Monkeys suddenly appeared in South America about 40 million years ago. Unlikely though it may seem, they probably sailed there from Africa

"In 1492 Columbus sailed the ocean blue." That much everybody knows for sure.

Earlier transatlantic travellers may have beaten him to it, of course: the Vikings almost certainly made the crossing, and there are claims that the Egyptians and all manner of other groups did too. But if most of these pre-Columbian ocean voyages sound outlandish and unlikely, they are nothing compared to a transatlantic journey that appears to have taken place about 40 million years ago.

Midway through the Eocene, a crew of monkeys sailed the ocean, er, green.

Like later primates including Christopher Columbus and Leif Ericson, this intrepid band set off in search of glory and riches on the other side of the ocean. Well, sort of.

The evolutionary history of primates has received plenty of scientific attention over the years. This is unsurprising: their history is our history, and in the course of investigating humanity's roots, researchers have revealed a lot about our distant ancestors too.

We know, for example, that primates probably have their origins in Asia, and thanks to the latest sophisticated studies we also have fairly accurate estimates for when different groups and species appeared.

One thing that has consistently baffled researchers, however, is how primates arrived in South America.

Geological rumblings in the 1950s and 1960s seemed to provide an explanation. This was when ideas of continental drift and plate tectonics were refined. The phenomenon soon became a catch-all explanation for many of the Earth's more incongruous species distributions.

In the case of the monkey puzzle the reasoning was simple. In the distant past there was no Atlantic Ocean – Africa and South America formed part of a much larger landmass called Gondwana. So the primitive precursor species of Old World and New World monkeys could literally have walked – or swung – to what is now South America's east coast.

But molecular clock estimates now date the last common ancestor for New and Old World monkeys to a time about 100 million years after the continents had split apart. So that idea has gone out the window.

Scientists considered alternative theories. Perhaps the monkeys crossed from places other than Africa – via North America, for example, or even through Antarctica. But there are no fossils to support these ideas, despite the primate fossil record being one of the most complete of all major mammalian groups. These ideas do not really stand up to scrutiny.

Unlikely though it sounds, the monkeys simply have to have crossed the Atlantic. Last year, new evidence emerged that reignited the debate and pushed this transatlantic crossing theory to the forefront.

A chain of volcanic islands might have roughly linked Africa and South America

A research team led by Mariano Bond of the National University of La Plata in Buenos Aires, Argentina, unearthed a handful of surprisingly familiar monkey teeth while digging in the Peruvian Amazon.

"One of the teeth is very, very similar to a fossil tooth from Africa," enthused team member Ken Campbell, a curator at the Natural History Museum of Los Angeles.

A picture emerged of a small, marmoset-like creature that the team christened Perupithecus ucayaliensis. It bore a striking resemblance to Talahpithecus, a genus of monkey that lived in northern Africa in the Eocene.

Perupithecus dates to the very end of the Eocene, about 36 million years ago, and is therefore the most primitive New World monkey ever found. More importantly, it provides for the first time a direct link between the ancestors of today's New World monkeys and the ancient monkeys of Africa.

And with Africa established as a kind of universal monkey homeland from where New World monkeys spread somewhere between 40 and 44 million years ago, we are left with an enormous, watery elephant in the room: the Atlantic Ocean.

The Atlantic Ocean gets very slightly larger each year. In the Eocene it was certainly smaller than it is today. But it was still pretty big – at least 1,400km wide. How did these primitive monkeys cross this seemingly insurmountable distance?

There are basically two possible explanations.

The Eocene Atlantic was a veritable thoroughfare

One is island hopping. Throughout the Earth's history ocean levels have risen and fallen. Land has emerged and then been consumed by the waves once more. In theory, when sea levels were relatively low, a chain of volcanic islands might have roughly linked Africa and South America, making an oceanic crossing considerably easier.

However, even if there was once a handy chain of volcanic islands, our pioneering primates still needed a mode of transport to get from island to island. And this is where the monkey sailors come in.

While it may seem pretty far out, the second key idea is oceanic dispersal via rafting, a concept with a surprising amount of biological precedent. First suggested by one of the forefathers of evolutionary biology, Alfred Russell Wallace, rafting has been used to explain everything from garter snakes in Baja California to the mammalian fauna of Madagascar.

Before the theory of plate tectonics and continental drift came along, rafting was the go-to explanation for any unusual geographical species distributions.

Given that plate tectonics cannot explain how monkeys reached South America, rafting has to have played a part. In fact, it has been suggested that rafting events are also responsible for seeding South America with the ancestors of its rodents and hoatzin birds. Clearly, the Eocene Atlantic was a veritable thoroughfare for nautical creatures.

This sort of adventitious migration is dragged in when necessary to explain away any facts that contradict the main thesis

So what would rafting mean in practice? Certainly the tiny creatures found by Bond and his team in Peru would not have been capable of fashioning a raft. Garter snakes would have found the task even more of a challenge. In fact the proposed "raft" is something more akin to a sizeable floating island, similar to the one depicted in the children's TV show, Noah's Island.

