Koalas have a long evolutionary history with fossils extending all the way back to the late Oligocene, around 25 million years ago. They’ve been an integral part of Australian forests throughout that time but were once much more diverse. When much of Australia was covered by dense forest 20-25 million years ago there were as many as five different genera of koalas, compared to the single species in the genus Phascolarctos that is still with us today.
Our knowledge of early koala evolution comes from a few key sites in the Lake Eyre and Frome Basins in South Australia as well as from the deposits of Riversleigh in northern Queensland.
It’s not clear at what point koalas became eucalypt specialists but their ancestors probably looked something like a Brush-tailed Possum and certainly weren’t restricted to eating just a few species of gum leaves.
Even the modern koala species was once much more widely distributed, with fossils showing that it occurred in Western Australia and in South Australia at least as far north as the Flinders Ranges.