Marsupials are one of three broad groups in which mammals are classified, the other two being placental mammals and egg-laying mammals. The term comes from the Greek word for pouch on account of the fact that certain members of this group have pouches which serve as a protective means of carrying about their young. This group of mammals are also known as didelphia, derived from the Greek from double womb, and this is actually a better description since the characteristic that is common to all members of this group of animals is a double canalled genital track, the pouch being present as a specialized feature that is fully developed only amongst some of the less primitive members of the group.
Marsupials are not as widely spread as placental mammals, although the egg layers are by far the smallest group. As for Marsupials, there are just somewhat over 300 species in existence with about two thirds being native to Australia and some of the nearby islands, such as New Guinea and about a hundred or so native to the Americas, most of them in South America, a dozen or so in Central America and one in North America.
It had been thought at a time that marsupials were primitive forerunners of placental animals, but the fossil record indicates that this is not so, both groups seeming to have developed at about the same time, i.e. towards the end of the Mesozoic era, indicating that the separation between the groups took place considerably earlier. In any case, marsupials seemed to have flourished for a long period alongside placental mammals in Europe, North America and Asia. The earliest known marsupial, Sinodelphys szalayi lived in China some 125 million years ago and it would seem that it was from here that marsupials spread around the world, but their journey to the areas that they dominate today was an extremely long one. At that time, China, Europe and North America are believed to have been linked together in one giant continent ( the southern continents, Africa, South America, Antarctica and Australia were also joined together at this time in another great continent) and marsupials seem to have trekked through Europe and entered North America.
A few million years later, both of these great land masses had started to fragment and the result was that South America, with Antarctica and Australia still attached, had broken off from Africa and drifting westward had collided with and attached itself to North America which had broken off from the Eurasian landmass. During this period, the marsupials continued there trek and entered into South America.
Some 50 or so million years ago, the marsupials had crossed over to Antarctica which was still attached to South America until just about 35 million or so years ago. Not long after these animals had entered Antarctica, Australia, which was still joined to Antarctica, broke of and started its float to its present destination with a full cargo of marsupials.
The breakaway of Australia and the remoteness of the southernmost parts of South America helped to ensure the survival of marsupials, for elsewhere, the soon lost the competition for survival to placental mammals and soon became extinct. In Australia, however, the marsupials were the dominant group until the arrival of man and the placental mammals that have come over with him once again put these animals at risk. Similarly, in South America, the inhospitability of the areas where the marsupials inhabited contributed to their survival, but the sort of dominance that the group developed in its isolated Australian home was never developed, and the South American marsupials are not only fewer in absolute numbers, but also are more backwards than their Australian cousins in evolutionary terms. Just as in Australia, man’s increasing encroachment into the habitats of these animals, and with man more and more species of placental mammals, have once again placed these marsupials under the threat of extinction that the escaped from just a few million years back.