A recent study published in the journal "Molecular Biology and Evolution" shows that Australia's iconic koala species experienced a severe population collapse about 100,000 years ago, which was long before humans landed in Australia. This basically rules out the theory that human activities caused the ancient koala population to plummet.By accurately measuring the genetic mutation rate of koalas and analyzing 457 koala genomes collected across Australia, the researchers reconstructed the ancient population history of the species. The results showed that the number of koalas declined sharply about 100,000 years ago. Many previous studies have believed that this turning point occurred about 40,000 years ago, after humans arrived in Australia.
Koalas are almost ubiquitous in contemporary cities: they appear on T-shirts, keychains, stationery and all kinds of souvenirs, but in parts of eastern Australia, it is increasingly difficult to see a real koala in the wild. Currently, koala populations in Queensland, New South Wales and the Australian Capital Territory have been included in the "endangered" species list under the framework of national environmental laws, reflecting a new round of survival crisis faced by this species in modern times.
Because koala fossil records are extremely sparse, it is difficult to accurately estimate its ancient population size and changes using traditional methods, so the scientific research team turned to genomics. The researchers pointed out that the genome is like a "historical archive" that preserves the genetic signals left by changes in ancestral population size. As long as the mutation rate is mastered, these signals can be converted into specific population curves on the timeline.
Previous studies based on genetic data have inferred that the number of koalas dropped significantly about 40,000 years ago, which coincided with the arrival of humans in Australia. Therefore, many scholars regard early human activities as one of the potential reasons for the extinction of many large animals, including koalas. However, the real driving factors for the demise of ancient Australian megafauna ("giant marsupials", etc.) have been controversial. There is no unified conclusion as to who is more important than human hunting, habitat changes and climate shocks.
To clarify the timeline, the research team first constructed a mutation rate benchmark for koalas themselves, rather than continuing to use reference values for placental mammals such as humans or mice. They sequenced the genomes of 12 koalas from three locus families, including 7 parents and 5 offspring, and counted the number of new mutations that appeared in each generation by comparing the DNA differences between the parents and offspring one by one.
The results showed that there are about 3.4 billion base sites in the koala genome that may be mutated, but only 25 new mutations were detected in each offspring. The researchers vividly compared this work to searching for only 25 mistakenly typed letters in more than 1,000 sets of the "Lord of the Rings" trilogy. This shows the scarcity of mutation signals and the sophistication of the screening work.
After obtaining this species-specific mutation rate, the team applied it to 457 previously collected koala genome data, covering all major groups of koalas in their current range in Australia. Through model inference, they reconstructed the effective population change curve of koalas over the past hundreds of thousands of years, and locked in that the large-scale population decline occurred about 100,000 years ago, not 40,000 years ago as in past literature.
This time revaluation directly pushed the collapse of the koala population back by about 60,000 years, meaning that it occurred before the first humans arrived in Australia, thus "exonerating humans" in time and almost ruling out the possibility that humans directly caused the sharp decline in ancient koala numbers. In addition, this is also the first measured mutation rate result in the order Diprotodon (including marsupials such as wombats, kangaroos, and possums), providing an important benchmark for future analysis of the evolutionary history of other Australian marsupials.
The study also pointed out that previous efforts to estimate the size of the ancient koala population were mostly forced to use mutation rate parameters of distantly related species such as humans and mice, which would inevitably introduce systematic bias in time scale inference. Now, with the addition of the koala's own mutation rate, the model's timeline has been completely revised, and the genetic timeline of the species' rise and fall on the Australian continent has been "rewritten."
So, since they are not humans, what is the real reason for the sudden decline in the number of koalas 100,000 years ago? The research team believes that this was highly synchronized with the dramatic environmental and climate changes experienced by Australia at that time. During the Pleistocene period, which spanned about 2.5 million to 11,700 years ago, the Earth's climate repeatedly oscillated between long-term glacial periods and interglacial periods, with cold-dry and warm-wet conditions alternately dominating the ecological pattern of the world and the Australian continent.
As the climate turned colder and drier, the land in southern Australia gradually desiccated, and large areas in the interior and south were replaced by semi-arid bushland, including the massive expansion of the now famous Nullarbor Plain. This vast dry barrier not only significantly reduces the area of forests and woodlands suitable for koalas to survive, but also geographically isolates the koala populations on the east and west sides of Australia.
The study points out that the koala population west of the Nullarbor Plain was later thought to be a different species from the modern eastern koala, and the western lineage eventually became extinct about 28,000 years ago. In contrast, although the eastern koala population is compressed into a small forest area on the east coast of Australia, it has still survived multiple rounds of severe ice ages and retained the "fire" for the continuation of the species.
Subsequently, as the climate shifted from cold and dry to relatively warm and humid over the past 17,000 years or so, the remaining koala populations in the east began to expand again, spreading to a wider area along the east coast. Genetic analysis shows that this process gradually formed and differentiated into five major genetic groups that are now distributed along the east coast of Australia, constituting the overall population structure of modern eastern koalas.
The study's authors said they next hope to apply similar genome analysis methods to other Australian species, particularly living species closest to extinct megafauna, to test whether these animals also experienced significant declines long before the arrival of humans. If similar patterns are observed repeatedly, it will further strengthen the idea that natural climate cycles and environmental changes played a central role in the rise and fall of ancient species in Australia.
However, for koalas, the ancient crisis has not become the end of history. Now they are once again facing the pressure of survival. Data cited in the study pointed out that koala populations across Australia are experiencing a new round of population decline today, with endangered assessments and calls for protection continuing to escalate.
There is an important similarity between the past and present declines: habitat loss remains one of the main causes. In ancient times, the global glacial-interglacial cycle and the resulting continental desiccation and vegetation replacement were almost unavoidable and were natural processes driven by changes in the Earth's orbit. In modern times, large-scale deforestation, urban expansion, and land development in human history and reality have caused equally serious or even more dramatic habitat loss in a short period of time.
The study pointed out that the threats faced by modern koalas are not limited to habitat reduction, but also include the accumulation of multiple pressures such as historical and ongoing hunting, disease transmission, road traffic deaths, wild dog attacks, and the increase in the frequency and intensity of bushfires. The combination of these factors has caused koala populations in some areas to not only plummet in number, but their genetic diversity has also begun to show signs of decline.
On a lighter note, genetic analyzes show that most of the loss of genetic diversity in koala populations occurred within a relatively recent window, meaning there is still a chance to prevent further genetic degradation and the risk of inbreeding through rapid and effective conservation measures. Scientists call for protecting and restoring suitable habitats, controlling key threat factors, and implementing scientific population management and ex-situ protection when necessary, which is expected to help koalas survive this modern "bottleneck period."
The study concluded that the severe climate and environmental changes of the Pleistocene once pushed the koala to the brink of extinction, but the eastern population eventually survived and spread again, forming the koala distribution pattern that people are familiar with today. After an ancient collapse that was "rehabilitated" by the timeline, the species is now at a crossroads again, and the scientific community and the public are paying close attention to whether the eastern koala can once again overcome the difficulties and continue its presence in Australian forests.