Scientists were floored after coming across seven fossilized monkey teeth exposed by the Panama Canal expansion project — indicating that monkeys may have come to North American a lot longer ago than thought.

The monkey teeth were found in rocks aged 21 million years old, even though scientists had thought monkeys hadn’t come from South America into North America until about 4 million years ago, according to a Smithsonian Tropical Research Institute statement.

The monkeys probably resembled capuchin monkeys, and are the only mammals known to have crossed a water boundary so early on. Today, the two continents are connected by the Isthmus of Panama, but back then they would have had to find a way to raft across 162 kilometers of open water.

Scientists didn’t think it was possible for monkeys to cross such a great distance, and it certainly doesn’t look like swimming was a possibility. But they may have rafted over on a mat of vegetation, which can happen when there are hurricanes or huge earthquakes resulting tsunamis.

“We suggest that Panamacebus was related to the capuchin (also known as “organ-grinder” monkeys) and squirrel monkeys that are found in Central and South America today,” Jonathan Bloch, curator of vertebrate paleontology at the Florida Museum of Natural History on the University of Florida campus and lead author on the study, said in the statement. “Prior to this discovery, New World monkeys were thought to have evolved in isolation on South America, cut-off from North America by a wide seaway.”

The statement added: “Before the monkey teeth were discovered, the oldest evidence of movement of a mammal from South to North America are 8.5–9 million-year-old fossil remains of giant sloths. The authors of this report suggest two explanations: 1) that mammals from South America were more adapted to life in the South American derived forests still found in Panama and Costa Rica than to other forest types characteristic of Northern Central America or 2) that the lack of exposed fossil deposits throughout Central America means that evidence of these dispersals has yet to be revealed.”