Koalas Nearly Went Extinct Before Humans Arrived, DNA Study Reveals

A major genomic analysis is rewriting the evolutionary history of the koala. The research reveals that the species underwent a severe population decline approximately 100,000 years ago, well before humans arrived on the Australian continent. Every koala alive today appears to descend from a single ancestral population that endured dramatic environmental shifts, including intense glacial periods.

Researchers from the University of Sydney and Texas A&M University led the study, published in Molecular Biology and Evolution. By calculating the mutation rate of modern koala populations for the first time, they were able to reconstruct population sizes going back 100,000 years — revealing a bottleneck far more ancient than previously assumed.

Three things worth knowing:

Earlier theories suggested that koala numbers collapsed only after humans reached Australia, due to hunting and habitat modification. The new timeline pushes the near-extinction event back to the Pleistocene, when glacial cycles caused massive environmental upheaval across the continent. The surviving population was so small that it left a permanent genetic signature in every koala born since.

Modern koalas now face a different set of threats: habitat loss from land clearing, catastrophic bushfires, chlamydia outbreaks, and climate-driven heat stress. The low genetic diversity inherited from that ancient bottleneck makes them more vulnerable to these pressures, since the population has less variation to adapt to new challenges.

The findings have immediate conservation implications. If today's koalas are all descendants of a single resilient lineage, protecting their remaining genetic diversity becomes even more critical. Conservation strategies may need to prioritize genetic management alongside habitat protection, ensuring that the species does not lose the adaptive capacity it needs for the rapidly changing conditions ahead.