It’s surprising how easy it is to see a koala every day in Australia’s major cities.

The cute, grey marsupial can be found on t-shirts, hanging off people’s bags and pencils, and decorating any decent souvenir shop. But seeing a real koala in the wild has become increasingly tricky in some parts of the country. The iconic marsupial is now listed as endangered in Queensland, New South Wales and the Australian Capital Territory.

But koalas have been in a similar situation before.

As my new study published in the journal Molecular Biology and Evolution shows, koalas experienced a population crash about 100,000 years ago. This finding rewrites our understanding of the genetic history of koalas in Australia – and overturns previous theories about what caused their decline in ancient times.

Turning to the genome

Fossil records of koalas are extremely rare. This makes it difficult to estimate how many koalas were present in the past.

Instead, genomes provide important clues about their evolutionary history. The genome acts as a historical record. It preserves genetic information from ancestral populations that can be used to determine their population size.

Previous genomic studies of koalas have estimated koalas experienced a major population decline roughly 40,000 years ago. This was shortly after the arrival of humans in Australia, suggesting........

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