At the end of the nineteenth century, Florentino Ameghino, a well-known naturalist in Argentinean history, started an international scientific controversy by declaring that Patagonia was the center of origin of all mammals. This article studies the history of Ameghino’s theories and the urgency with which various U.S. paleontologists opposed them. This story allows us to better understand the role that science played in the powerful expansion of the United States in Latin America over the nineteenth and twentieth centuries. It also illuminates the influence that Argentina’s economic and political context had on the development of Ameghino’s ideas and his scientific career.