Millions of years ago, the small arc of volcanic land that became Panama collided with the tectonic plate of South America, joining the continents and allowing once-isolated species to encounter each other for the first time. That turned out to be bad news for many of South America’s unique species—elephant-sized sloths, for example, and flightless, sprinting terror birds—as they were eventually outcompeted by North American natives like saber-toothed cats. But when exactly did the Panama arc meet South America, beginning this fateful encounter? Fossil evidence suggested the bridge formed between 2.5 million and 3 million years ago. But a new geological study challenges that long-accepted idea. When a team of researchers examined the layers of rock in an ancient riverbed in Colombia, they found that volcanic minerals suddenly appeared between 13 million and 15 million years ago. The only nearby volcanoes that could have created such rocks were in the Panama arc. In order for the minerals to reach Colombia, they would have had to flow down a river—and rivers don’t flow across oceans. The existence of a river between Panama and Colombia implies that at least part of the Panama isthmus had formed 10 million years earlier than most scientists thought, the researchers report online today in Science. The new findings are part of a suite of discoveries—including fossils recovered from the expansion of the Panama Canal (pictured)—that point to an earlier closure for the isthmus, potentially rewriting the biogeographical history of the Americas.