Fossil evidence does not support the once-common belief that marsupials were a primitive forerunner of the placental mammals: both main branches of the mammal tree appear to have evolved at around the same time, toward the end of the Mesozoic era, and have been competitors since that time. In most continents, placentals were much more successful and no marsupials survived; in South America the opossums retained a strong presence; in Australia's harsh climate the placentals died out and only marsupials survived.
The early birth of marsupials removes the developing young much sooner than in placental mammals, and marsupials have not needed to develop a complex placenta to protect the young from its mother's immune system. Early birth places the tiny new-born marsupial at greater risk, but significantly reduces the risk of pregnancy, as there is no need to carry a large foetus to full-term in bad seasons.
Because a newborn marsupial must climb up to its mother's nipples, the otherwise minimally developed newborn has front limbs that are much better developed than the rest of its body. This requirement is responsible for the more limited range of locomotory adaptations in marsupials than placentals; marsupials must retain a grasping forepaw and cannot develop it into a hoof[?], wing, or flipper as some groups of placental mammals have done.
There are between 260 and 280 species of marsupials, almost 200 of them native to Australia and nearby islands to the north. There are also many extant species in South America and one species, the Virginia Opossum, native to North America.
The are two primary divisions of Marsupialia: the Ameridelphia[?], the American marsupials; and the Australidelphia[?], the Australian marsupials. Order Micorbiotheria (which has only one species, the Monito del Monte) is found in South America but is believed to be more closely related to the Australidelphia.