Unlike the Old World and the Americas, the continent of Australia has been geographically isolated for hundreds of millions of years, breaking off of the Gondwana super continent only shortly after the first mammals evolved (short at least in evolutionary terms).
Upon this remote continent over the hundreds of millions of years that followed life evolved a unique branch of species unlike no place on earth. Those that came to dominate the landscape were Marsupials, a subclass of mammals that tend to carry their young within a pouch. Some of these have become icons of Australia, like the Kangaroo, Koala, and Wombat.
Sharing a common marsupial ancestor, the species branched and morphed to fit the various niches within the food-chain left by the great triassic-jurassic extinction of 200 millions years ago. Some of these had even taking the role of top predators, such the tasmanian tiger and marsupial lion, but have since disappeared in a more recent wave of extinctions.
Coexisting along side of the marsupials are a few remaining species of Monotremes, an order of animals that only still exists in Australia and New Guinea. These bizarre egg laying mammals like the echidna and platypus are truly odd. Their reptilian reproduction serves as a reminder of an ancient time when mammals where at an interim state, before mammals had evolved the ability to gestated young within their bodies. Meanwhile the lines of Australian reptiles that survived into today bear resemblance to the dinosaurs from before the continental split, with little change in physiology since that prehistoric epoch, with even their size seeming larger than typical.
As with our arrival in the new world however, the arrival of humans into Australia resulted in mass extinction event. The Aborigines came as early as 50,000 years ago on land bridges from what now is New Guinea and Timor, made possible by a lower sea levels. Many species couldn't adapt quick enough to survive the weapons and intelligence of these new predators. The megafauna were hit the hardest. Huge beasts that roamed the land, like the diprotodon, a hippo sized relative of the wombat, were easy targets for the Aboriginal hunters.
Further competitive pressure came with the dingos, wild dogs that arrived on the same land bridges. Occupying the same niche as these rival predators, the native tasmanian tiger vanished from the mainland, only to be finally driven into extinction within Tasmania by hunters and farmers this century, who saw it's presence as a threat to local livestock.
For decades since many of the other marsupial species had been in sharp decline, their range rapidly shrinking. This trend accelerated as human development expanded and invasive species encroached. Until a public outcry resuled in their protection, Koalas were being hunted almost to extinction for their fur, with up to one million killed per year. Today only around 100,000 remain.
Environmental pressures continue today, compounded with increasing pressure from wildfires and drought. Some of Australia's unique species may not make it to the end of this century. But many populations of species have stabilized, in large part due to an environmental movement that began to value protecting them. A collection of national parks and wilderness protection areas has been set aside as native habitat. The Australian people now tend to embrace their wildlife as part of the national identity, with the government attempting to balance environmental concerns with economic ones.
Today, in spite of these ongoing challenges, the Australian wilderness still supports an incredible range of biodiversity. It remains a spectacle that should be witnessed firsthand by anyone with an interest in Earth's natural history.