You wouldn't know it from their relatively paltry numbers today, but marsupials (the kangaroos, koalas, wombats, etc. of Australia, as well as the opossums of the western hemisphere) have a rich evolutionary history. As far as paleontologists can tell, the distant ancestors of modern opossums diverged from the distant ancestors of modern placental mammals about 160 million years ago, during the late Jurassic period (when pretty much all mammals were the size of mice), and the first true marsupial appeared during the early Cretaceous, about 35 million years later. (Here's a gallery of prehistoric marsupial pictures and profiles and a list of recently extinct marsupials.)
Before we go any further, it's worthwhile to review what sets marsupials apart from the mainstream of mammalian evolution. The vast majority of mammals on earth today are placental: fetuses are nurtured in their mother's wombs, by means of a placenta, and they're born in a relatively advanced state of development. Marsupials, by contrast, give birth to undeveloped, fetus-like young, which then must spend helpless months suckling milk in their mothers' pouches. (There's also a third, much smaller group of mammals, the egg-laying monotremes, typified by platypuses and echidnas.)
The First Marsupials
Because the mammals of the Mesozoic Era were so small--and because soft tissues don't preserve well in the fossil record--scientists can't directly examine the reproductive systems of animals from the Jurassic and Cretaceous periods. What they can do, though, is examine and compare these mammals' teeth, and by that criterion, the earliest identified marsupial was Sinodelphys, from early Cretaceous Asia. The giveaway is that prehistoric marsupials possessed four pairs of molars in each of their upper and lower jaws, while placental mammals had no more than three.
For tens of millions of years after Sinodelphys, the marsupial fossil record is frustratingly scattered and incomplete. We do know that early marsupials (or metatherians, as they're sometimes called by paleontologists) spread from Asia to North and South America, and then from South America to Australia, by way of Antarctica (which was much more temperate at the end of the Mesozoic Era). By the time the evolutionary dust had cleared, by the end of the Eocene epoch, marsupials had disappeared from North America and Eurasia but prospered in South America and Australia.
The Marsupials of South America
For most of the Cenozoic Era, South America was a gigantic island continent, completely separated from North America until the emergence of the Central American isthmus about three million years ago. During these eons, South America's marsupials--technically known as "sparassodonts," and technically classified as a sister group to the true marsupials--evolved to fill every available mammalian ecological niche, in ways that uncannily mimicked the lifestyles of their placental cousins elsewhere in the world.
Examples? Consider Borhyaena, a slouching, 200-pound predatory marsupial that looked and acted like an African hyena; Cladosictis, a small, sleek metatherian that resembled a slippery otter; Necrolestes, the "grave robber," which behaved a bit like an anteater; and, last but not least, Thylacosmilus, the marsupial equivalent of the Saber-Tooth Tiger (and equipped with even bigger canines). Unfortunately, the opening of the Central American isthmus during the Pliocene epoch spelled the doom of these marsupials, as they were completely displaced by better-adapted placental mammals from up north.
The Giant Marsupials of Australia
In one respect, the marsupials of South America have long since disappeared--but in another, they continue to live on in Australia. It's likely that all of the kangaroos, wombats, and wallabies Down Under are descendants of a single marsupial species that inadvertently rafted over from Antarctica about 55 million years ago, during the early Eocene epoch. (One candidate is a distant ancestor of the Monito del Monte, or "little bush monkey," a tiny, nocturnal, tree-dwelling marsupial that today lives in the bamboo forests of the southern Andes mountains.)
From such unprepossessing origins, a mighty race grew. A few million years ago, Australia was home to such monstrous marsupials as Diprotodon, aka the Giant Wombat, which weighed upwards of two tons; Procoptodon, the Giant Short-Faced Kangaroo, which stood 10 feet tall and weighed twice as much as an NFL linebacker; Thylacoleo, the 200-pound "marsupial lion"; and the Tasmanian Tiger (genus Thylacinus), a fierce, wolf-like predator that only went extinct in the 20th century. Sadly, like most megafauna mammals worldwide, the giant marsupials of Australia, Tasmania, and New Zealand went extinct after the last Ice Age, survived by their much more petite descendants.