Then at some stage an ancestral species migrated to Australia and gave rise to the various families found there now.
When exactly this happened is still unknown, as this kind of analysis does not show when in evolutionary time the retroposons were inserted.
"Maybe it's around 30-40 million years ago, but we cannot say because jumping genes do not give this information," Dr Schmitz told BBC News.
"It's now up to other people, maybe from the palaeontology field, to find out when exactly it happened."
The overall marsupial history is virtually a circular migration.
The earliest identified species (Sinodelphys szalayi) is known from 125-million-year-old fossils found in China.
Subsequently the family - or perhaps a single species - moved across the super-continent of Gondwana into what is now South America.
The marsupial family began expanding about 70-80 million years ago.
After crossing into Australia, they penetrated north into the Indonesian archipelago - almost returning to their Chinese homeland.