http://www.afrol.com/features/35163
As the evolution of species through natural selection had been scientifically accepted, Darwin's theories seemed to fit perfectly to the large island of Madagascar. The island's isolated fauna seemed to have frozen a moment of evolution when it drifted away from Africa, back to the time when lemurs had yet to evolve into monkeys and apes. Even modern encyclopaedias refer to this age-old theory, stating that "the resulting isolation left Madagascar's plants and animals to evolve independently" (encyclopedia.com).
Indeed, Madagascar has more unique species of animals than any location except Australia, which is 13 times larger. The island's population includes 70 kinds of lemurs found nowhere else and about 90 percent of the other mammals, amphibians and reptiles are unique to its 587,000 square kilometres.
The original theories about the origin of Madagascar's unique fauna stem from an age when the processes of plate tectonics were not well known. Critically, scientists of those times were unable to date geological processes.
Dating processes of fossils and sediments however improved. As it became more and more probable that Madagascar drifted away from Mozambique before the lemurs had evolved in Africa, the theory had to be altered. Scientists now held that the animals arrived on Madagascar via a land bridge that was later obliterated by shifting continents.
Yet, the land bridge hypothesis also is problematic in that there is no geologic evidence that such a bridge existed during the time in question. Also, there are no large mammals such as apes, giraffes, lions or elephants, indigenous to Madagascar. Only small species such as lemurs - the island's signature species - hedgehog-like tenrecs, rodents, mongoose-like carnivores and similar animals populate the island.
But already a century ago, some scientists started doubting the prevailing split-and-isolation theory. In 1915, the first alternative theory was launched, saying many of the animals found in Madagascar could have rafted to the island.
Rafting would have involved animals being washed out to sea during storms, either on trees or large vegetation mats, and floating to the mini-continent, perhaps while in a state of seasonal torpor or hibernation.
As improved dating made the land bridge theory even less plausible, the very influential palaeontologist and evolution theorist George Gaylord Simpson in 1940 launched a more detailed rafting theory. Mr Simpson introduced the concept of a "sweepstakes" process to explain the chance of raft colonisation events taking place through vast stretches of geological time. Once the migrants arrived on the island, their descendants evolved into the distinctive and sometimes bizarre forms seen today.
Though influential, Mr Simpson failed to convince all scientists - and laymen at large. His theory had a major flaw. The prevailing currents and winds in Mozambique Channel - the straight splitting Madagascar from the continent - flow and blow south and southwest, away from, not toward, the island.
Since this controversy, no generally accepted theory could explain how the lemurs, flying foxes and narrow-striped mongooses got to the large, isolated island of Madagascar sometime after 65 million years ago. The split-and-isolation theory
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Different species of endemic mouse lemurs (Microcebus) occurring in the western half of Madagascar. Microcebus are the world's smallest living primates and some species are limited to small geographic areas. |
| © Peggy Macnamara/The Field Museum |
Professor Jason Ali of the University Hong Kong, who has a research focus in plate tectonics - the large-scale motions of the Earth's outer shell - a few years ago caught interest in this unsolved major evolutionary mystery.
Mr Ali kept running across the land bridge hypothesis in the course of his work. The question intrigued him because the notion of a bridge between Madagascar and Africa appeared to break rules of plate tectonic theory. A background in oceanography also made him think ocean currents between Africa and Madagascar might have changed over time.
"Critically, Madagascar and Africa have together drifted more than 1,600 kilometres northwards and could thus have disrupted a major surface water current running across the tropical Indian Ocean, and hence modified flow around eastern Africa and Madagascar," says Mr Ali, an earth sciences professor. Maybe, he thought, Mr Simpson's rafting theory from 1940 still could prove right.
That led the Hong Kong professor to contact Professor Matthew Huber, a palaeoclimatologist who reconstructs and models the climate millions of years in the past, at the US Purdue University. Mr Huber has a particular interest and expertise in ocean currents and had recently developed a very potent programme modelling ancient ocean currents and climates.
The Purdue professor was able to show that 20 million to 60 million years ago, when scientists have determined ancestors of present-day animals likely arrived on Madagascar, currents flowed east, toward the island. Climate modelling showed that currents were strong enough - like a liquid jet stream in peak periods - to get the animals to the island without dying of thirst. The trip appears to have been well within the realm of possibility for small animals whose naturally low metabolic rates may have been even lower if they were in torpor or hibernating.
Mr Huber's computer modelling also indicates that the area was a hotspot at the time, just as it is today, for powerful tropical cyclones capable of regularly washing trees and tree islands into the ocean.
"It seems likely that rafting was a distinct possibility," their study concludes. "All signs point to the Simpson sweepstakes model as being correct: Ocean currents could have transported rafts of animals to Madagascar from Africa during the Eocene."
The new theory, published in the renowned journal 'Science' on 4 February 2010, has already found some support. "The raft hypothesis has always been the most plausible," says Anne Yoder, director of the Duke University Lemur Center. She specialises in the evolutionary history of Madagascar. "But Ali and Huber's study now puts hard data behind it," she adds.
The theory would also solve a problem related to new knowledge about Madagascar's evolutionary history. The island's animals appear to have arrived in occasional bursts of immigration by species rather than in a continuous, mixed migration. They likewise appear to have evolved from single ancestors, and their closest relatives are in Africa, scientists say. All of which suggests Mr Simpson's old theory was correct.
While the first reception in the scientific community seems to be positive, it remains to be seen whether the currently applied computer models of the region's palaeoclimate and ancient currents will have to be corrected. But surely, the popular split-and-isolation theory prevailing in most textbooks, in encyclopaedias and in popular belief, will not disappear for years to come.
Also surely, Madagascar's fascinating and poorly studied biodiversity will continue to pose challenges and present surprises for researchers and laymen in the future.
Only in 2006, three new species of lemurs were identified on the Great Island. Globally, new mammals are very nowadays very seldom identified, again showing how poorly Madagascar's amazing ecology has been studied.
Also in 2006, the first-ever comprehensive theory explaining the island's exceptionally rich biodiversity was presented. This is the only thorough study into Madagascar's evolutionary history and regional speciation. Also the island's geology and palaeoclimate is understudied.
Major discoveries are still to be expected on the Great Island.
By afrol News staff and Greg Kline
© afrol News
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