Marsupials are a branch of mammals that have pouches in which they carry and nurse their young. This family consists of creatures like the kangaroo, wombats, possums, bandicoots, and koalas. These creatures are different from their placental mammal cousins in the way their young are born. Marsupials give birth in a very short time, and the young completes its development inside the mother's pouch. There are over 300 species of marsupial. About 200 are native to Australia and the neighboring Asian islands. There are also about 100 extant American species; these are centered mostly in South America. Central America has 13 species, and North America has only one, the Virginia Opossum. Why is it that Australia is home to over half of the marsupial species? The answer lies in Earth's geological past.
It was once believed that marsupials were originally the predecessors for many of earth's mammals, however, fossil evidence contradicts this idea. There were discoveries showing that both marsupials and common placental mammals were branches developed around 250 million years ago, in the Mesozoic era. The earliest known marsupial, the sinodelpys, was found in what is now China, about 125 million years ago, placing it in the same time and area as some of the earliest placental mammals. This discovery suggests that marsupials passed through Southeast Asia on their migration to Australia. However, the fossil record has some minor contradictions: There are marsupial fossils in some of the nearby islands that are younger then those found in Australia, an odd complication in this theory.
Another theory suggests that marsupials evolved in the Americas, dispersing to Asia and Africa by passing through Europe. This theory is fairly well supported by fossil record, showing a very large marsupial population during the Mesozoic era in the Americas. During this time, Marsupials thrived in North and South America as well as Australia. However, about eight million years ago, Placental mammals began arriving in bigger numbers in North America via Europe. The apparent competition resulted in the marsupials losing land and resources, eventually going extinct, except for the opossum ancestor. Eventually, the land bridge between North America and South America was joined, allowing the placental mammals to migrate into territory once taken by the marsupials. Marsupials were no longer the dominant species in South America by two million years ago.
Most major theories can agree on one thing: Placental mammals were much more successful on most continents than their marsupial counterparts, out-surviving them across the board. Placental mammals had lower infant mortality rates due to the longer gestation of their young. However, marsupials were more successful in Australia and other hot climates due to their lower metabolic rates. Thus, marsupials flourished in these areas, and went extinct in others. While more research must be done on the subject to come up with something conclusive, most theories hold valid points, and few contradictions. It is up to you to decide which one is most convincing.