Madagascar mammals traced to African ancestor
Date: 14-Feb-03
Country: UK
Author: Patricia Reaney
Because there are few fossil records of Madagascar's land mammals, determining how, when and from where the creatures, which are found nowhere else in the world, came from has been one of the great unsolved mysteries of natural history.
One theory was that the carnivora, an evolutionary order of mammals that includes dogs, cats, bears and pandas, were already on Madagascar when it separated from the African continent 165 millions years ago.
Another hypothesis suggested the mammals travelled over a land bridge from Africa about 45 million to 26 million years ago.
But new DNA research by scientists from Yale University in Connecticut and The Field Museum in Illinois suggests neither theory is correct and that the so-called sweepstakes model of dispersal is the most probable.
"That certainly seems to be the best explanation of the data," Anne Yoder, an associate professor of evolutionary biology at Yale, told Reuters.
According to the research, carnivoran mammals are not old enough to have been present on Madagascar before the split with Africa and if there was a land bridge other species probably would have also crossed it.
"That leaves us with this alternative hypothesis of being swept out to sea and managing to survive this rather extraordinary voyage," Yoder added.
John Flynn, of the Field Museum, said all the species of Madagascar's canivoran mammals represent a unique evolutionary branch formed by a significant one-time event.
"In fact, all 100 or so known species of terrestrial mammals native to Madagascar, which fall in four orders - carnivorans, lemurs, tenrecs and rodents - can now be explained by only four colonisation events," he said.
The researchers have not calculated when tenrecs, hedgehog-like animals that eat mainly insects, and rodents arrived on the island located 250 miles (402 km) from Africa.
But the research reported in the science journal Nature indicates lemurs crossed the ocean and arrived on the island between 62 million - 66 million years ago.
Madagascar is considered a treasure trove of information for evolutionary biologists because it has been completely isolated from other land masses for 88 million years.
The scientists used DNA sequences of four genes from 20 different mammal species from Madagascar, Africa and Asia to determine which are most closely related and fossil data from around the world to estimate when they diverged.
Their findings suggest the animals, which couldn't swim the distance, hitched a ride on floating vegetation and went into a tupor, a mechanism to survive for periods without food or water.
"The kinds of mammals that can make it across those open ocean crossings might have been species that had that adaptation," Flynn explained.
