlers could have arrived so far south so quickly. These early people might have used the two continents' west coast as a pathway to settlement but, as Waters noted, it would mean those early explorers would have been "paddling as fast as they could to get down to the southern tip of South America," passing up a lot of awfully nice places on the North American coast along the way—such as the Columbia River, San Francisco Bay and San Diego, "where I would have stopped," Waters said, half jokingly.

Researchers have also yet to find strong technological links between Clovis technology and same-period stone tools in northeastern Asia. "There are a lot of problems with the Clovis-first model," Waters said, adding that it is "time to abandon [it] once and for all."

Some of the pre-Clovis tools found at the Buttermilk Creek site, such as "bladelets," do show similarities with bifacial (two-sided) techniques found in Asia, suggesting a deeper history. But, as Waters pointed out, known tools from that period in Siberia and northeastern Asia are relatively scant.

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. "I think it's kind of been waiting to be found," he says of a substantial pre-Clovis site.

The presence of a settlement in the middle of North America by 15,500 years ago gives "ample time for Clovis to develop" and plenty of time for people to reach the South American sites in Monte Verde, Waters said.

But such an early, glacial-period arrival poses some problems for the overland route through the Laurentide and Cordilleran ice sheets, the corridor between which would have been closed until about 15,000 years ago. "As you push it back," Bamforth says of the early settlement, "they have to come down the coast" before penetrating the continental interior. Recent descriptions of relatively sophisticated stone tools from California's Channel Islands also add strength to a costal path.

Diligent dating
Surrounding any ancient artifact is a slurry of questions and doubts as to whether the place they are found reflects when and where they were originally discarded. 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. The technique is "not as precise as radiocarbon by a long shot," Bamforth says. 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. So if no biological material was available for handy radiocarbon dating, researchers would have had no way to gauge exactly when an assemblage of tools was made. And even the team behind the new paper, Bamforth points out, "they wouldn't have had pre-Clovis dates on this site without OLS."

The new dating development, along with the apparent lack of a hallmark tool or style means that "the possibility is that pre-Clovis is all around us, and we just can't recognize it," Waters said.

A new old Clovis?

Many of the rough, chipped chert chunks at Buttermilk Creek might look crude, especially when compared with a refined prototypical fluted Clovis spear point. But, as Waters pointed out, most Clovis sites are littered with "utilitarian chips like we found at Buttermilk Creek."

Better preserved ancient Homo sapiens, such as those found frozen, reveal that stone tools might represent little more than 5 percent of a culture's material relics. The rest, whether textiles or other more delicate artifacts, would have been destroyed by time had it not been for protective permafrost. So, Waters noted, "we have to be careful about how we interpret the evidence that we have."

Under the microscope, the researchers found that some tools had likely been used to carve hard material, such as bone or wood, and others seem to have been applied to softer surfaces, such as animal hides.

As Waters and his colleagues described in their study, the people who made these tools were already using some similar, if less exacting, techniques as the later Clovis patterns, such as bifacial points.

The rough similarities between some of these earlier tools with later, more sophisticated Clovis technologies has Bamforth pondering whether distinguishing this assemblage as "pre-Clovis" is necessary. "I wonder why it's all just not Clovis," he says. "It's not a critique of their work—it might just be a new way of thinking about it," he says, noting the distinctive "style zones" found in different ancient groups working stone in different regions.

Shea notes that it can get tricky when trying to ask these big human questions of chips of cold stone. "As to whether it is 'ancestral' to Clovis in a meaningful sense" is a knotty question, he says. Unlike genetic links among people, tools "can't have ancestor–descendant relationships with one another."

The research team is not arguing that there was a definite disconnect between these earlier Buttermilk Creek visitors and Clovis-era peoples—or for a separate settling of the continent. Rather, as Waters noted, these earlier groups would likely have eventually developed into the groups that produced the Clovis tools.

Firmer answers should emerge as other sites are discovered—and as more genetic data is gathered from ancient remains. But it is always going to be more challenging to affix firm start and end dates to the earliest settlement groups, because the older the site, the more likely it is to have been wrecked in the intervening epochs. And populations were so few and far between that they were not likely producing the sheer quantity of stone tools that were later made in the Clovis style. In order to land firmly on science's radar groups "have to cross the threshold of archaeological visibility," Bamforth says.

Given the new Texas find and the other accumulating evidence, Bamforth says, it looks like "the data are hanging together pretty well" to support an earlier occupation of North America than traditional models had suggested. But as tempting as it might be to string sites and theories together neatly, "each site has to stand on its own and by its own merits," Waters noted.

But, he adds, the Buttermilk Creek site "takes us a long way down the trail" to understanding the first inhabitants of the Americas.