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By Root 1911 0

characteristics—incisor shape, canine size, molar root number, the presence or absence of grooves on tooth faces—differ slightly in ways that are consistent within ethnic groups. In a fantastically painstaking process, Turner measured “28 key crown and root traits” in more than 200,000 Indian teeth. He discovered that Indians formed “three New World dental clusters” corresponding to Greenberg’s Aleut, Na-Dené, and Amerind. By comparing tooth variation in Asian populations, Turner estimated the approximate rate at which the secondary characteristics in teeth evolved. (Because these factors make no difference to dental function, anthropologists assume that any changes reflect random mutation, which biologists in turn assume occurs at a roughly constant rate.) Applying his “worldwide rate of dental microevolution” to the three migrations, Turner came up with roughly similar dates of emigration. Amerinds, he concluded, had split off from northeast Asian groups about fourteen thousand years ago, which fit well “with the widely held view that the first Americans were the Clovis-culture big-game-hunting paleo-Indians.”

The article provoked vigorous reaction, not all of the sort that its authors wished. In hindsight, a hint of what was to come lay in its third section, in which Arizona State geneticist Stephen L. Zegura conceded that the “tripartite division of modern Native Americans is still without strong confirmation” from molecular biology. To the authors’ critics, the lack of confirmation had an obvious cause: the whole three-migrations theory was wrong. “Neither their linguistic classification nor their dental/genetic correlation is supported,” complained Lyle Campbell, of the State University of New York at Buffalo. Greenberg’s three-family division, Campbell thought, “should be shouted down in order not to confuse nonspecialists.” The Amerind-language family was so enormous, Berkeley linguist Johanna Nichols complained, that the likelihood of being able to prove it actually existed was “somewhere between zero and hopeless.”

Although the three-migrations theory was widely attacked, it spurred geneticists to pursue research into Native American origins. The main battleground was mitochondrial DNA, the special DNA with which Pena, the Brazilian geneticist, hoped to find the Botocudo. As I mentioned before, a scientific team led by Douglas Wallace found in 1990 that almost all Indians belong to one of four mitochondrial haplogroups, three of which are common in Asia (mitochondria with similar genetic characteristics, such as a particular mutation or version of a gene, belong to the same haplogroup). Wallace’s discovery initially seemed to confirm the three-migrations model: the haplogroups were seen as the legacy of separate waves of migration, with the most common haplogroup corresponding to the Clovis culture. Wallace came up with further data when he began working with James Neel, the geneticist who studied the Yanomami response to measles.

In earlier work, Neel had combined data from multiple sources to estimate that two related groups of Central American Indians had split off from each other eight thousand to ten thousand years before. Now Neel and Wallace scrutinized the two groups’ mitochondrial DNA. Over time, it should have accumulated mutations, almost all of them tiny alterations in unused DNA that didn’t affect the mitochondria’s functions. By counting the number of mutations that appeared in one group and not the other, Neel and Wallace determined the rate at which the two groups’ mitochondrial DNA had separately changed in the millennia since their separation: .2 to .3 percent every ten thousand years. In 1994 Neel and Wallace sifted through mitochondrial DNA from eighteen widely dispersed Indian groups, looking for mutations that had occurred since their common ancestors left Asia. Using their previously calculated rate of genetic change as a standard, they estimated when the original group had migrated to the Americas: 22,414 to 29,545 years ago. Indians had come to the Americas ten thousand years before