A collection of stone spear points, with a distinctive groove down the center of each one, has for the longest time been the earliest sign of the earliest Americans. These “Clovis points” appear in 13,000-year-old archaeological sites over most parts of the country, and have led researchers to argue they are the first signs of human habitation.
That barrier has now been broken—by perhaps 2,500 years, according to a team of university scientists that has been digging in the dirt just northwest of Austin, Tex. In a paper published on Thursday in Science, they claim that a group of about 15,000 man-made stone flakes, and 56 actual tools, are as old as 15,500 years and were made by groups of hunter-gatherers who repeatedly came back to a campsite in an area called Buttermilk Creek. “Together with other pre-Clovis sites, this shows people were all over the Americas 15,000 years ago, so it’s time to put ‘Clovis first’ to rest,” says Michael R. Waters, a professor of anthropology at Texas A&M University and the leader of the research team. The date also indicates that full-blown Clovis culture may have evolved from people who crossed a Bering Sea land bridge, carrying something like this older tool kit, thousands of years before Clovis appeared.
The claim has fired up both fans and critics. “This is the smoking point of the smoking flake. It really nails pre-Clovis,” says Daniel H. Sandweiss, a professor of anthropology and climate studies at the University of Maine at Orono who specializes in American prehistory. “Other pre-Clovis sites have all had questions about them, but this has very obvious human artifacts” in layers that are clearly below Clovis layers.
But other scientists have raised questions about the accuracy of the dates in Texas, and also contend the Clovis-first theory has already been discarded by many researchers. “This is really much to do about little,” says Tom D. Dillehay, a professor of anthropology at Vanderbilt University who has worked on a site called Monte Verde in Chile that has been dated—somewhat controversially—at 14,500 years old.
The dating at Buttermilk Creek site does have some problems, says Jon M. Erlandson, a professor of anthropology at the University of Oregon, “but over all the authors have made a reasonable case for a pre-Clovis occupation.”
The tools at the new site lie in a 20-centimeter-thick layer of sediment, directly beneath another layer holding Clovis-type tools. “We found knives and choppers and blades and ‘bladelets,’” says Mr. Waters. They were all bifacial, meaning the toolmakers had knocked chips off of both sides of the stone to shape it. They are similar to tools in the higher Clovis layer, though the Clovis tools showed a more refined manufacturing technique that is seen only with Clovis technology. (There were, however, no classic Clovis points in the higher layer.)
Debate Over Dating Methods
The layer was dated by a technique called optically stimulated luminescence. It used grains of quartz from the lower layer as tiny clocks. Electrons within these grains accumulate in “traps” inside the molecular structure at a steady, clocklike rate, explains Steven L. Forman, a geochronology specialist on the team and a professor at the University of Illinois at Chicago. Light releases them, so the accumulation indicates how long it has been since the grains last saw the light of day. Scientists measure that electron accumulation by exposing the grains to light in a lab, releasing the particles. In this case, Mr. Forman says, the number of escapees indicated the grains had been buried with the tools between 13,200 and 15,500 years ago, give or take a margin of error of about 1000 years.
That margin is too much for some critics. “It’s a big fudge factor,” says Gary Haynes, a professor of anthropology at the University of Nevada at Reno who has voiced doubts about other pre-Clovis sites. “And they are not dating the artifacts. They are dating the dirt.” Dates on any charred wood from fire hearths, analyzed with different techniques, would be more definitive, agrees Mr. Erlandson, noting that luminescence dates have been variable and questioned at other sites.
Mr. Forman responds that the team was very conservative in its dating estimates; in addition, the researchers got consistent dates from a variety of samples taken from different areas at the site. And there was no charred wood, so other dating techniques could not be used. “Science is a progress report,” Mr. Forman says. “I hope that other people will get out and do better.”
Actually, the team has already done a pretty good job, says David G. Anderson, a professor of anthropology at the University of Tennessee at Knoxville. It has multiple lines of research, adding analysis of the artifact-shaping methods and the sequence of the tool-bearing layers to the sediment dating “which combine to provide pretty convincing evidence of human occupation,” he says.
If the dates are correct, says Mr. Sandweiss, they help to clear up a Clovis puzzle. At 13,000 years ago, the continent was getting significantly colder, “so why would people show up just as the climate got worse?” But earlier, at 14,000 to 15,000 years ago, the Americas were warmer, he says, “and would have been a more hospitable place.” As for the dating, he says, “even if its off by a millennium it doesn’t matter. It’s still enough to break the Clovis barrier.”
