Evolutionary biologists agree, based on evidence from molecular DNA, that the island's major fauna made their way from continental Africa in four stages.

A first wave of lemur-like animals appeared between 60 and 50 million years ago, followed by tenrecs some 42 to 25 million years ago, carnivores just after that, and then rodents.

Tenrecs are insect-eating relatives of hedgehogs, shrews and opossums.

But just how these fauna -- many of them today threatened with extinction -- got there has been sharply debated.

The so-called "sweepstakes hypothesis," first laid out some 70 years ago, speculated that the forebear of today's Madagascar mammals came via driftwood "rafts" across the 430-kilometre- (267-mile-) wide Mozambique Channel.

The theory is consistent with a relatively small number of mammalian families, suggesting rare opportunities for migration.

All of the species that landed during the 40-million year period in question were also semi-aquatic. In other words, being able to swim at least a little bit might have been a required skill for the perilous voyage.

But there has always been a problem with this notion: the currents swirling in the channel and the surrounding Indian Ocean would make it virtually impossible for a floating Noah's Ark of vegetation to reach Madagascar's shores.

A competing theory of land bridges argues that portions of the Davie Ridge running on the sea floor along the channel separating Madagascar and Africa may have been partially exposed.

This would have creating a causeway linking the two bodies of land.

But the bridge hypothesis, in turn, raises questions.

If animals could simply walk to Madagascar, why did so few species do so?

In addition, given the plate tectonics of the region, it seems unlikely that if some peaks broke the waterline they could have created a contiguous pathway to the island.

In the new study, Matthew Huber of Purdue University in West Lafayette, Indiana, and Jason Ali of the University of Hong Kong say land bridges were improbable and argue they have an explanation which revives the raft theory.

What if, they speculated, the ocean currents in the area were radically different tens of millions of years ago?

After all, over the last 60 million years, Australia and India have respectively shifted 2,200 kilometers (1,370 miles) and 4,000 kilometers (2,500 miles) northward, and six major ocean gateways have opened or closed, changing the pattern of ocean currents.

Critically, Africa and Madagascar have both moved north some 15 degrees, or around 1,600 kilometers (1,250) miles over the same period.

The researchers ran computer simulations based on what is known about the position of continents at the time.

"In all our simulations, the large-scale ocean current systems in the Eocene epoch [55 to 34 million years ago] were robustly different from modern observed and modelled circulations," they report in the British journal Nature.

Their findings point to an occasional eastward flow from what is today Mozambique and Tanzania.

Tropical storms generating large, floating "tree islands" could also contribute to making an ocean passage possible, the study said.

"Successful rafting may have involved the fortuitous coincidence of transient storms and ocean current activity," the researchers conclude.

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