1491: New Revelations of the Americas Before Columbus, by Charles C. Mann, New York: Alfred A. Knopf, 465 pages, $30 In 1950 the anthropologist Allan R. Holmberg published his classic text Nomads of the Longbow, a study of the Bolivian natives known as the Sirionó. Holmberg had lived with the Indians and studied their habits for two years. His assessment, which generations of scholars took as gospel and applied to other indigenous groups, was that the Sirionó were an unimpressive people who had existed for thousands of years without innovation or progress. He claimed the Sirionó had no real history...