In the late 1970s, the U.S. archaeologist Tom D. Dillehay and his Chilean colleagues began excavating what appeared to be an ancient settlement on a creek bank at Monte Verde, in southern Chile. Radiocarbon readings on organic material collected from the ruins of a large tent-like structure showed that the site was 14,800 years old, predating Clovis finds by more than 1,000 years.
At the Buttermilk Creek Complex archaeological site north of Austin, Texas, in a layer of earth beneath a known Clovis excavation, researchers led by Waters over the past several years found 15,528 pre-Clovis artifacts—most of them toolmaking chert flakes, but also 56 chert tools. Using optically stimulated luminescence, a technique that analyzes light energy trapped in sediment particles to identify the last time the soil was exposed to sunlight, they found that the oldest artifacts dated to 15,500 years ago—some 2,000 years older than Clovis.
Stanford and Bradley say evidence for the Solutreans’ presence in America includes stone artifacts gathered by archaeologists at several sites on the eastern shore of Chesapeake Bay, all producing dates more than 20,000 years old. Most of the dates were derived from organic material found with the artifacts. The exception was a mastodon tusk with attached bone and teeth netted by a fisherman in 1974, along with a laurel leaf-shaped stone knife. Stanford found the tusk to be 22,760 years old.
The key question is precisely when the migration occurred. To be sure, there were constraints imposed by North America’s glacial history. Researchers suggest that it happened sometime after gradual warming began 25,000 years ago during the depths of the ice age, but well before a severe cold snap reversed the trend 12,900 years ago. Early in this window, when the weather was very cold, migration by boat was more likely because immense expanses of ice would have turned an overland journey into a nightmarish ordeal. Later, however, the ice receded, opening up plausible land bridges for trekkers coming across the Bering Strait.
The idea that the Clovis people, as they came to be known, were the first Americans 13,000 years ago incorrectly won over the research community in 1929. “The evidence was unequivocal,” said Ted Goebel, a colleague of Waters at the Center for the Study of the First Americans. Clovis sites, it turned out, were spread all over the continent, and “there was a clear association of the fauna with hundreds, if not thousands, of artifacts,” Goebel said. “Again and again it was the full picture.”
The peopling of the Americas, scholars now tend to agree, happened sometime in the past 25,000 years. In what might be called the standard view of events, a wave of big game hunters crossed into the New World from Siberia at the end of the last ice age, when the Bering Strait was a land bridge that had emerged after glaciers and continental ice sheets froze enough of the world’s water to lower sea level as much as 400 feet below what it is today.