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Libby won a well-deserved Nobel Prize in 1960. By that time, carbon dating was already revolutionizing archaeology. “You read books and find statements that such and such a society or archaeological site is 20,000 years old,” he remarked. “We learned rather abruptly that these numbers, these ancient ages, are not known.” Archaeologists had been making inferences from limited, indirect data. With radiocarbon, these numbers, these ancient ages, could be known, and with ever-increasing accuracy.

One of the first tasks assigned to the new technique was determining the age of the Clovis culture. Much of the work occurred at the University of Arizona, in Tucson, which in 1958 established the world’s first major archaeological carbon-dating laboratory. At the new lab was a doctoral student named C. Vance Haynes. Haynes was a mining engineer who became fascinated by archaeology during a stint in the air force. While serving at a base in the Southwest, he began collecting arrowheads, a hobby that ultimately led to his abandoning geology and coming to the University of Arizona as a graduate student in archaeology. As the Clovis-culture dates crossed his lab bench, Haynes was struck by their consistency. No matter what the location of a site, carbon dating showed that it was occupied between 13,500 and 12,900 years ago.*17 To Haynes, with his geologist’s training, the dates were auspicious. The Clovis culture arose just after the only time period in which migration from Siberia seemed to have been possible.

During the Ice Ages so much of the world’s water was frozen into glaciers that sea levels fell as much as four hundred feet. The strait between Siberia’s Chukotsky Peninsula and Alaska’s Seward Peninsula is now only 56 miles wide and about 120 feet deep, shallower than many lakes. The decline in sea levels let the two peninsulas join up. What had been a frigid expanse of whale habitat became a flat stretch of countryside more than a thousand miles wide. Beringia, as this land is called, was surprisingly temperate, sometimes even warmer than it is today; masses of low flowers covered it every spring. The relative salubriousness of the climate may seem incredible, given that Beringia is on the Arctic Circle and the world was still in the throes of the Ice Ages, but many lines of evidence suggest that it is true. In Siberia and Alaska, for instance, paleoentomologists—scientists who study ancient insects—have discovered in late-Pleistocene sediments fossil beetles and weevils of species that live only in places where summer temperatures reach the fifties.

C. Vance Haynes

Beringia was easily traversable. Western Canada was not, because it was buried beneath two massive, conjoined ice sheets, each thousands of feet deep and two thousand miles long. Even today, crossing a vast, splintered wilderness of ice would be a risky task requiring special vehicles and a big support staff. For whole bands to walk across it with backpacks full of supplies would be effectively impossible. (In any case, why would they want to do it?

There was a short period, though, when the barrier could be avoided—or at least some scientists so believed. The Ice Ages drew to a close about fifteen thousand years ago. As the climate warmed, the glaciers slowly melted and sea levels rose; within three thousand years, Beringia had again disappeared beneath the waves. In the 1950s some geologists concluded that between the beginning of the temperature rise and the resubmergence of the land bridge the inland edges of the two great ice sheets in western Canada shrank, forming a comparatively hospitable pathway between them. This ice-free corridor ran down the Yukon River Valley and along the eastern side of the Canadian Rockies. Even as the Pacific advanced upon Beringia, these geologists said, plant and animal life recolonized the ice-free corridor. And it did so just in time to let paleo-Indians through.

In a crisply argued paper in Science in 1964,