Around 2.4 billion years ago, the air over Earth began to change. Microbes in the oceans, the cyanobacteria, had been running a chemical reaction that split water and released oxygen as waste. For a long time that oxygen was mopped up almost as fast as it was made. Then the sinks filled, and it began to build up in the sea and the atmosphere, a gas that was poison to most of the life then living.

This is the Great Oxidation Event, sometimes called the oxygen catastrophe. It is often described as the first mass extinction in Earth’s history, a die-off caused not by an impact or an eruption but by life altering its own planet. The poisoning is well grounded in chemistry. The scale of the die-off is an inference, and a much shakier one, for reasons worth being clear about from the start.

How we know the air changed

The strongest evidence for the timing does not come from fossils. It comes from sulfur.

In rocks older than about 2.4 billion years, sulfur isotopes carry a pattern, known as mass-independent fractionation, that can only form when ultraviolet light reaches sulfur dioxide in an atmosphere with no oxygen and therefore no protective ozone. The signature was identified by James Farquhar and colleagues in a 2000 paper in Science. After roughly 2.4 billion years ago it disappears from the record, and that disappearance is now the most widely used marker for the arrival of free oxygen in the air. A 2023 paper in Nature Communications describes the signal as a fingerprint of an oxygen-free atmosphere, while also noting that reading it is less straightforward than it once seemed.

A second line of evidence sits in the iron. Before oxygen accumulated, the oceans held large amounts of dissolved iron. As oxygen spread, that iron reacted and settled out, laying down the banded iron formations that geologists still mine today.

Why oxygen was a poison

Oxygen is reactive.

In cells that evolved without it, it produces what are now called reactive oxygen species, fragments that damage proteins, membranes and genetic material. Many organisms that dominated the early Earth lacked the defences needed to cope with it.

So as oxygen built up, much of the anaerobic world died back. Some lineages did not vanish so much as retreat, into ocean sediments, deep water and other pockets where oxygen did not reach. Their descendants still live in those anoxic refuges. The microbes that caused the crisis, meanwhile, kept producing the very gas that was lethal to their neighbours.

The cold may have done as much as the oxygen

There was a second effect, and it may have been the more destructive one.

The early atmosphere was rich in methane, a strong greenhouse gas that helped keep the planet warm at a time when the Sun was fainter than it is now. Oxygen destroys methane. As oxygen rose, the methane greenhouse collapsed, and the Earth fell into the Huronian glaciation, a run of ice ages spanning roughly 2.4 to 2.1 billion years ago and among the longest and most severe in the planet’s history. Work by Robert Kopp, Joseph Kirschvink and colleagues, published in PNAS, argued that the spread of oxygenic photosynthesis triggered that near-global freeze. If the reading holds, the organisms that poisoned the air also helped chill the surface.

Two killing mechanisms, chemical and climatic, would have arrived together. Neither leaves the kind of record that lets us count the dead.

What the record can and cannot tell us

This is where the popular story needs handling. The microbial life of 2.4 billion years ago did not leave shelly fossils that can be tallied bed by bed, the way the victims of later extinctions can. As the American Society for Microbiology puts it, working out which lineages were lost has proved difficult precisely because the fossil evidence is so thin. Oxygen as a poison is sound. “The first mass extinction,” with the confidence that phrase carries, is a reconstruction laid over a very sparse record.

The “filled the air” part also needs care. Early oxygen levels were a small fraction of today’s, not a breathable atmosphere, and the rise was neither smooth nor in one direction. A 2021 study in Nature led by Simon Poulton found that oxygen fluctuated for around 200 million years before it became a permanent feature of the air, and a 2017 PNAS study by Ashley Gumsley and colleagues spread the onset and tempo of the change across a long interval rather than a single moment. The name suggests an event. The evidence describes a long, uneven transition.

None of this softens the underlying point. Something in the chemistry of the planet changed, life itself caused it, and a great deal of what was alive could not survive the new conditions.

The same gas that ended their world is the one we now cannot live without. Our own lineage runs through organisms that eventually learned not just to survive oxygen but to use it, turning a planetary poison into one of the great engines of complex life. When that turn happened, and how much was lost along the way, are questions still being dated in the rocks.