Marsupials are a group of mammals best known from Australia, but are also present in South America and up to the southern and eastern parts of the USA. Despite their current geographical distribution, metatherians (the group containing marsupials and other marsupial-like mammals) were once much more cosmopolitan; the earliest fossil evidence being from the Cretaceous of China, in the Northern Hemisphere.

The story of marsupial evolution is therefore much more complex than is first apparent: When did metatherians and eutherians (placental mammals) diverge? Why are eutherians much more common? Why are metatherians restricted to the Southern Hemisphere? To answer some of these questions we spoke to Dr. Robin Beck, an expert on marsupial and metatherian phylogenetics, from the University of New South Wales, Sydney, Australia.