Life's first catastrophe was self-inflicted and accidental
Around 2.4 billion years ago cyanobacteria filled the atmosphere with oxygen — a waste product lethal to most organisms then alive. The chemistry that made the planet habitable for us began as pollution.
The marker is a change in sulfur isotopes: before the transition, sulfur in sediments shows mass-independent fractionation, which can only occur under an ozone-free sky. It disappears from the record between roughly 2.45 and 2.32 billion years ago. Oxygen also stripped out the atmospheric methane that had been keeping the young, dim sun's Earth warm, and the planet fell into the Huronian glaciations.
The largest environmental catastrophe in the planet's history had no villain, no intent, and no plan. It was a metabolic byproduct at scale. Every framework that locates ecological harm in motive is missing the mechanism that has actually done the most damage.
Ultraviolet light striking sulfur dioxide sorts sulfur isotopes in a way that ignores their mass, which ordinary chemistry cannot do. Ozone blocks that ultraviolet. So the presence of mass-independent sulfur in ancient rock is a direct proxy for an atmosphere with effectively no free oxygen, and its disappearance is the clearest single line drawn across Earth's chemical history.
Oxygen rose, then stalled. The interval from about 1.8 to 0.8 billion years ago is called the Boring Billion: stable chemistry, stable climate, almost no evolutionary innovation visible in the record. Complex life did not follow from oxygen promptly. It waited roughly as long as everything since has taken.
Calling it a mass extinction is an inference, not an observation.
There is no fossil census of Archean microbial diversity to compare before and after. What we actually have is a geochemical transition plus the reasonable claim that anaerobes are poisoned by oxygen. The death toll is a story told over a chemistry result.
Molecular-clock or biomarker evidence showing anaerobic lineages passed through 2.4 Ga without a bottleneck.