The world’s marsupials – including Australia’s iconic kangaroos, wallabies, Tasmanian devils – all originated in what was once South America.
So says Maria Nilsson at the University of Munster in Germany in a paper published this week in PLoS Biology.
By comparing unique genes called retroposons in the genome of 22 marsupials, Nilsson concluded that ancient South American marsupials migrated to Australia more than 80 million years ago – when the continents were part of a supercontinent, called Gondwana.
Not surprisingly, some Australians aren’t impressed.
More seriously, Mike Archer, an evolutionary biologist of the University of New South Wales, Australia takes issue with the findings. He told the Australian Broadcasting Corporation Online “They just assume Australia is the arse-end of the world and gets everyone else’s leftovers.” So where did Skippy come from?
Skippys with sombreros
Marsupials, mammals that carry their young in front pouch, are Australia’s pride and joy, but there are seven existing Marsupial orders, three from the Americas and four from Australia.
According Matt Phillips an evolutionary biologist at the Australian National University in Canberra, who was not involved in the work,“there’s not much debate” about the fact that marsupials originated from South America.
The real debate, he says, is whether some marsupials once in Australia crossed Gondwana once more, returning to South America.
It’s been difficult to decipher early migration patterns of marsupials because ancient fossil records for marsupials are limited. So, Nilsson and her team, tackled the issue by analysing retroposons. These “jumping genes” copy and paste themselves from one location to another within an animal’s genome.
Since the insertion sites of retroposons are effectively random, finding them in similar places in different species suggests a common ancestor.
To begin their search for marsupial retroposons the German team analysed the genome sequences of the South American opossum (Monodelphis domestica) and the Australian tammar wallaby (Macropus eugenii).
Los Angeles Times reports
“The scientists found 53 similar retroposons in the opossum and wallaby, verifying their common ancestry.
The team then compared the wallaby and opossum data to the DNA of 20 other marsupial species, including the wallaroo, the common wombat, and the marsupial mole, to find out which marsupial lineages are more closely related and which split off first.
They found that all of the species had common retroposons, and thus a common ancestor. Closer analysis revealed that the South American opossum order, Didelphimorphia, was the oldest living marsupial order, indicating that all marsupials originated in South America.”
Super two- way highway
But, Archer told the ABC that the study doesn’t give credence to evidence that Gondwanan migration was a “not just a one-way highway, it was a super two-way highway.”
In 2008, Archer and colleagues found fossils demonstrating that ancestors of the little mountain monkey Dromiciops gliroides, now found in the rainforests of Chile and Argentina lived in Australia 55 million years ago.
Phillips says while Nilsson’s paper correctly identifies which marsupial species diverged first, but without fossil evidence, “it’s difficult to interpret locations.”
Photo by cleopold73