Koalas experienced a major demographic collapse long before humans arrived in Australia. This discovery overturns the understanding of their evolutionary history.
To study this phenomenon, scientists from the University of Sydney and Texas A&M sequenced the genomes of koala families to directly measure their mutation rate. Their findings, published in the journal
Molecular Biology and Evolution, challenge the idea that koala decline was linked to human presence.
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The team compared the genomes of four parent-offspring trios and counted the new mutations that appeared. The mutation rate in koalas turned out to be two times lower than that of humans. Applied to 457 other genomes, this rate made it possible to reconstruct the size of ancient populations.
The analysis revealed a severe bottleneck 60,000 years ago, linked to climatic changes of the late Pleistocene. Glacial cycles and the drying of the continent fragmented the habitat, well before any human intervention.
The formation of the Nullarbor Plain 70,000 years ago separated the eastern and western koalas. The western population disappeared, while a small eastern population survived under the harshest conditions.
Between 16,500 and 6,000 years ago, with the return of a more favorable climate, the surviving koalas expanded and gave rise to the five genetic groups found today along the eastern Australian coast.
This new mutation rate offers a valuable tool for current conservation. It allows a more precise assessment of genetic diversity and the adaptive capacity of koalas facing modern threats such as habitat loss and wildfires.