The koala population crash, long blamed on early human hunting, is now revealed to have begun much earlier, driven by a drying climate and shrinking forests. This new study, led by researchers at the University of Sydney and Texas A&M University, challenges the long-held view that human arrival in Australia 65,000 years ago was the catalyst for the koala's decline. Instead, the genomic study, published in Molecular Biology and Evolution, suggests the decline started around 100,000 years ago, with a critical bottleneck close to 60,000 years ago, well before any human contact. This finding is significant as it shifts the blame from early humans to natural environmental factors. The study's mutation rate calculation, about half that of humans, was derived from sequencing parent-offspring trios and counting new genetic changes. This is the first time such a mutation rate has been calculated for koalas or any species in the Diprotodontia order, which includes wombats, kangaroos, and possums. The timing of the koala's decline coincides with a period of intense late Pleistocene upheaval, characterized by glacial cycles that left Australia colder, drier, and more fire-prone. As the Nullarbor Plain expanded, it split the koala's range, leading to the extinction of the western population and the survival of a small eastern group. This group eventually expanded and diversified into five genetic populations between 16,500 and 6,000 years ago, forming the koala populations along the east coast today. The study's findings are not only significant for understanding the koala's history but also for tracking living populations and testing whether other Australian species also declined before human arrival. The koala's current endangered status, due to land clearing, bushfires, vehicle strikes, and diseases like chlamydia, highlights the ongoing threats to the species. The study's mutation rate provides a valuable tool for researchers to monitor and protect koala populations, as well as to explore the broader implications for other Australian species.
