LOS ANGELES — The kangaroo, a beloved national symbol of Australia, may in fact be an ancient interloper.

A study published this week in the online journal PLoS Biology suggests that 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 another 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 South America and four from Australia.

Three theories

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 together into 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 until now, it had been hard to verify the correct theory, said Matt Phillips, a biologist from 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.

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.

DNA evidence

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 gene” that pass virtually unchanged from mother to offspring. When two species share retroposons with very similar genetic sequences, it is likely they are derived from the same ancestor. The scientists found 53 similar retroposons in the opossum and wallaby.

The team then compared the wallaby and opossum data to the DNA of 20 other marsupial species, including the wallaroo and the common wombat, to find out which marsupial lineages are more closely related and which split off first.

They found that all of the species shared 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.