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The most prominent speaker in Philadelphia was Aleš Hrdlička, then sixty-eight. Hrdlička gave Clovis the ultimate accolade: silence. Before one of the biggest archaeological audiences in history, Hrdlička chose to discuss the skeletal evidence for Indians’ early arrival in the Americas. He listed every new find of old bones in the last two decades, and scoffed at them all. “So far as human skeletal remains are concerned,” he concluded, “there is to this moment no evidence that would justify the assumption of any great, i.e., geological antiquity” for American Indians. Every word Hrdlička said was true—but irrelevant. By focusing on skeletons, he was able to avoid discussing Clovis, the focus of the conference, because Howard had found no skeletons there.
^*16

Clovis culture had a distinctive set of tools: scrapers, spear-straighteners, hatchetlike choppers, crescent-moon-shaped objects whose function remains unknown. Its hallmark was the “Clovis point,” a four-inch spearhead with a slightly cut-in, concave tail; in silhouette, the points somewhat resemble those goldfish-shaped cocktail crackers. Folsom points, by contrast, are smaller and finer—perhaps two inches long and an eighth of an inch thick—and usually have a less prominent tail. Both types have wide, shallow grooves or channels called “flutes” cut into the two faces of the head. The user apparently laid the tip of the spear shaft in the flute and twisted hide or sinew repeatedly around the assembly to hold it together. When the point broke, inevitable with stone tools, the head could be loosened and slid forward on the shaft, letting the user chip a new point. A paleo-Indian innovation, this type of fluting exists only in the Americas.

Clovis (left) and Folsom points (shown to scale; fluting at bases)

With Blackwater Draw as a pattern, scientists knew exactly what to look for. During the next few decades, they discovered more than eighty large paleo-Indian sites throughout the United States, Mexico, and southern Canada. All of them had either Folsom or Clovis points, which convinced many archaeologists that the Clovis people, the earlier of the two, must have been the original Americans.

Nobody really knew how old the Clovis people were, though, because geological strata can’t be dated precisely. Figgins surmised that Folsom had been inhabited fifteen to twenty thousand years ago, which meant that Clovis must be a little before that. More precise dates did not come in until the 1950s, when Willard F. Libby, a chemist at the University of Chicago, invented carbon dating.

Libby’s research began in the global scientific race during the 1930s and 1940s to understand cosmic rays, the mysterious, ultrahigh-velocity subatomic particles that continually rain onto the earth from outer space. Like so many bullets, the particles slam into air molecules in the upper atmosphere, knocking off fragments that in turn strike other air molecules. Along the way, Libby realized, the cascade of interactions creates a trickle of carbon-14 (C
¹⁴
), a mildly radioactive form of carbon that over time disintegrates
—decays,
as scientists say—back into a form of nitrogen. Libby determined that the rate at which cosmic rays create C
¹⁴
is roughly equal to the rate at which it decays. As a result, a small but steady percentage of the carbon in air, sea, and land consists of C
¹⁴
. Plants take in C
¹⁴
through photosynthesis, herbivores take it in from the plants, and carnivores take it in from them. In consequence, every living cell has a consistent, low level of C
¹⁴
—they are all very slightly radioactive, a phenomenon that Libby first observed empirically.

When people, plants, and animals die, they stop assimilating C
¹⁴
. The C
¹⁴
already inside their bodies continues to decay, and as a result the percentage of C
¹⁴
in the dead steadily drops. The rate of decline is known precisely; every 5,730 years, half of the C
¹⁴
atoms in nonliving substances become regular carbon atoms. By comparing the C
¹⁴
level in bones and wooden implements to the normal level in living tissues, Libby reasoned, scientists should be able to determine the age of these objects with unheard-of precision. It was as if every living creature had an invisible radioactive clock in its cells.

In 1949 Libby and a collaborator ascertained the C
¹⁴
level in, among other things, a mummy coffin, a piece of Hittite floor, an Egyptian pharaoh’s funerary boat, and the tomb of Sneferu of Meydum, the first Fourth Dynasty pharaoh. Archaeologists already knew their dates of construction, usually from written records; the scientists wanted to compare their estimates to the known dates. Even though Libby and his collaborator were still learning how to measure C
¹⁴
, their estimates were rarely more than a century off—a level of agreement, they wrote dryly, that was “seen to be satisfactory.”

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, Haynes drew attention to the correlation between the birth of “an ice-free, trans-Canadian corridor” and the “abrupt appearance of Clovis artifacts some 700 years later.” Thirteen thousand to fourteen thousand years ago, he suggested, a window in time opened. During this interval—and, for all practical purposes,
only
during this interval—paleo-Indians could have crossed Beringia, slipped through the ice-free corridor, and descended into southern Alberta, from where they would have been able to spread throughout North America. The implication was that every Indian society in the hemisphere was descended from Clovis. The people at Blackwater Draw were the ancestral culture of the Americas.

Haynes was the first to put together this picture. The reaction, he told me, was “pretty gratifying.” The fractious archaeological community embraced his ideas with rare unanimity; they rapidly became the standard model for the peopling of the Americas. On the popular level, Haynes’s scenario made so much intuitive sense that it rapidly leapt from the pages of
Science
to high school history textbooks, mine among them. Three years later, in 1967, the picture was augmented with overkill.

If time travelers from today were to visit North America in the late Pleistocene, they would see in the forests and plains an impossible bestiary of lumbering mastodon, armored rhinos, great dire wolves, sabertooth cats, and ten-foot-long glyptodonts like enormous armadillos. Beavers the size of armchairs; turtles that weighed almost as much as cars; sloths able to reach tree branches twenty feet high; huge, flightless, predatory birds like rapacious ostriches—the tally of Pleistocene monsters is long and alluring.

At about the time of Clovis almost every one of these species vanished. So complete was the disaster that most of today’s big American mammals, such as caribou, moose, and brown bear, are immigrants from Asia. The die-off happened amazingly fast, much of it in the few centuries between 11,500 and 10,900
B.C.
And when it was complete, naturalist Alfred Russell Wallace wrote, the Americas had become “a zoologically impoverished world, from which all of the hugest, and fiercest, and strangest forms [had] recently disappeared.”

The extinctions permanently changed American landscapes and American history. Before the Pleistocene, the Americas had three species of horse and at least two camels that might have been ridden; other mammals could have been domesticated for meat and milk. Had they survived, the consequences would have been huge. Not only would domesticated animals have changed Indian societies, they might have created new zoonotic diseases. Absent the extinctions, the encounter between Europe and the Americas might have been equally deadly for both sides—a world in which
both
hemispheres experienced catastrophic depopulation.