When Central America rose from the ocean three million years ago, horn snails divided into two species, one in the Pacific and one in the Atlantic. Smithsonian Tropical Research Institute scientists and others collected snails from 29 locations to study genetic variation. They found that twice, 750,000 years ago and 72,000 years ago, the marine snails crossed the isthmus. How? They likely flew, hitching a ride on birds’ legs.