The genome is a historical record written in mutations
Long before humans walked the Australian continent, the koala had already learned to survive the end of a world. New genomic research published in Molecular Biology and Evolution reveals that the species suffered a catastrophic population collapse around 100,000 years ago—driven not by human hands, but by the slow, grinding force of glacial cycles that dried the land and severed ancient habitats. By precisely measuring the koala's mutation rate for the first time, scientists have rewritten a chapter of evolutionary history, replacing a story of human culpability with one of deep geological time. The finding carries a quiet urgency: the animal that endured one planetary transformation now faces another, this time measured not in millennia but in years.
- A species already listed as endangered across three Australian states has now been revealed to carry the genetic memory of a far older catastrophe—one that nearly erased it from the continent entirely.
- Previous scientific consensus blamed human arrival 65,000 years ago for the koala's ancient decline, but new analysis of 457 genomes pushes that collapse back to 100,000 years ago, dismantling the human-culpability theory.
- To unlock this timeline, researchers had to solve a foundational puzzle first—counting just 25 mutations per offspring across 3.4 billion genetic sites, a precision equivalent to finding 25 misspelled words across more than a thousand copies of a trilogy.
- The ancient western koala population, isolated by an expanding arid Nullarbor Plain, disappeared entirely around 28,000 years ago, while eastern survivors clung to a coastal forest refuge before slowly expanding as the climate softened.
- Today's crisis mirrors the ancient one in cause but not in pace—habitat destruction, disease, and climate pressure are compressing into decades what once unfolded across tens of thousands of years.
- Scientists offer a narrow but real window of hope: most koala populations have not yet lost critical genetic diversity, meaning accelerated conservation action could still prevent the inbreeding spiral that extinction follows.
You can buy a koala on a t-shirt in any Australian souvenir shop, yet finding one in the wild has grown steadily harder. The species is now listed as endangered across Queensland, New South Wales, and the Australian Capital Territory. What new research makes clear, however, is that this is not the first time the animal has stood at such a precipice.
A study published in Molecular Biology and Evolution has established that koalas underwent a catastrophic population collapse approximately 100,000 years ago—long before humans arrived in Australia around 65,000 years ago. For decades, that human arrival had been the leading suspect in the ancient decline. The new findings dismiss that explanation entirely.
Because koala fossils are vanishingly rare, scientists turned to genomes instead. DNA encodes ancestral population sizes in the mutations that accumulate across generations, but reading that record accurately requires knowing the species' mutation rate—something never precisely measured for koalas. Researchers sequenced the genomes of 12 animals across three families and found that only 25 new mutations appear per offspring across 3.4 billion genetic sites: like locating 25 misspelled words scattered through more than a thousand copies of a trilogy.
Armed with that rate, the team analyzed 457 koala genomes spanning the species' full range. The data pointed to a collapse aligned with a period of intense Pleistocene climate upheaval, when Australia grew colder and drier. The Nullarbor Plain expanded into a vast semi-arid barrier, fragmenting populations and cutting eastern koalas off from western ones. The western lineage—now recognized as a distinct species—vanished around 28,000 years ago. Eastern koalas survived in a small coastal refuge, then expanded and diversified into five genetic groups as the climate warmed over the last 17,000 years.
The parallel to today is difficult to ignore. Both crises share a root cause—habitat loss—but the ancient collapse unfolded across geological time, while the modern one is compressing into decades. Hunting, disease, vehicle strikes, bushfires, and land clearing have created a bottleneck more severe and more sudden than anything the species has previously endured. Yet most koala populations have only recently begun losing genetic diversity, which means recovery remains biologically possible. The eastern koala has survived one planetary transformation. Whether it survives this one will be decided in the years immediately ahead.
You can buy a koala on a t-shirt in any Australian souvenir shop. The animal is everywhere in the cities—plush toys, pencil toppers, bag charms—a symbol so familiar it barely registers. But finding an actual koala in the wild has grown harder. The species is now listed as endangered across Queensland, New South Wales, and the Australian Capital Territory. Yet this is not the first time the animal has faced such a crisis.
