From fossil evidence, it appears that the marsupials began to follow a different course of evolution from other mammals (the placental mammals) some 100 million years ago.

　　The earliest remains of marsupial animals have been unearthed in North America, but as the placental mammals developed, the marsupials died out in that continent. In South America they survived - facing only limited competition from the placental mammals - and still exist as the opossums of today. However, it is in Australia that the group has truly flourished.

　　Australian fossils found during the 1950s and 1960s show that about a hundred million years ago there were several species of monotremes on the continent, but no marsupials. The first sign of marsupials in Australia comes from a site in south Queensland, where many marsupial fossils dating back just over 50 million years have been discovered. It is thought that marsupials evolved originally in the north hemisphere and then spread to Australia via South America.