Why do koalas live only in Australia? (and other animals as well)

Australia and the islands of Oceania are ancient lands that have been isolated from other continents for millions of years. That is why 84% of Australia’s mammals and plants and almost half of its birds cannot be found anywhere else in the world. These include the koala, kangaroo, platypus, echidna, brush-tailed possum and wombat, and birds like the emu and the kookaburra. Other native animals include reptiles like crocodiles, snakes and lizards.Note: The tuatara, the most ancient reptile, is found in New Zealand. It can live for up to 100 years.