Rafting? Really?

It never fails. Whenever I give a talk about South American mammals, like I did the other day for a group of undergraduates, the show-stopper is when I mention how caviomorph rodents and monkeys rafted from Africa to South America tens of millions of years ago.

Mara Dolichotis patagonum 1.jpg
A Patagonian mara (Dolichotis patagonum), a close relative of guinea pigs and capybaras that is a particularly charismatic species of caviomorph rodent. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

 

“Wait… how did they get there? On a raft? Across the Atlantic Ocean? Are you kidding me?”

I go on to explain how the Atlantic Ocean wasn’t as big then as it is now, so the trip would not have been quite so long. I describe how these were natural rafts – basically large chunks of earth, roots, and trees – not pieces of bamboo lashed together in a style reminiscent of the Kon-Tiki. I clarify that rodents and primates probably did not come over on the same raft, even though they may have arrived in South America about the same time.

But the skepticism persists.

“Do scientists really believe that?!”

Yes, we do.

Leontopithecus.jpg
A golden lion tamarin (Leontopithecus rosalia). Tamarins and marmosets are very small monkeys characteristic of the New World tropics. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

 

Even though this scenario sounds unlikely or even impossible, the alternatives are even less likely. Science isn’t about having a good explanation for the facts. It is about having the better explanation for them. Those facts are:

• The oldest rodent fossils in South America are just over 40 million years old. The oldest monkey fossils are probably at least 30 million years old. Thus, rodents and primates got to South America sometime during the Eocene.

• Eocene rodent and primate fossils have also been found in North America, Eurasia, India, and Africa. However, these mostly belong to groups that are only distantly related to South American rodents (caviomorphs) and South American primates (platyrrhines). Close fossil relatives of these South American groups have only been found in Asia and Africa.

• Rodents and primates are the only two groups of terrestrial mammals that suddenly appear in the Eocene fossil record of South America. This argues against the existence of a land connection with another continent, which presumably would have allowed more than two types of terrestrial mammals to reach South America. The only other option is over-water dispersal.

• No caviomorph rodent or monkey fossil has ever been found in Antarctica or Australia.

In addition to information from the fossil record, other types of evidence favor a trans-Atlantic dispersal from Africa:

• A gigantic tectonic feature known as the Mid-Atlantic Ridge has been slowly pushing South America and Africa apart for some 150 million years. Today, the two continents are separated by some 3,000 km of ocean at their closest points. Fifty million years ago, this distance was only 1,000 km – not a small ocean, but a far cry from the modern Atlantic.

2008_age_of_oceans_p1024.jpg
Location of the Mid-Atlantic Ridge (white arrows) on a diagram illustrating the ages of oceanic tectonic plates. “Hot” colors (red through yellow) indicate oceanic crust that has been created in the past 60 million years, most of which would not have been present in the Eocene. Modified from the NOAA.

• Models of winds and ocean currents indicate that crossing from Africa to South America would have taken a mere 5-10 days during the early to middle Eocene, thanks to a smaller ocean and the predominantly east-to-west winds and currents.

• A trans-Atlantic dispersal would not be the only example of rodents and primates crossing a large expanse of water. Primates crossed the Mozambique channel at least once to get to Madagascar from Africa, and both rodents and primates dispersed to various Caribbean islands in a similar fashion, probably from South America.

• Similarly, this would not be the only example of a land-living vertebrate crossing the Atlantic from Africa to South America. A similar scenario has been proposed for “worm lizards” (amphisbaenians), some true lizards, and a group of strange, nearly flightless birds known as hoatzins.

Nevertheless, even if you believe the evidence and arguments presented above, you may still have a tough time believing the proposed mechanism. I think that is mainly due to terminology and probability.

“Rafting” is a nice, shorthand way to refer to how these animals crossed the Atlantic, but it implies some sort of vessel. It also has a connotation of intentionality on the part of the animals. These so-called rafts were likely small floating islands made of large chunks of earth and perhaps entire trees that were ripped from the shores of a large tropical river during the rainy season and eventually washed out to sea. Such floating islands occur today, and storms are one way that small vertebrates such as lizards are known to cross expanses of ocean.

Floating island.jpg
A small collection of tropical vegetation like this is probably not how small mammals were able to cross the Atlantic Ocean. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

 

More importantly, we humans often have a difficult time with probabilities and large numbers. What are the odds that a family of rodents would end up on a floating island and survive the journey to South America 45 million years ago? One in a million? One in a billion? Certainly they are low. It isn’t a coincidence that the famous evolutionary biologist George Gaylord Simpson named such events “sweepstakes routes.” Nevertheless, just as the odds of winning the lottery increase substantially if you can purchase a million tickets, events that are incredibly improbable during your lifetime become much less so when dealing with millions of years. If you consider all the mammals in all of Africa over all the Eocene, the fact that two successfully made the crossing seems about right.

Simpson 1940 Sweepstakes.jpg
George Gaylord Simpson’s fanciful illustration of a sweepstakes route for mammals between Africa and Madagascar. The estimated dates of arrival are no longer accurate, but the analogy is appropriate. From Simpson (1940:fig. 6).

We will never know how many other types of African mammals bought sweepstakes tickets to South America only to be disappointed. Some never found themselves on a floating island, and many others probably had the misfortune of being buried at sea. Monkeys and caviomorph rodents were the only ones with winning tickets, but we humans are the truly lucky ones; we now get to see the incredible diversity of species to which these hopeful castaways gave rise.

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