I don't care if this gets removed.

Originally, Australia was part of a large landmass known as Gondwana, there were several genus (genuses? genus') known as Procoptodons, who were large ancestors of the kangaroos (wallabies have also been included in this broad spectrum). They were part of a large group of marsupials that inhabited Gondwana. These marsupials were part of a family of marsupials (family in the scientific classification term) - you see macropods such as possums, opossums, and so on all around the continents of Gondwana. As Australia moved northwards, it's environment changed from a cool, wet, rainforest to warm, dry sclerophyll. Genus such as the procoptodon (or specifically, P. goliah) that were the ancestors of modern kangaroos such as the red or grey kangaroos adapted to this new environment over millions of years, thus answering your question. Similar to the kangaroo, the koala was part of a group of marsupials that evolved separately due to the isolated nature of Australia.

In a word? Pangaea.

It's the theory that all of the continents were once one large landmass. This landmass split along seismic fault lines and drifted apart into the continents we know today, bringing with them all of the life that had already evolved there.