The marsupials (in the subgroup Metatheria) are a sister group to the placental mammals and like them, they have nipples and a womb for infant nursing and development. However the similarity ends there. They produce their young prematurely, meaning their gestation period is short. The offspring develop further by climbing to a teat (usually in a pouch) and nurse for a longer period of time.
The oldest marsupials date back to the Cretaceous period from mammals such as Sinodelphys. This group has fossil and living representatives from almost all continents, even Antarctica.