Why does Australia have such unique biodiversity (variety of animals and plants) in its fauna and flora?

Australia has unique biodiversity in its fauna and flora because it has been separated from the rest of the world (from other lands) for millions of years.

Explanation:

Australia is located in the Oceania continent. It's the country that has the most unique biodiversity and environment in the entire continent. Being isolated from the rest of the world, and also being completely surrounded by oceans, made Australia a place where plants and animals evolved.

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