A new genetic study has challenged the long-standing view that the arrival of humans in Australia around 65,000 years ago drove a major decline in ancient koala populations, instead pointing to extreme climate change as the primary cause.
Researchers say prehistoric environmental shifts across Australia, occurring long before humans first reached the continent, were responsible for a dramatic reduction in koala numbers. The findings come after scientists directly measured koala mutation rates for the first time, allowing for a more precise reconstruction of their evolutionary history.
Mutation rates — the frequency at which new genetic changes occur across generations — are a key tool in tracing species evolution. Because these rates vary between species, accurate measurement is critical for reconstructing population history.
According to findings cited by SciTechDaily, researchers sequenced the genomes of four koala trios — parent and offspring groups — to identify newly arising mutations. They found koala mutation rates are roughly half those observed in humans, a result that significantly alters earlier population models.
The study marks the first direct measurement of mutation rates in koalas and, more broadly, in members of the marsupial order Diprotodontia, which includes kangaroos, wombats and possums.
Previous research had suggested koala populations declined sharply following human arrival in Australia. However, those earlier studies relied on “borrowed” mutation rates from distantly related species such as mice and humans, leading to less accurate estimates.
When researchers applied the newly derived koala-specific mutation rate to a dataset of 457 koala genomes, the species’ demographic history changed substantially. The revised analysis suggests koala populations began declining around 100,000 years ago, reaching a severe bottleneck roughly 60,000 years ago.
That timing predates human arrival and coincides with major environmental upheaval during the late Pleistocene epoch. Scientists say Australia’s landscapes have undergone profound transformation over millions of years, shifting from widespread rainforest coverage during the Paleogene period (66 million to 23 million years ago) to increasingly arid conditions as the continent drifted north during the Miocene epoch.
Further climatic oscillations during the Pleistocene (2.5 million to 11,700 years ago), alternating between cold, dry glacial phases and warmer, wetter interglacial periods, reshaped ecosystems across the continent. Over time, Australia became progressively drier and more prone to wildfires.
Around 70,000 years ago, the expansion of the Nullarbor Plain created vast semi-arid landscapes that fragmented koala habitats and separated eastern and western populations. The western population ultimately went extinct, while a small eastern group survived the harshest climatic conditions.
As conditions later improved, surviving koala populations expanded again. Between about 16,500 and 6,000 years ago, they diversified into five genetically distinct groups that form the basis of modern koala populations along Australia’s east coast.
Researchers say the findings raise broader questions about whether similar pre-human population declines affected other native Australian species, including relatives of now-extinct megafauna.
“Given these results, we are now curious to see whether other Australian species, including relatives of extinct megafauna, also experienced population declines before human arrival,” said Toby G. L. Kovacs, one of the study’s lead authors.
Beyond rewriting koala evolutionary history, the newly established mutation rate could have implications for conservation efforts, as koalas face ongoing threats from habitat loss, disease and climate change.
“Our team is generating a huge genomic resource for koalas, but to fully understand what these data can tell us, we also need to know how fast new genetic changes arise in the species,” Kovacs said.
He added that improved mutation estimates will strengthen efforts to reconstruct koala population history and guide future conservation decisions.
“Estimating mutation rates improves our ability to reconstruct koala population history, understand their capacity to adapt and make better conservation decisions for the future,” Kovacs said.
