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.
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.