Dating stone tools

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.

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.

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.