Marsupials Not From Down Under After All

A pair of Kangaroos cavort. Marsupials like these, once thought to originate in Australia, may actually have come from South America. CREDIT: PLoS

All living marsupials - such as wallabies, kangaroos and opossums - all originated in South America, a new genetic study suggests.

Yep – the animals most famous for populating Australia actually started out on another continent altogether. But marsupials – a group of mammals known for toting their young in belly pouches on the females – are still common in South America, too.

The recent study used new genetic data about some of these species to trace the family tree.

"The two recently sequenced marsupial genomes of the South American opossum (Monodelphis domestica) and a kangaroo, the Australian tammar wallaby (Macropus eugenii), provide a unique opportunity to apply a completely new approach to resolve marsupial relationships," the researchers, led by Maria A. Nilsson of Germany's University of Munster, wrote in a paper published July 27 in the journal PLoS Biology.

The South American opossum (commonly called simply "possum") looks like a large, furry mouse. Meanwhile, the Australian tammar wallaby is a small member of the kangaroo family that hops around on two legs.

The scientists analyzed genes from these species for special genetic markers called retroposons that can reveal how much the two genomes share in common. They found that these animals – and all living members of the marsupial family – must have originated from one branch of mammals, because they all share special retroposon patterns that no other mammals have.

The results suggests marsupials started out from a common ancestor in South America, and one major branching-off took place long ago when South America, Antarctica and Australia were all connected to each other as part of a large landmass called Gondwana. This fork would have allowed the animals to populate Australia.

This finding goes against previous ideas that marsupials originated in Australia. Under this scenario, some groups of marsupials would have split off when the landmasses of South America, Antarctica and Australia split around 80 million years ago. The situation is complicated by a lack of strong fossil evidence of this group from ancient times.

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