Scientists have found seven tiny teeth in Panama that could change the way we think about how monkeys came to be in North America. The ancient monkey fossils indicate the primate at one point made a 100-mile ocean crossing between North America and South America into modern-day Panama, meaning monkeys swung around jungles on the North American continent far earlier than previously thought.

Scientists believed until now that the animals began crossing between the continents about 3.5 million years ago, but the monkey teeth are at least 21 million years old. The fossils were recovered during excavations related to the expansion of the Panama Canal, according to the University of Florida, which is affiliated with several of the researchers who published their findings in the journal Nature Wednesday.

The new genus and species has been dubbed Panamacebus transitus, with the latter term based on the Latin word for transit, meaning crossing. It’s unclear how the monkey crossed the sea between North America and South America during the early Miocene period. It may have swam 100 miles or unintentionally rafted across on mats of vegetation, said Jonathan Bloch, curator of vertebrate paleontology at the Florida Museum of Natural History on the University of Florida campus.

“We hope to find more monkey fossils, but time is definitely a factor,” Bloch said. “We’re fighting against the forest that wants to grow over the rocks again. The expansion of the Panama Canal provides a once-in-a-century opportunity for these kinds of exciting discoveries. But we can’t assume we’ll always be able access these rock exposures.”

The finding represents an “utterly bizarre” chapter in the history of New World monkeys, Bloch said. The researchers said the animals may have weighed about 5.9 pounds each.

“Somehow they made a transoceanic journey from Africa, then they dispersed throughout South America,” Bloch said. “Now we see that they, as far as we know, are the only mammal that successfully crossed the early Miocene Central American Seaway into present-day Panama. So how were monkeys able to do this? Hopefully, future fossil discoveries will help us better understand this extraordinary history.”

Other animals, such as amphibians, reptiles, freshwater fishes and insects, are also believed to have made ocean crossings between North America and South America during the early Miocene. The two continents likely remained separated until about 3.5 million years ago.

 “Uncovering a monkey this old in Central America, at the southernmost point of the North American landmass, is similar in some ways to finding Homo erectus, an extinct human ancestor known only from Africa and Asia, in Australia,” Bloch said.

New World monkeys nowadays largely live in tropical forests from Brazil to Mexico. It’s unknown why they are not found farther north, given that were at one point in Panama, said study co-author Aaron Wood, who discovered the first teeth as a Florida Museum of Natural History postdoctoral researcher in 2012.

“While the fossil mammals found with P. transitus include horses, camels and squirrels that look like what paleontologists have found in the early Miocene of Mexico, Texas and Florida, the new monkey was limited to the southernmost point of the continent,” said Wood, now a paleontologist at Iowa State University. “The ancient South American-derived forests found in Panama were absent in northern Central America at the time, preventing monkeys from moving north, even though climate and geographic barriers like oceans did not wholly restrict their northward movements.”