A study published in Molecular Biology and Evolution reveals that koalas experienced a catastrophic population collapse roughly 100,000 years ago. The finding upends decades of scientific thinking about what triggered that ancient decline and reshapes our understanding of the species' genetic history in Australia. For years, researchers had pointed to human arrival on the continent—which occurred around 65,000 years ago—as the likely culprit. The new work pushes back against that explanation entirely.
Fossil records for koalas are vanishingly rare, making it nearly impossible to count how many animals lived in the distant past. Genomes, however, tell a different kind of story. DNA preserves a record of ancestral populations encoded in the mutations that accumulate across generations. By reading that genetic text, scientists can estimate population sizes stretching back hundreds of thousands of years. Earlier genomic studies had suggested koalas declined sharply around 40,000 years ago—shortly after humans arrived—implying a causal link. The new research challenges that timeline entirely.
To rewrite the koala's genetic history, researchers needed to establish the species' mutation rate: how many genetic changes appear in each generation. This is fundamental to reading the genome as a historical document, yet it had never been precisely measured for koalas. Scientists sequenced the genomes of 12 animals from three families—seven parents and five offspring—and counted the new mutations that appeared in each generation. Across the koala's 3.4 billion genetic sites, only 25 mutations per offspring emerged. Finding those 25 errors is like searching for 25 misspelled words scattered across more than 1,000 copies of The Lord of the Rings trilogy.
With that mutation rate in hand, the team analyzed 457 koala genomes sampled across the entire range of the species. The data revealed that the population crash occurred around 100,000 years ago—tens of thousands of years before humans set foot in Australia. That timing effectively eliminates humans as a cause. Instead, the collapse aligns with a period of intense environmental upheaval. During the Pleistocene, Australia cycled between glacial periods of cold and dryness and interglacial periods of warmth and moisture. Around 100,000 years ago, the continent grew drier. The Nullarbor Plain expanded into a vast semi-arid shrubland across the south, fragmenting koala habitat and isolating eastern populations from western ones. The western population, recently identified as a distinct species, vanished entirely around 28,000 years ago. The eastern koalas, confined to a small patch of forest on the coast, survived the harsh glacial conditions. As the climate warmed and wetted over the last 17,000 years, they expanded and diversified into the five genetic groups now distributed along the eastern seaboard.
Today, koalas face a second population crisis. The parallel is striking: both ancient and modern declines stem from habitat loss. But where the ancient collapse unfolded over tens of thousands of years as a result of Earth's orbital cycles, the modern one is compressing into decades. Habitat removal by humans, combined with hunting, disease, vehicle strikes, feral dog attacks, and bushfires, has created a bottleneck far more severe and far more rapid. The good news is that most koala populations have only recently begun losing genetic diversity. If populations recover quickly, further loss and inbreeding can still be prevented. The eastern koala has survived one climate catastrophe. Whether it survives this one depends on what happens in the next few years.
Citas Notables
The ancient decline was driven by global glacial cycles—an unavoidable result of Earth's orbit. However, recent declines have generated a similar bottleneck over a much shorter time window, due to the historical and continued removal of suitable koala habitat.— Study author
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter that the crash happened 100,000 years ago instead of 40,000?
Because it changes who we blame. For decades, the story was that humans arrived and koalas collapsed. This finding says the collapse was already underway before humans got here—it was climate, not people.
But doesn't that let us off the hook for what's happening now?
The opposite, actually. It shows that koalas are vulnerable to habitat loss itself, regardless of the cause. Ancient climate cycles did it. We're doing it now, just faster.
How do you even know what happened 100,000 years ago? There are no koala fossils.
The genome is the fossil. Every mutation that accumulates in DNA is a clock. If you know how fast mutations happen, you can read backwards through time.
And they found that out by looking at baby koalas?
Exactly. They sequenced parents and offspring, counted the new mutations in each generation. Only 25 per koala. That tiny number is what lets them see so far back.
So the western koalas just vanished?
Around 28,000 years ago, yes. The Nullarbor Plain dried out and cut them off from the east. The eastern population hung on in a small forest patch and eventually expanded again.
And now we're doing the same thing—fragmenting habitat—but on purpose?
And faster. The ancient process took tens of thousands of years. We've done comparable damage in a few decades. The difference is we can still stop it.