In a strange twist of evolutionary history, the ancestors of modern South American monkeys such as the capuchin and woolly monkeys first came to the New World by floating across the Atlantic Ocean on mats of vegetation and earth. According to a new study, they were not the only primates to make the trip. A fossil find in Peru suggests that a different, entirely extinct family of primates undertook the same kind of oceanic voyage more than 30 million years ago.

On the banks of Río Yurúa, close to the border of Peru and Brazil, University of Southern California paleontologist Erik Seiffert documented a fossil site that contains a mix of the strange and familiar. Here, roughly 32 million-year-old rock preserves the remains of bats, relatives of capybaras, and early New World monkeys. They also found evidence of a second primate group, one thought to have lived only in Africa.

Described today in Science, the key fossils are a set of four teeth. While teeth may be small, they are often crucial pieces of the mammalian fossil record. The natural durability of teeth gives them a better chance at lasting for millions of years. Plus, mammal teeth change rapidly over evolutionary time and are often distinct. A single molar can be more useful in identifying a fossil than a set of ribs or a leg bone.

Seiffert and colleagues propose the primate teeth they found in Peru belonged to a now-extinct group of monkeys called parapithecids. To a casual observer, Seiffert says, these primates would have looked somewhat similar to today’s New World monkeys. “It’s only when we look into the details of the teeth, crania, and long bones that we see there are important differences,” he notes, with the arrangement of bumps and troughs on the teeth acting as a reliable guide to which fossil belonged to which family.

“The new molars were almost identical to those of the parapithecid Qatrania, which is known from sites that I worked in the Fayum area of Egypt,” Seiffert says. Now it seems parapithecids were present in South America by about 32 million years ago. The researchers have given the name Ucayalipithecus perdita to the teeth representing the new species.

“Parapithecid teeth are distinctive,” says Wake Forest University paleoprimatologist Ellen Miller, who was not involved in the research. It’s unlikely that another form of mammal, or even another form of monkey, independently evolved teeth the same shape and size as those of the parapithecids that were alive at the same time, Miller notes. The sudden appearance of Ucayalipithecus in South America, far from its closest relatives in prehistoric Africa, hints that these monkeys the same sort of transatlantic journey that the ancestors of New World monkeys must have endured. No, these primates were not lashing together rafts and intentionally setting sail for uncharted territories. The process was entirely accidental, relying on luck and the fact that the world was different 32 million years ago.

Back then, during a time known as the Late Eocene, Africa and South America were significantly closer. The span of the Atlantic Ocean between the two continents measured about 930 to 1,300 miles apart compared to the modern expanse of 1,770 miles. In addition, the buildup of glaciers in Antarctica around that time caused sea levels to drop, making the passage shorter than it is today. During this window of prehistory, the path between the continents was passable by sea.

“I think everyone kind of shakes their heads at primates rafting long or even moderate distances,” Miller says, but such events have happened at other times and are still going on today. Animals such as tenrecs and lemurs arrived on Madagascar by rafting from mainland Africa across a distance of more than 260 miles, for example, and small lizards island-hop in the Bahamas on natural rafts.

The lack of comparable primate fossils in other parts of the world helps to cement the case. If the ancestors of New World monkeys or Ucayalipithecus had spread through Europe and island-hopped to North America before heading south, or if they had taken the southern route via Antarctica, there would be a fossil trail of related primates in these places. Not to mention, Seiffert says, that the world was cooling during the time primates traveled across the Atlantic and many ancient primate species were going extinct in Europe, Asia, and North America. To date, paleontologists have found no evidence of an alternate route.

“I have to admit that I was much more skeptical about rafting until I saw a video of mats of vegetation floating down the Panama Canal, with trees upright and maybe even fruiting,” Seiffert says.

The trip must have been harrowing. The working hypothesis is that monkeys living along the Atlantic coast of Africa were swept up in intense storms and found themselves at sea. These primates clung to storm debris that formed natural rafts. Currents carried these platforms of vegetation across the ocean. Upon arrival in a new continent, the surviving monkeys found a suitable new home and began to proliferate.

“The discovery of Ucayalipithecus reveals that, for the last century or so, we have been missing a whole chapter in the chronicle of primate evolution in South America,” Seiffert says. These monkeys lived alongside and competed with the ancestors of today’s New World monkeys, helping to shape the evolution of plant and animal life as a hidden part of South America’s ancient ecology.

Ucayalipithecus was discovered at a location far inland that would have been 2,400 miles from the eastern coast of South America. This fact offers some evidence that parapithecids thrived for a time after their arrival, occupying some of the same habitats as the ancestors of New World monkeys.

“The implications of this research should be a game-changer in primate biogeography,” Miller says. Primates made oceanic journeys to new places over and over again through prehistoric time, and paleontologists may find additional evidence of these dispersals in the fossil record. “I think more researchers will become interested in modeling these events,” Miller says, “saying ‘Okay, we know this happens, so under what circumstances might we expect it to occur?’”