Four million years ago, the tectonic plates underlying North and South America crashed into each other, creating the Isthmus of Panama. Genetic research suggests that this was the moment that monkeys crossed into regions of present-day Central America from their ancestral homes to the south. But seven teeth found in Panama suggest that the primates headed north much earlier, and that they crossed 100 miles of ocean to reach their new world.

According to a paper published this week in Nature, seven monkey teeth were discovered encased in 21-million-year-old rock found during an excavation of material from the Panama Canal expansion. That means the primates must have crossed an ocean barrier once thought insurmountable for most animals. “It’s fantastic,” Marcelo Tejedor, who studies primates at Argentina’s National Patagonian Center in Chubut and who wasn’t involved in the study tells Lizzie Wade at Science. “[This] opens up a heap of possibilities we never expected.”

It’s unlikely that the little primates swam to Panama, so it’s believed they must have floated over, perhaps on a mat of vegetation. “When there are events like hurricanes or major earthquakes or tsunamis, vegetation and clumps of dirt can get washed off the shore. Animals can come along with it,” Siobhán Cooke, a paleobiologist at Northeastern Illinois University who was not part of this study tells Eva Botwin-Kowacki at The Christian Science Monitor. “Monkeys are pretty good dispersers. It isn't surprising that they were able to disperse to North America.”

The history of monkey dispersal is filled with mysteries. The fossil record shows that monkeys evolved in Africa. Between 34 and 37 million years ago, they somehow found a way to South America. Many believe they crossed the Atlantic on similar mats of vegetation, Wade points out, but that is not certain.

It was thought monkeys came to Central America during an event dubbed the Great American Biotic Interchange, which took place after the Isthmus of Panama connected the two continents, allowing relatives of opossums, armadillos and porcupines to head north and deer, cats, raccoons, bears, and other species to cross the isthmus to the southern continent. This new discovery means researchers need to second-guess those assumptions.

The seven monkey teeth found in Panama suggest the ancient species, dubbed Panamacebus transitus, was related to present-day capuchin and squirrel monkeys. It’s not known how large the population of monkeys was in Panama, and digs all around Central America have never produced any evidence that the ancient monkeys made it further north. Jonathan Bloch, the curator of vertebrate paleontology at the Florida Museum of Natural History and lead author of the study says that’s one of the biggest questions the research raises. “[This monkey species] can disperse everywhere, but it just can’t go north on this continent. What’s the problem?” he tells Wade.

In Panama, he suggests, the monkeys found vegetation and fruits similar to what they ate in South America. Once they headed north to Costa Rica and Nicaragua, however, they probably didn’t know how to handle the changing ecosystem.