Monkeys resembling today's capuchins accomplished the astonishing feat of crossing at least 100 miles (160 km) of open ocean 21 million years ago to get from South America to North America eons before the two continents joined together.
Scientists said Wednesday they reached that conclusion based on the discovery of seven little teeth during excavations involving the Panama Canal's expansion, showing monkeys had reached the North American continent far earlier than previously known.
The teeth belonged to Panamacebus transitus, a previously unknown medium-sized monkey species. South America at the time was secluded from other continents, with a strange array of mammals evolving in what 20th century American paleontologist George Gaylord Simpson called "splendid isolation."
How Panamacebus performed the feat is a bit mysterious.
After all, seagoing simians seem somewhat suspicious.
"Panama represents the southernmost extreme of the North American continent at that time," said Jonathan Bloch, a vertebrate paleontology curator at the Florida Museum of Natural History on the University of Florida campus.
"It may have swum across, but this would have required covering a distance of more than 100 miles, a difficult feat for sure. It's more likely that it unintentionally rafted across on mats of vegetation," Bloch added.
Bloch said as far as anyone knows, these monkeys were the only mammals that managed to cross the seaway from South America to reach present-day Panama. While South American giant ground sloths managed to reach North America about 9 million years ago, it was not until about 3.5 million years ago that the Isthmus of Panama formed, allowing animals to begin trekking in large numbers between the continents in one of the biggest mixing of species on record.
Bloch said learning that monkeys lived in North America that long ago was a "mind-bending discovery" because it had long been accepted that they simply did not exist there at that time.
It would be akin to learning that Australia's kangaroos and koalas live in the wilds of Asia today.
Monkeys originated in Africa and later spread to other parts of the world. Scientists believe monkeys made an even lengthier transoceanic voyage, perhaps 37 million years ago, when they transited from Africa to South America, also probably on floating debris.
Bloch said the seven teeth, the largest of which were molars about one-fifth of an inch (5 mm) long, were unmistakable as belonging to a South American monkey, and their shape showed Panamacebus had a diet of fruit in its tropical forest environment.
The research was published in the journal Nature.