The kangaroo, a beloved national symbol of Australia, may in fact be an ancient interloper.
A study published Tuesday in the online journal PLoS Biology suggests that Australian marsupials — kangaroos, wallabies, Tasmanian devils and more — evolved from a common South American marsupial ancestor millions of years ago. The finding, by researchers at the University of Munster in Germany, indicates that the theory that marsupials originated in Australia is incorrect.
Marsupials are characterized by distinctive frontal pouches in which they carry their young. There are seven existing orders, three from the Americas and four from Australia.
One prominent theory, now validated by the new study, suggested that ancient South American marsupials migrated across Antarctica to Australia more than 80 million years ago when the continents were connected in a supercontinent known as Gondwana. But scientists had also theorized that the first marsupials migrated from South America to Australia and then back again.
A third theory was that marsupials originated in Australia and then traveled to South America.
Up till now, it had been hard to verify any of the theories, said Matt Phillips, a biologist at the Australian National University in Canberra, who was not involved in the study.
"Ancient fossil records for marsupials are very poor, particularly in Australia," Phillips said. "This has made it hard to understand early migration patterns and relationships amongst the species."
Previous studies had tried to tackle the question by comparing small bits of DNA or physical differences between marsupials, such as ankle joint characteristics, Phillips said. The new study, in contrast, examines large chunks of marsupial genomes for evolutionary clues.
The team started by analyzing the genome sequences for the South American opossum and the Australian tammar wallaby. They specifically looked at DNA features called retroposons, types of "jumping genes" that pass virtually unchanged from mother to offspring. When two species share retroposons with very similar genetic sequences it is likely that they are derived from the same ancestor. 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.
"Scientists had always suspected there was a common ancestor between South American and Australasian marsupials but now we finally understand where they may have originated and how they branched off from one another," said study lead author Maria Nilsson, a postdoctoral researcher at the University of Munster.
The study also cleared up years of confusion about where to group a marsupial called the monito del monte (mountain monkey). Although this creature is native to South America, it has more characteristics in common with Australian marsupials, and so scientists had debated its closest relatives for many years, Phillips said.
The DNA comparisons clearly showed that the mountain monkey belongs to the South American group on the marsupial evolutionary tree.