There's a pretty wild theory about how we think monkeys came to inhabit the Americas. And it’s also a weird one, because we think they got there by sailing across the seas.

Our story begins 140 million years ago during the Cretaceous, when the continents of Africa and South America first split through continental drift. This caused the diversification of many species as the South Atlantic Ocean opened, ending any land link between these two locales.

But it wasn’t until 45 million years ago during the Eocene that the first primates, Eosimias, emerged in China. At some point in the following 5 million years, the descendants of these primates called Talahpithecus made their way into Africa.

Here comes the problem. Not too long ago, we discovered monkeys in the New World, the Amazon, dating back to about 36 million years. According to the fossil record, no primates predate these animals in South America.

Then, 21 million years ago, monkeys (Cebidae) sprung up in North America. These two continents would not be connected by land until 3 million years ago, when the Isthmus of Panama arose.

So the question is, how did primates travel around the world when there was no land for them to walk on? The answer, my friends, is they sailed on rafts across the ocean, surviving the elements to thrive in a new land. Primate conquistadors, if you will.

“It is one of the most bizarre examples of what looks like an ocean crossing,” Alan de Queiroz, evolutionary biologist and author of The Monkey's Voyage, told IFLScience. “Initially a lot of people were kind of incredulous, they didn’t think that it was likely that something like a monkey could cross the Atlantic Ocean.”

But that is exactly what it looks like. It explains how we find primate fossils in the most bizarre places. Other explanations, such as land bridges or island hopping, simply come up short when looking at geological history.

An important discovery supporting the raft theory came in 2012 when the Panama Canal was widened. Researchers used this opportunity to excavate the land and discover previously hidden fossils. And they were stunned to find what looked like primate teeth (Perupithecus ucayaliensis) dating back 21 million years.

“We bounced ideas with colleagues, we thought they could be from a bat,” said Jonathan Bloch from the University of Florida in Gainesville, the lead author on the paper describing that finding. “But all of a sudden it clicked, and it was actually a piece of primate. And that was very exciting, because up until this discovery there hadn’t been any primates found on the North American landmass.”

So, let’s go through this. How did these monkeys come to find themselves on a raft, let alone survive the crossing of the Atlantic Ocean – estimated to be a journey of hundreds of miles – and later the Central American Seaway?

The primates that made the journey in this story were small, about the size of a squirrel. They did not construct their own rafts of course, but rather the very ground beneath their feet would have been swept away, leaving them stranded on a floating island.

Estimates for how many animals of a species you need to colonize a new world vary, from the dozens to the hundreds. Let’s picture a scene here of maybe 100 monkeys hanging out near the western coast of Africa. Suddenly, disaster strikes. A tsunami, or perhaps a powerful storm, batters the land. A huge chunk of the ground is torn away, creating a large floating platform. This may have been a sort of mat of vegetation, or perhaps it was more soil-based.

“I would assume that those rafts were pretty dramatic events,” said Alexandre Antonelli, Professor in Systematics and Biodiversity at the University of Gothenburg in Sweden. “It could have been a tsunami or a really big storm. So I don’t think they were having a good time on the island. But it probably was pretty fast and dramatic.”

Floating rafts like these are not unprecedented. Years ago, we spotted one in the Atlantic that measured about 840 square meters (9,000 square feet) in size. It was seen in two places 1,600 kilometers (1,000 miles) apart, meaning it had survived a pretty long time. This is not even the biggest raft that has been found.

“It’s the dirt and the roots that are holding it together, and then the vegetation on top,” said de Queiroz. “So it makes this pretty substantial island, and it’s not just vegetation.”

Some theories suggest that these mats could even support trees. If that’s the case, they could act as sails, carrying the island at speeds above that of just the ocean current, reducing the time any animals aboard would need to survive.

Alain Houle, from the University of Montreal, calculated how long it would take to cross the Atlantic Ocean under these conditions. He suggested the journey from Africa to South America may have been as short as 14.7 days, with a similar amount of time for the subsequent journey to North America 20 million years later. The quicker the better in terms of survival.

But the monkeys also probably benefited from being omnivorous, being able to eat pretty much anything they could find on the raft. “Assuming some fresh water could be found in the leaves, roots, insects, fruits, bark, and other similar food items, an animal like a paleomonkey could survive those two weeks,” Houle told IFLScience. With enough rain, it’s possible they could have survived for quite some time.

“Even a month or two months, if the raft is big enough and has all kinds of vegetation on it, there could be enough food for a group of monkeys to actually survive,” added de Queiroz. “Maybe they wouldn't even be that stressed.”

Once they arrived in the Americas, it looks like they took to their surroundings pretty quickly. With a similar climate and abundant food, they spread far and wide. For some reason, though, the descendants of these sailing monkeys did not venture further into North America after their second voyage.

“Once the monkeys crossed the [Central American Seaway], they found a rainforest just like the one they left and the fruits they were used to eating,” said Bloch. But if they’d headed north, they would have started to find the available food changing to things like acorns, which were not in their diet, and thus stopping them going much further.

All in all it's a rather bizarre story, and not everyone agrees with it. Some have proposed that maybe our dates for continental drift are wrong, or perhaps we simply haven't found the extent of the fossil record yet. The rafting theory is sometimes derided for being a catch-all for when we can't explain an animal's migration.

Perhaps more bizarrely, though, is that we can't better the raft theory. And monkeys are not the only life for which it has been proposed. Going back as far as Charles Darwin in the 19th Century, it’s been used to explain why we find lizards, rats, plants, and more spread across seemingly unconnected locations. Monkeys are some of the larger animals thought to have been involved in a raft crossing.

“Rafting in general is something that comes up quite a lot,” Antonelli said. “When people started to use DNA, they noticed that many [dispersal] events had been long after the continents had separated.”

Primates have one of the most detailed fossil records, so we can paint a pretty good picture of where they’ve been. The fact they sprung up in the Americas, with seemingly no land bridge to get there, suggests that this rafting idea really is true, no matter how odd.

“Monkeys are the weirdest thing,” said Bloch. “Somehow they managed to get from Africa to South America across the Atlantic. Then they dispersed throughout South America, going to the very highest latitudes, all the way down to the southernmost tip of Argentina. Then they somehow became the only mammal to cross the Central American Seaway.”

There’s just one more final twist in the tale. Remember at the start of this article, when we told you about primates originating in Asia and making their way to Africa? Well, the thing is, they may have also relied upon an ocean crossing to do that, across the Tethys Sea about 45 million years ago.

That means that the dawn of humanity itself may very well be dependent on a fluke of nature, when our distant primate ancestors were ripped from their homeland and thrown across the ocean. A monkey raft could be the reason you are sitting here right now, reading this.

“If that’s true, then all monkeys and apes are the result of that earlier ocean crossing, and that includes humans,” said de Queiroz. “Our existence was directly dependent on one of these rafting voyages. All humans are dependent on that event.”