If this is starting to sound silly to you, then you are in good company. In his comprehensive analysis of the topic, Alain Houle of the University of Montreal admonishes his predecessors for consistently using rafting as a fix-all solution without considering its practicalities.

Even the hugely influential palaeontologist George Gaylord Simpson, who championed the idea of rafting in the 1940s, acknowledged that "it has been claimed [that] this sort of adventitious migration is dragged in when necessary to explain away any facts that contradict the main thesis".

With this in mind, Houle attempted to quantify the actual likelihood first of such of an island ever forming, and second of it successfully ferrying a healthy population of mammals half way across the world.

The first step is to establish what these so-called "floating islands" would look like. The image that comes up most frequently in the literature is a section of land, or at the very least a large mass of vegetation, being dragged out to sea during violent storms at river mouths.

Such events have been documented, albeit rarely. Tantalisingly, there have even been reports of vegetation mats carried by the South Equatorial Current between the Niger and Congo Rivers of Africa and the coast of Brazil – exactly the kind of occurrence required for our monkey odyssey.

40 million years ago, the Atlantic Ocean could have been crossed on a raft in 14.7 days

If these hypothetical rafts had vertically standing trees on them – as historical rafts reportedly have had – Russell Ciochon, now at the University of Iowa, and Brunetto Chiarelli from the University of Florence suggest that it would even have been possible for them to act as sails.

Research into the ancient flow of currents (examining geological features such as sedimentary structures) indicates that strong currents in a westerly direction from West Africa did indeed exist in the late Eocene, as they do today. Nevertheless, crude estimates made by palaeontologist Elwyn L Simons indicated that relying on currents alone, the transatlantic journey would take a minimum of 60 days – probably longer than even the toughest of monkeys could last.

This is why, according to Houle, the sail idea is in fact crucial to the whole theory. In his analysis, he factors in the considerable effects of winds on these hypothetical sails, based on modern Atlantic wind velocities. He estimates that, 40 million years ago, the Atlantic Ocean could have been crossed on a raft in 14.7 days.

It is worth mentioning that Houle, along with many other scholars who have considered the idea in detail, is generally in favour of one long crossing as opposed to multiple smaller ones via long-gone volcanic island chains. Consider the likelihood of a raft forming just once and making successful contact with land, he reasons, and then consider the likelihood of that happening multiple times with the same population of creatures. It is, perhaps, stretching the imagination to breaking point.

Some mammals are actually quite good at being dehydrated

The next conundrum is the wellbeing of the passengers aboard this floating island. If these hypothetical monkeys were in it for the long haul, it is important to consider whether they possessed the physiological characteristics necessary for survival.

The 14.7 days given by Houle makes a crossing far more feasible than earlier estimates, but the monkeys would still have to contend with dehydration, starvation and sun exposure.

For obvious reasons, it is impossible to say for sure how they would have responded to their new nautical lifestyle, and it is ethically dubious to find out by populating rafts with monkeys and casting them adrift. But we can infer their survival chances based on a more general understanding of mammalian physiology.

Campbell, one of the members of the Peruvian Amazon tooth-finding team, and an ardent advocate of the raft hypothesis, points out that these animals were small – around the size of squirrels – and would have had correspondingly limited requirements for both food and water. That being said, comparative studies of relative resilience to water deprivation have indicated that smaller mammals tend to be far less capable of dealing with dehydration.

But some mammals are actually quite good at being dehydrated; those hailing from arid regions perform pretty well when deprived of both food and water. If these monkeys were of hardy West African stock, then there is every chance that they were well adapted to survival in harsh and unpredictable environments.

A comparative study undertaken by Arturo Cortes and a team from the University of Chile demonstrated that degus – rodents about the size of small monkeys that inhabit semi-arid regions of Chile – can survive for nearly two weeks without water.

South America's rainforests are filled with the screams and hoots of everything from tiny tamarins to raucous howler monkeys

Concrete evidence is hard to come by for such an unusual occurrence. But given the feasibility of both a floating island's formation and its capacity to carry a healthy(ish) population of monkeys, it can at least be said to work in theory.

Oceanic rafting has received its share of criticism over the years, but the more its effects can be properly quantified, the more it is turning from a convenient go-to explanation for bizarre animal distributions into a well-tested and legitimate hypothesis. The "monkey sailor" idea, while bizarre, is no longer as nonsensical as it first appears.

In fact, author of The Monkey's Voyage Alan de Quieroz describes what he sees as a "counterrevolution" against the simple explanations of animal distributions offered by continental drift. He suggests a more complex model in which evolutionary biologists embrace the seemingly wacky ideas underpinning oceanic dispersal as key components in our understanding of life's rich tapestry.

Today, South America's rainforests are filled with the screams and hoots of everything from tiny tamarins to raucous howler monkeys, hiding in holes and swinging though the trees.

It is incredible to think that these diverse, charismatic animals can be traced back to a few soggy pioneers, stepping groggily off their accidental vessel into a new world all those millions of years ago.

They may not be as well-known as Columbus and his fellow hairless primates, but these seafaring monkeys deserve their own place in history.