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About 100,000 years ago, modern human beings started spreading out from their initial homeland in Africa to occupy Europe, Asia and, by sea, even Australia, displacing or absorbing Neanderthals and other archaic hominid species. That diaspora took about 70,000 years, and when it was completed our ancestors stood triumphant.

The peopling of the Americas, scholars 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.

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.

For decades the most compelling evidence of this standard view consisted of distinctive, exquisitely crafted, grooved bifacial projectile points, called “Clovis points” after the New Mexico town near where they were first discovered in 1929. With the aid of radiocarbon dating in the 1950s, archaeologists determined that the Clovis sites were 13,500 years old. This came as little surprise, for the first Clovis points were found in ancient campsites along with the remains of mammoth and ice age bison, creatures that researchers knew had died out thousands of years ago. But the discovery dramatically undermined the prevailing wisdom that human beings and these ice age “megafauna” did not exist in America at the same time. Scholars flocked to New Mexico to see for themselves.

The idea that the Clovis people, as they came to be known, were the first Americans quickly won over the research community. “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.”

Furthermore, the earliest Clovis dates corresponded roughly to the right geological moment—after the ice age warming, before the great cold snap. The northern ice had receded far enough so incoming settlers could curl around to the eastern slope of North America’s coastal mountains and hike south along an ice-free corridor between the cordilleran mountain glaciers to the west and the huge Laurentide ice sheet that swaddled much of Canada to the east. “It was a very nice package, and that’s what sealed the deal,” Goebel said. “Clovis as the first Americans became the standard, and it’s really a high bar.”

When they reached the temperate prairies, the migrants found an environment far different from what we know today—both fantastic and terrifying. There were mammoths, mastodons, giant sloths, camels, bison, lions, saber-toothed cats, cheetahs, dire wolves weighing 150 pounds, eight-foot beavers and short-faced bears that stood more than six feet tall on all fours and weighed 1,800 pounds. Clovis points, finely made and strong, were well suited for hunting large animals.

The hunters spread through the United States and Mexico, the story went, pursuing prey until too few animals remained to support them in the last cold snap. Radiocarbon dates show that most of the megafauna became extinct around 12,700 years ago. The Clovis points disappeared then as well, perhaps because there were no longer any large animals to hunt.

The Clovis theory, over time, acquired the force of dogma. “We all learned it as undergraduates,” Waters recalled. Any artifacts that scholars said came before Clovis, or competing theories that cast doubt on the Clovis-first idea, were ridiculed by the archaeological establishment, discredited as bad science or ignored.

Take South America. 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. The 50-foot-long main structure, made of wood with a hide roof, was divided into what appeared to be individual spaces, each with a separate hearth. Outside was a second, wishbone-shaped structure that apparently contained medicinal plants. Mastodons were butchered nearby. The excavators found cordage, stone choppers and augers and wooden planks preserved in the bog, along with plant remains, edible seeds and traces of wild potatoes. Significantly, though, the researchers found no Clovis points. That posed a challenge: either Clovis hunters went to South America without their trademark weapons (highly unlikely) or people settled in South America even before the Clovis people arrived.

There must have been “people somewhere in the Americas 15,000 or 16,000 years ago, or perhaps as long as 18,000 years ago,” said Dillehay, now at Vanderbilt University.