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 unique branches of species unlike those of no other place on earth. The animals that came to dominate the landscape were mostly marsupials, a subclass of mammals that tend to carry their young within pouches. Some of these have become icons of Australia, like the kangaroo, koala, and wombat.
Sharing a common marsupial ancestor, their descendent species branched and morphed to fit the various niches within the food-chain left by the great triassic-jurassic extinction of 200 million years ago. Some of these had even taken the role of top predators, such the Tasmanian tiger and marsupial lion. But these have since disappeared in a much more recent wave of extinctions.
Coexisting alongside the marsupials are a few remaining species of Monotremes, an order of animals that only still exists in Australia and New Guinea. These unique egg laying mammals like the echidna and platypus are truly bizarre, almost seeming like a cross between various other unrelated branches of the evolutionary tree. 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 ancient creatures that existed from before the continental split, with little change in physiology since that prehistoric epoch.
The Aborigines came as early as 50,000 years ago by boat. During that time sea levels were much lower, and Australia had merged into a larger landmass with New Guinea and Timor, referred to as Sahul. As with people’s arrival in the New World, the migration into Australia is suspected by some researchers to have resulted in a mass extinction event. They suggest that these species couldn't adapt quick enough to survive the weapons, intelligence, and strategic use of fire employed by these new predators. The megafauna were hit the hardest. Huge beasts like the diprotodon, a hippo sized relative of the wombat, were easy targets for the Aboriginal hunters.
Further competitive pressure came with dingos. Since they arrived during the same time period, these dogs were likely introduced by humans via boat. Occupying the same niche as these rival predators, the native Tasmanian tiger were outcompeted and vanished from the mainland, only to be finally driven into extinction within Tasmania by hunters and farmers this century, who saw its presence as a threat to local livestock. Some locals suspect that a few Tasmania tigers may be still hiding out deep within the outback.
For decades since, many other marsupial species had also been in sharp decline, with their geographic range rapidly shrinking. This trend accelerated as human development expanded and invasive species encroached. Until a public outcry resulted 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 due to a warming climate. But many populations of species have stabilized, in large part due to an environmental movement that began to advocate for their protection. A collection of national parks and wilderness protection areas has been set aside as native habitat. The Australian people now widely embrace their local wildlife as part of the national identity, with the government attempting to balance environmental concerns with economic ones.