Animals on rafts: How Floating Debris Likely Shaped Animal Evolution

Baby Ring-tailed Lemur
Baby Ring-tailed Lemur

Going into this week, I was prepared to tell you about lemurs, but then I learned that lemurs are not alone in one of the things that make them really interesting. But before I dive into too much detail, imagine this: the year is 65 million years ago, heavy monsoon rains are hitting Eastern Africa and Southern Asia and India (which, of course, weren’t were you would think of them today), and that means trees and other vegetation are being washed out into rivers and eventually into the sea. This is exactly what was happening and, if you were a primate, the circumstances are going to be particularly important in the path of your evolution.

What I’m talking about is the theory, and a very well-supported one, of large rafts made of branches and vegetation carrying animals across the oceans to islands or other continents. This is also called “dispersal” or “Sweepstakes evolution”, because of the great risk and chance of complete failure and enormous benefit in making the trip. Since the early 1900s, this has been an accepted theory to explain the variety of fauna found only on Madagascar, along with the land-bridge theory. However, there has been no proof of such a land-bridge ever existing and there is evidence of sea-faring ancestors of the species that now populate the island.  To be clear, the animals didn’t make the raft or even make a conscious decision to set out.  They were just in the right place at the right time to head out for adventure.

The Island of Madagascar
The coastline of Madagascar

For one thing, scientists who study ocean currents say that, rather than a Southern-flowing ocean current in that part of the ocean today, the water in fact flowed East, towards Madagascar, making it possible for material such as dirt, leaves, and animal-carrying rafts to head towards the island. These scientists also show that the currents were stronger than they are today, getting the rafts to land again much quicker than you might think. They wouldn’t be out on the sea long enough to die of dehydration and, if they were small and/or hibernating, it would have been even easier to survive. The globe was much warmer at this time, though, so the reason for their hibernation would be unknown.

Lemurs, which were much larger at the time, weren’t the only passengers going to Madagascar, though. With them were the ancestors of bats and mongooses (and mongoose relatives such as the fossa). And Madagascar wasn’t the only destination these rafts were heading. Evidence shows that large apes also made the trip from Asia to Africa, because their evolutionary paths show a sudden appearance in Africa and a divergence around 40 million years ago. So, knowing that we share a common ancestor with apes and knowing that the hominids came from Africa, we can conclude that our ancestors were probably some of those rafters who left Asia.

A raft theory has also been described in explaining the unique diversity in the Galapagos Islands. Most of islands’ population are either birds, who would have flown in, or reptiles. The trip would have been much longer than the one to Madagascar, and the rafters would have had to survive what was potentially weeks at sea. Scientists say that reptiles are best adapted to be able to withstand these conditions, and it explains why there are so many reptiles and so few mammals and amphibians on the Galapagos Islands.

Full Grown Capybara
An adult capybara is closely related to the guinea pig.

Some very famous mammals did make a long raft-trip, too. The capybara is the largest rodent in the world, native only to South America, and, believe it or not, a close relative to the Guinea pig. They are called the caviomorphs, and they rafted to South America from Africa around 40 million years ago. Scientists have determined this as the most-likely scenario through studies of their genes rather than a fossil record. Caviomorphs are very unique and interesting in other ways, many of which are so different from rodents in all other parts of the world that scientists think the divergence from African rodent lines may have occurred even longer ago than suspected.

Was there anything you have a question about or something you know to add to the discussion? Comment down below! Thank you for reading and, until next time, goodbye!

2 thoughts on “Animals on rafts: How Floating Debris Likely Shaped Animal Evolution”

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.