Scientists have discovered the earliest known traveler between South and North America: a new species of monkey. It lived in Panama 21 million years ago, long before most scientists believe there was a land bridge connecting the continents. So how did it get there? By floating on rafts across the sea.

Take to trip to Central America today, and you’re likely to see monkeys swinging through the trees in tropical forests and even in cities. But scientists thought that millions of years ago, the region was free of the mischievous primates. A plethora of monkey species lived in South America at the time, but it was then an island continent, cut off from its northern neighborhood by a seaway at least 160 kilometers wide.

So Jonathan Bloch was shocked when a postdoc sent him photos of fossils he had dug up while exploring ancient sediments in the newly expanded Panama Canal: They were monkey teeth. The teeth, encased in 21-million-year-old rock, were far older than any monkey teeth in Central America should be. In fact, until this find, the oldest monkeys in Central America were presumed to be less than 5 million years old, when many mammals stampeded over a land bridge between the continents. Bloch, a vertebrate paleontologist at the University of Florida in Gainesville, knew he had something that would rewrite the (pre)history books.

The seven fossil teeth, described online today in Nature, represent the earliest evidence for mammals moving from South America to North America, as well as the oldest known ancestor of today’s New World monkeys. “It’s fantastic,” says Marcelo Tejedor, a paleontologist who studies New World primates at Argentina’s National Patagonian Center in Chubut and who wasn’t involved in the study. When it comes to ancient primates in the Americas, the fossil record is very scarce. A discovery like this one, he says, “opens up a heap of possibilities we never expected.”

Bloch and his colleagues aren’t sure how large the Panamanian population of monkeys was, or for how long it may have survived. But when it comes to what they looked like, “you can say a lot from seven teeth,” Bloch says. The teeth all look very similar to one another, meaning they likely belonged to the same medium-sized primate, probably weighing about 3 kilograms. Bloch and his team put their new find in the Cebidae family, a group of Central and South American primates that today includes capuchin and squirrel monkeys. The find “represents the oldest fossil record … of the group that gave rise to all of the living South American monkeys,” Bloch explains. His team named the new species Panamacebus transitus, combining “Panama” with the Latin root for “Cebidae” and referring to its voyage across what was previously thought to be an impenetrable stretch of ocean.

But just how did Panamacebus make that journey, and why does it appear to be the only mammal that traveled from South to North America at such an early date? Bloch proposes that they floated to Panama from South America on mats of dirt and vegetation. “Primates are known for doing this,” agrees Lauren Gonzales, a primate paleontologist at Duke University in Durham, North Carolina, who wasn’t involved in the study. In fact, scientists think primates first traveled from Africa to South America 40 million years ago by floating across the Atlantic on the same kinds of mats.

A debate has raged in recent years about whether Panama may have collided with South America millions of years before scientists previously thought. But Gonzales doesn’t think Panamacebus can be used to bolster the theory of an earlier overland route between the continents, which some propose emerged between 13 million and 15 million years ago. “If there was a land bridge, you would see other larger animals making it across. And you don’t. You just find these primates,” she says.

The real question, Bloch says, is if these monkeys were such intrepid travelers, why didn’t they make it any farther north than Panama? “It can disperse everywhere, but it just can’t go north on this continent. What’s the problem?” Bloch wondered.

Pollen records offered a clue. They show that around 21 million years ago, the plants in Panama’s forests were similar to those in South America. So when the South American monkeys disembarked in Panama, “they found all the fruits and things they were used to eating,” Bloch says. But as they moved north into Costa Rica and Nicaragua, they would have found themselves forests with a very different evolutionary history. Plants in those places had spread down from North America, and therefore might not have offered Panamacebus the plethora of fruit trees it needed to survive.

“Getting there is just half the task. The other half is to establish yourself and survive,” says Alexandre Antonelli, an evolutionary biologist at the University of Gothenburg in Sweden who wasn’t involved in the research. An ocean couldn’t thwart Panamacebus. But a forest without their favorite fruit trees? That would do in any monkey.