By Mark Kinver
Science and environment reporter, BBC News
Lemurs are among Madagascar's unique mammals
The ancestors of the current mammals found on the island of Madagascar could have been transported on floating vegetation from Africa, a study says.
Researchers modelled ancient ocean currents and found that favourable conditions existed in the same period as when mammals arrived on the island.
The idea of "rafting" first emerged in 1940, but some argued that a "land bridge" allowed animals to walk there.
The findings have been published online on the Nature website.
Madagascar, the fourth largest island on the planet, is deemed one of the world's biological hotspots.
Because of its isolation, most of its mammals, half its birds, and many of its plant species exist nowhere else on Earth.
The first mammals are believed to have appeared on the island about 60 million years ago, 100 million years after the landmass was thought to have separated from Africa.
This led to the emergence of two main hypotheses on how mammals managed to inhabit the island: via a "land bridge" or floating vegetation.
Ticket to ride
Using a climate model used by the Intergovernmental Panel on Climate Change (IPCC), co-author Matthew Huber - a palaeoclimate modeller at Purdue University in Indiana, US - adapted it to shed light on the past.
"I had been doing these simulations for some time," he told BBC News.
"The paper's lead author (Dr Jason Ali from the University of Hong Kong) asked me to look at the Madagascar region because he thought that the ocean currents were different during that time.
"I looked, and - sure enough - the ocean currents went in the opposite direction than they do today," he explained.
"The reason is primarily because, in the past, both Madagascar and Africa were 15 degrees further south.
This meant that the 430km (270 mile) Mozambique Channel that separates the two landmasses was located in a different ocean "gyre" (circular ocean current), which had an important impact on the direction and strength of the currents within the channel.
Dr Huber said that the model showed that this provided the right conditions to allow mammals to be transported across the channel.
"What the model suggests is that occasionally - say one month in 100 years - the currents were strong enough to allow a raft, for example a large log, carrying a family of lemurs to make the journey in about three weeks," he explained.
"Biologists and palaeontologists say that rafting is the only sensible way for this [dispersal] to have happened. But the problem has always been the currents."
"When you looked at present ocean currents, the journey is impossible. "So scientists have been stuck because when you are faced with impossibilities, what do you do?"
As a result, a number of scientists favoured the theory that a land bridge existed in the past.
But the theory would have required a "radical rethinking of the region's plate tectonics", Dr Huber explained.
"What we have done is resolved this conundrum by saying that ocean currents were actually different in the past.
"So it was possible - not probable, but possible."
The idea of mammals being transported on "rafts" of vegetation was first mooted back in 1940 by US researcher George Simpson.
He developed the "sweepstakes" hypothesis because the biodiversity on Madagascar was unique, lacking "megafauna" such as elephants, lions and zebras.
If the animals had reached Madagascar via a "land bridge" - meaning the landmass was connected to the African continent - Simpson argued that large mammals would have also made the journey.
He added that the match between the currents and the arrival of new mammals on Madagascar was "pretty good".
It is understood that the common ancestor of present-day lemurs arrived on Madagascar between 60 million and 50 million years ago; tenrecs (such as hedgehogs) appeared 42-25 million years ago, and rodents between 24 million and 20 million years ago.
"About 20 million years ago, the 'flow' of species stopped," Dr Huber observed.
"When I look at my simulations for 20 million years ago, the currents are going the same way as they do today."
He explained that change in the direction of the current in the Mozambique Channel was a result of the slow northward movement of Africa and Madagascar.
This meant that the influence of the southern oceanic gyre was gradually weakened, causing the "sweepstake" route to be closed.
He said: "The 'switch' might have turned off gradually between 50 million and 20 million years ago, but by 20 million years ago, the journey was impossible."