Most of the deep branches in the marsupial family tree, in other words, are American, which is one reason why we think the marsupials migrated to Australinea from America, rather than the other way around. But the Australinean branch of the family diversified mightily after their homeland became isolated. The isolation came to an end around 15 million years ago when Australinea (specifically New Guinea) reached close enough to Asia to allow the arrival of bats and (presumably island-hopping) rodents.
