Marsupials are unique among mammals because their young are born at an extremely early stage of development, attach to their mother's teats and complete their subsequent development while in a protective pouch.
Marsupials and the ancestors of placental mammals diverged 180 million years ago. By comparing the opossum and human genomes, researchers were able to pinpoint genetic elements that are present in placental mammals, but missing from marsupials - that is, the genetic factors that may underlie many of the differences between the two types of mammals.
Scienists are still searching for the koala "cuteness" gene.