Koala populations began collapsing 100,000 years ago — long before humans reached Australia

A newly calibrated genomic mutation clock shows koala numbers falling to about a tenth of their peak roughly 100,000 years ago, tens of thousands of years before humans are thought to have set foot on the continent.

Noodle News data desk · reading time ~3 min

10%

Where koala numbers bottomed out, relative to their historical peak, in genome-modelled population history.¹

35,000+ yrs

Minimum gap between the start of the decline and humans' earliest proposed arrival in Australia.¹

457

Wild koala genomes analysed with the new mutation-rate clock.¹

What the data shows

A recalibrated clock rewrites the timeline

Earlier studies concluded that koala numbers fell after modern humans arrived in Australia around 65,000 years ago, but those estimates borrowed mutation rates from distantly related mammals — mainly humans and mice.¹ The new study measured the koala's own mutation rate directly, sequencing four parent-offspring trios, and found it runs at roughly half the human rate.¹ Applying that koala-specific clock to 457 wild genomes from across the species' five surviving populations produced a markedly different demographic picture.¹

Modelled this way, koala numbers grew for hundreds of thousands of years, then entered a severe, prolonged decline roughly 100,000–120,000 years ago — estimates vary slightly depending on the statistical method used — falling to about a tenth of their peak size by around 60,000 years ago.¹ That is tens of thousands of years before humans are thought to have reached the continent, in a window most commonly dated to between 65,000 and 47,000 years ago.¹

The timing points to a cause other than people. The decline coincides with the onset of the most recent glacial period, when Australia grew colder and drier and forest habitat fragmented across the continent.¹ Around 70,000 years ago the Nullarbor Plain dried into semi-arid shrubland, splitting a once-continuous range into an eastern population — ancestor of every koala alive today — and a western one that later died out.² The study's authors point to this climate-driven habitat loss, not human arrival, as the more likely driver of the crash.¹

Exhibit 01

Koala population history vs. human arrival, 700,000 years ago to present

Source: Modelled from genome-wide coalescent analysis (PSMC and SMC++) of 457–458 wild koala genomes; human-arrival window from established archaeological estimates cited in the same paper.¹ Spans are estimated periods, not exact single years.

Key finding. However precisely it is dated, the collapse in koala numbers came first, driven by climate, not people — the population was already small and isolated by the time humans are thought to have arrived.²

What we don't know

PSMC and SMC++, the two coalescent methods used, agree the decline predates human arrival but disagree on its precise shape and timing — and neither can fully separate a genuine drop in population size from reduced gene flow as populations became isolated from one another; the study's authors say both processes probably acted together.¹ The date humans first reached Australia is itself debated among archaeologists, with credible estimates spanning close to 20,000 years; a later arrival date would narrow the gap described above, though not close it.¹

Caveat. The strongest objection: these coalescent methods infer population size from patterns of shared ancestry, so they cannot fully rule out that growing isolation between koala groups — rather than a true crash in numbers — produces part of the "decline" signal. The paper's authors regard a real population drop as the more likely explanation, but say both effects probably occurred together.¹

Go deeper

Left out here to hold length:

Humans and the western population. Climate split off the western koala population tens of thousands of years before people arrived, but humans have since been credited — in other research cited by the study — with finishing off that already-isolated western lineage; a related 2026 Science study separately traces how today's surviving eastern populations are regaining some lost genetic diversity as they recover.¹³
Recombination maps. The same team used the new mutation rate to map recombination across koala immune genes — relevant to how well translocation programmes can restore lost genetic diversity.¹

Method. Population history is modelled from genome-wide coalescent analysis (PSMC and SMC++) using a koala-specific mutation rate — 6.12×10⁻&sup9; mutations per base pair per generation, measured directly from four parent-offspring trios — rather than the human or mouse rates used in earlier koala studies. The "10% of peak" and timing figures are as reported for the SMC++ and PSMC results in the source paper. The "35,000+ years" hero figure is the minimum gap obtained by comparing the most conservative (latest) decline-onset estimate against humans' earliest proposed arrival date; using the paper's earlier PSMC decline estimate against the most recent arrival estimate widens the gap to roughly 70,000 years. No projected or scenario data appears anywhere in this piece — all spans shown are the study's estimated historical periods.

Sources. Numbered, highest tier first; each inline citation points here.

T1Kovacs, T.G.L., Foley, N.M., Silver, L.W., McLennan, E.A., Murphy, W.J., Hogg, C.J. & Ho, S.Y.W. "Mutation rate estimate and population genomic analysis reveals decline of koalas prior to human arrival." Molecular Biology and Evolution 43(6): msag108 (2026). doi.org/10.1093/molbev/msag108 — retrieved 28 Jul 2026.
T2University of Sydney. "Koala population crash came before humans, genomic study reveals." News release, 9 June 2026. sydney.edu.au — retrieved 28 Jul 2026.
T1Ahrens, C.W., Miller, A.D., Silver, L.W., McLennan, E.A., Hogg, C.J. & Weeks, A.R. "Escaping bottlenecks: the demographic path to genetic recovery in koalas (Phascolarctos cinereus)." Science 391(6789): 1010 (2026). doi.org/10.1126/science.adz1430 — retrieved 28 Jul 2026.

Corrections. None. Material corrections will be logged here with date.

Figures to 29 Jul 2026. Noodle News · v1.0 · 2026-07-29-koala-population-collapse.

Koala populations began collapsing 100,000 years ago — long before humans reached Australia

A recalibrated clock rewrites the timeline

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