A koala population crash long blamed on the first people to reach Australia actually began some 40,000 years before humans settled the continent, driven instead by a drying climate and shrinking forests. That is according to researchers at the University of Sydney and Texas A&M University, whose genomic study dates the decline to roughly 100,000 years ago and overturns the long-held view that early Australian hunting set it in motion.
The findings, published in Molecular Biology and Evolution by Oxford University Press, rest on the DNA of 457 koalas drawn from across the country. By calculating how quickly genetic mutations arise in the species, the team reconstructed a timeline that extends deep into the koala’s past and estimated the size of its ancient populations.
Lead author Toby Kovacs, a PhD candidate at the University of Sydney, said the work resets what scientists thought they knew about when the marsupial first began to struggle. The study rewrites the timeline for the koala’s genetic history in Australia, Kovacs said.
The mutation rate the team measured, about half that seen in humans, came from sequencing four parent-offspring trios and counting the new genetic changes that appear in each generation. It is the first time researchers have calculated a mutation rate directly for koalas, or for any species in Diprotodontia, the marsupial order that also includes wombats, kangaroos and possums.
Earlier studies had pinned the koala’s decline on modern humans reaching the continent roughly 65,000 years ago, but those estimates leaned on mutation rates borrowed from distantly related mammals such as humans and mice. The Sydney-led analysis instead places the onset at about 100,000 years ago, with a critical bottleneck close to 60,000 years ago, both well before any human contact.
The timing matches a stretch of intense upheaval during the late Pleistocene, when repeated glacial cycles left much of Australia colder, drier and increasingly fire-prone. As the Nullarbor Plain expanded around 70,000 years ago, a vast semi-arid shrubland split the koala’s range in two, and the western population eventually died out whilst a small eastern group clung on through the harshest conditions.
As the climate warmed into the current interglacial, the surviving eastern koalas expanded and split into five genetic populations between 16,500 and 6,000 years ago, forming the groups strung along the east coast today.
The result shifts the blame from the first Australians, who reached the continent around 60,000 years ago, long after the crash was already underway. Joe Melton of the Australian Science Media Centre (AusSMC), speaking to Renee Krosch on ABC NSW & ACT Evenings, said it looks like people were not to blame for these declines.
The same genomes carry a more hopeful thread, with every koala alive today descended from a single group that endured both searing heat and severe ice ages. Melton called them the descendants of some pretty tough survivors.
The dangers facing koalas now look nothing like the slow climate forces of the deep past, running from the colonial fur hunters who shot millions in the late 19th and early 20th centuries to the land clearing, bushfires, vehicle strikes and disease such as chlamydia of the present. The species has been listed as endangered across Queensland, New South Wales and the Australian Capital Territory since 2022.
Beyond the deep past, the new mutation rate gives researchers a tool to track living populations and to test whether other Australian species also declined before humans. Kovacs said the latest generations show Queensland and New South Wales koalas still falling whilst Victoria’s recover, each population a remnant of the single lineage that clawed back from near-collapse 100,000 years ago.
