If you were dropped into virtually any region of North America 56 million years ago, you probably would not recognize where you had landed. Back then, at the dawn of the Eocene epoch, the earth was warmer and wetter than it is today. A sea had just closed up in the middle of the Great Plains, and the Rocky Mountains had not yet attained their full height. The continent's plant and animal communities were dramatically different. In the Canadian High Arctic, which today harbors relatively few tundra plant species, year-round temperatures above freezing nurtured a rich and diverse flora; Ellesmere Island in far northern Canada, across from the northwestern coast of Greenland, was home to alligators and giant tortoises. What is now the southeastern U.S. was dominated by tropical rain forest, complete with primates. The northeastern U.S., for its part, ranged from broad-leaved (as opposed to needle-leaved) evergreen forest to deciduous forests of ginkgo, viburnum, birch and elm, among other species. The deciduous broad-leaved forests that now cover 11 percent of North America north of Mexico were in their infancy. But that was about to change, with the spread and extraordinary diversification of what would eventually become some of the most ecologically and economically significant woody plants in the world: the acorn-bearing, wind-pollinated trees we call oaks.