For centuries, elite people in ancient Mesoamerica drank a foaming beverage made from the ground beans of the tropical Theobroma cacao tree. Served in cylindrical jars and spouted pitchers in grand cities and temples, this was a chocolate drink steeped in ritual. North America had its great prehistoric cities, too—for example, ancient Cahokia near St. Louis, Missouri, or Chaco Canyon in New Mexico—but for years most archaeologists assumed little contact between the cultures. Chocolate may change that picture, with the recent discovery of subtle residues of it in pots from Cahokia. Added to similar evidence from the Southeast and Southwest, the findings suggest regular trade in cacao—and movements of the people who imbued it with significance—between ancient Mesoamericans and their northern neighbors, says Dorothy Washburn, an archaeologist at the University of Pennsylvania Museum of Archaeology and Anthropology and co-author of a new paper. But the new work also reveals the complexities of such residue analysis, she says: Low levels of a certain alkaloid of T. cacao may be due to modern contamination.