Before there was continents and everything was still squished together, the marsupials came to Australia. It is believed that the kangaroos and marsupials came from South America to Australia around 60 million years ago. When Australia broke of from Antarctica (about 38 to 45 years ago) it drifted north. The climate of the continent repeatedly changed from wet and warm conditions to dry and cold conditions. Between 23 and 15 million years ago, Australia had a huge wet and warm condition phase. During that time marsupials, thunderbirds, flamingoes and other animals flourished. After 15 million yeas,r the rainfall and temperatures dropped, which still continues today. Australia dried out a lot and some species went extinct like the giant rat kangaroo and thunderbird. The red kangaroo, however, survived.
http://www.pbs.org/wgbh/nova/evolution/evolution-down-under.html
http://animals.nationalgeographic.com/animals/mammals/red-kangaroo/
http://animals.nationalgeographic.com/animals/mammals/red-kangaroo/