1491_ New Revelations of the Americas Before Columbus - Charles C. Mann [88]
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 (C14), 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 C14 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 C14. Plants take in C14 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 C14—they are all very slightly radioactive, a phenomenon that Libby first observed empirically.
When people, plants, and animals die, they stop assimilating C14. The C14 already inside their bodies continues to decay, and as a result the percentage of C14 in the dead steadily drops. The rate of decline is known precisely; every 5,730 years, half of the C14 atoms in nonliving substances become regular carbon atoms. By comparing the C14 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 C14 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 C14, their estimates were