But an article published Saturday in The Journal of Vertebrate Paleontology says the koalas from 5 million years ago did not have the same kind of specialized eucalyptus leaf diet as the modern koala.
The shift to eucalyptus diet was probably caused, the article says, by the drift north of the Australian continent, causing a retreat of its rain forests and dominance of eucalyptus trees.
Coping with the new diet caused adaptive changes in the digestive and anatomical structure of the koalas. When scientists measured skulls of ancient and modern animals, they noticed dramatic differences in the facial skeleton, which they noted were probably due to the change to a tougher diet of eucalyptus leaves.
And the low nutrition levels of eucalyptus leaves caused the animals to develop a more sedentary, energy-conserving lifestyle, they said.
"In order to accommodate both the mechanical demands of their new diet, as well as maintaining their auditory sophistication, the koala underwent substantial changes to its cranial anatomy, in particular that of the facial skeleton,"says Dr Julien Louys of University of New South Wales's School of Biological, Earth and Environmental Sciences..