Thursday, February 11, 2010

Lemurs on a raft!

“Any event that is not absolutely impossible ... becomes probable if enough time elapses”.

-George Gaylord Simpson

Madagascar is one of the world’s richest biological hotspots. It is home to 5% of the world’s flora and fauna, 80% of which are endemic, or only found on the island nation. One question that has been raised over and over is how did all these creatures get to Madagascar in the first place? This is especially true for Malagasy mammals. Excluding those introduced by humans, all native terrestrial mammals are found nowhere else in the world.

A few theories have been cast about attempting to explain this natural mystery. Could the mammals all have been present on the Gondwanan supercontinent and then separated over geological time? Perhaps they crossed on a land bridge left by shallow seas. While all possible, there is currently a lack of geological evidence to support these theories. If these scenarios were true, then we would expect to see more “typical” African mammals on the island – which is the exact opposite of what is found. The fossil record too for example does not support either of the above models - it only dates back a few thousand years for mammals on Madagascar. Meaning, we have no trace of ancestors from modern populations. Most surprisingly, the relatively recent appearance of mammals in the fossil records means they must have arrived long after the opening of the Mozambique Channel, a 430km physical barrier separating Madagascar from Africa.

While the fossil record is scarce for mammals, molecular dating from mammalian population on Madagascar shines a more complete light on their ancestry. Molecular systematics and divergence ages puts the founding populations for different groups arriving independently from Africa between 60-20 million years ago. Still long after Madagascar became an island.

A rather fanciful idea that has always floated around was that animals could have rafted from the continent to the island. Hell, even the Creation Museum uses that idea to justify how animals colonized the world after The Flood. But in the case of Madagascar, it may actually be true. In Nature this month, new computer models of Eocene ocean currents (and most likely throughout the Oligocene and Miocene) lends support for a rafting scenario. During this period which corresponds with the molecular dating of founding populations, not only were currents especially strong in the southwest Indian Ocean and Mozambique Channel, they were also much different directionally than their modern counterparts. Flowing from northwest to southeast and at a ripping 20cm/s at times, matted vegetation could have been transported from Africa to Madagascar in as little as 25-30days. And as the currents changed through time to their modern patterns, further colonization would have been cut off. This makes the superficially silly notion that animals surfed their way between islands, currently our best explanation. Don’t you love how science works in weird, cool ways like that? This work is however very preliminary, with the authors of the paper underscoring that a further 1-3 years of coupled models running on a supercomputer will be necessary for a more complete simulation. Nevertheless, this may lead to a more robust theory of how terrestrial organisms came to populate land separated by huge marine barriers independent of typical biogeographical models.

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Ali, J. R., and M. Huber. Mammalian biodiversity on Madagascar controlled by ocean currents. Nature 463:653-656.

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