Millions of years ago, a koala looked more like a possum. By studying rare skulls of the famous marsupial that date between 5 and 24 million years old, a team of Australian researchers propose how it got to looking like it does today, with findings published in the Journal of Vertebrate Paleontology.

Food was one driver, they say—millions of years ago koalas ate a variety of foods. The dietary switch to an exclusive eucalypt diet seems to have occurred during the late Miocene period, some 12 to five million years ago, when a drying climate made eucalyptus the dominant forest species [Canberra Times]. As a result, they lost their snouts and developed powerful jaw muscles.

But while koalas need to adapt to the changing food supply, they also needed to stay in communication, and began to develop the low-frequency calls that today can travel half a mile. The researchers hypothesize that the ancient koalas evolved their communication system at a time when the Australian continent was drying out and the koala habitat becoming less dense. By lowering the frequency of their calls, they were able to maintain communication in the sparser forests [Wired.com]. And to hear those low-frequency calls, koalas developed a middle ear with high volume compared to other marsupials.

“The unique cranial configuration of the modern koala is therefore the result of accommodating their masticatory adaptations without compromising their auditory system,” write the researchers [Wired.com]. In other words: you put those two things together and you get today’s adorable but strange marsupial.

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Image: flickr / tinyfroglet