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Preface In 1955, when I was entering my senior year in high school, unbeknownst to me the Ameri­ can Museum of Natural History archaeologist Gordon F. Ekholm wrote the following: “For many years it has been tacitly accepted as one of the basic tenets of Ameri­ canist [archaeological] studies that everything above the level of the simpler cultures such as could have existed at an early time in the subarctic regions of Siberia and Alaska was independently invented or developed in the New World. We have proceeded to study Ameri­ can Indian history with this as a basic assumption and have trained our students to look no further. . . . We of­ ten read that archaeological researches in the Americas have an especial value because they deal with an example of culture history isolated from the main stream of culture history in the Old World.”1 But—again, unknown to me—during the 1950s Ekholm and a few others were putting a major dent in the assumption of interhemispheric independence between the time that Ice Age hunters crossed the then-­ dry Bering Strait into America and the moment that Christopher Columbus arrived in the Bahamas in 1492. Little did I imagine that I was to become a major player in the study of the then (and still) much-­ reviled notion that a long history of pre-­ 1492 transoceanic voyaging to many of the pre-­ Columbian societies of the Americas had resulted in increments, that such increments may even have been a primary stimulus for the emergence of true civilizations in the West­ ern Hemisphere, and that these transoceanic contacts may have had consequential reciprocal effects on the history of the East­ ern Hemisphere. As an undergraduate majoring in geology at Princeton University in the late 1950s, I heard about a German meteorologist named Alfred Wegener, who in 1912 had proposed a theory that he called “continental drift.” He had noticed that the coasts of the continents on either side of the Atlantic seemed roughly parallel, so that if those landmasses could be pushed together, the parts would fit more or less snugly against one another. Wegener’s and others’ investigations indicated that the sequences of rock strata at certain spots on opposite sides, as well as the fossils in those strata, were identical, and that the distributions of certain families of wild plants seemed inexplicable except on the basis of the continents’ once having been joined. Accordingly, Wegener concluded that (long, long before the advent of humans on the planet) there had existed a supercontinent, which he called Pangaea, that subsequently broke up, its pieces then floating about over the ages and ultimately reaching their present positions. Geologists of Wegener’s time derided such ideas as “pseudoscience” and the “delirious ravings” of a mere meteorologist. And, as my geology professors said forty-­ five years later, in the interim virtually no reputable scientist had accepted the theory of continental drift. This was because no plausible mechanism had ever been proposed that could account for the continents’ plowing through thousands of miles of solid oceanic crust, a phenomenon that had never been observed. Geologists’ initial rejection of drift theory had not altered much over the years, even in light of the post-­ Wegener finding that the alignment of magnetic minerals (whose orientations indicate the directions of Earth’s magnetic poles at the time of the formation of the rocks) were everywhere consistent in very late rocks but became increasingly more divergent among the continents the farther back in time one went. I couldn’t believe what I was hearing. There seemed to be no way in which all of these independent phenomena could be accounted for other than by the continents’ having split and moved. There had to be a mechanism—the evidence showed that—we just hadn’t identified it yet. What was wrong with these doubting geologists? As it turned out, the chairman of our own department, Harry H. Hess, was, along with his colleagues, in the process of developing a hypothesis about a possible mechanism of movement: convection currents in Earth’s plastically deformable subsurface mantle that, as they diverged at the tops of their rising plumes, split overlying continents apart and, like snail-­ paced conveyor belts, carried the resulting pieces slowly in opposite directions. This concept, which came to be called “sea-­ floor spreading,” supplied the missing mechanism for continental movement. It galvanized geologists around the globe and set the stage for the novel concept of “plate tectonics.” This new...