Lemurs rafted to Madagascar millions of years ago, a new model suggests. Credit: Luci Betti Nash

Castaways are probably behind the island of Madagascar's incredible biodiversity. A new model provides strong evidence that lemurs and other small mammals first arrived millions of years ago, travelling on African logs that had washed out to sea.

The model, published today in Nature1, settles a long-running debate on how Madagascar's biodiversity came about. It may also provide clues about how prehistoric animals spread to other parts of the globe.

Madagascar is considered to be one of the world's most diverse — and imperiled — natural habitats. Its forests are home to more than 150 species of mammal alone — including ring-tailed lemurs, otter-like web-footed tenrecs, and giant jumping rats. But all of these mammals belong to just four orders.

Just why there are so many species and so few orders has been a mystery. Some researchers have proposed that there may have been a land bridge between Madagascar and the African mainland around 20-60 million years ago. But the geology of the region shows little evidence of such a link. Moreover, a land bridge would have allowed more orders of animal to reach the island.

An alternative hypothesis is that animals 'rafted' to the island. First mooted some 70 years ago by the American palaeontologist George Gaylord Simpson, the idea is that small mammals were inadvertently washed out to sea by storms and floated to Madagascar's shores. The survivors evolved over millions of years to fill numerous niches in the island's ecosystem.

A washed-up theory

The idea that animals could traverse hundreds of kilometres of open ocean by clinging to a log "sounds kind of dubious", admits Matthew Huber, a palaeoclimatologist at Purdue University in West Lafayette, Indiana, and an author on the Nature paper. "The key thing to remember is that it doesn't have to happen often." A few lucky lemurs over millions of years would have been all that was needed, he says.

But for that to happen even once in a million years would be impossible under today's conditions: the ocean currents that flow between Madagascar and the mainland flow away from the island, not towards it.

Palaeontologists were stumped by the troublesome current until Huber's co-author Jason Ali, a geologist at the University of Hong Kong in China, asked Huber to look at the problem. In less than a day, Huber had cracked the puzzle. Roughly 60 million years ago, both Madagascar and the African mainland were about 1,650 kilometres south of their current positions, and that, according to Huber's model, put them in a different ocean gyre. The gyre reversed the current, propelling castaway animals towards Madagascar. Not only were currents flowing in the right direction, they were much stronger than they are today, reducing the travel time of stranded animals to 30 days or perhaps less, especially if there were a strong tropical cyclone in the region.

"It's pretty straightforward actually," says Ian Tattersall, a palaeontologist at the American Museum of Natural History in New York City. Many biologists already favoured the rafting theory because it could explain present-day biodiversity, he says. The new analysis solves the last outstanding puzzle.

"For me, the debate is settled," agrees Anne Yoder, an evolutionary anthropologist at Duke University in Durham, North Carolina. Yoder believes that many other types of animal and plant also arrived on the island through rafting.

Given the simplicity of the solution to the problem, it might seem amazing that nobody has solved it before. But palaeontologists and ecologists aren't equipped to model ancient ocean currents, Tattersall says. "It really requires expertise different from those of people who normally agonize over this."

Huber says that now he's helped biologists, they may be able to help him. By studying the fossil record and biodiversity today, he says, he might learn more about how ancient ocean currents once flowed.