The gas that nearly wiped out early life is the reason you’re alive today

For most of the microbial life that existed on Earth roughly 2.4 billion years ago, oxygen was not a gift — it was a threat. As microscopic organisms called cyanobacteria began pumping oxygen into the atmosphere through photosynthesis, the gas proved toxic to many of the anaerobic microorganisms that had dominated the planet’s ecosystems up to that point.

Scientists call this turning point the Great Oxidation Event, and according to a review titled ‘The rise of oxygen in Earth’s early ocean and atmosphere’, published in Nature, it permanently changed Earth’s atmosphere, oceans, and the future evolution of life. Even as oxygen proved lethal to many existing organisms, it opened the door for a new kind of life — organisms capable of using oxygen far more efficiently to generate energy.

The Annual Review of Earth’s Planetary Sciences notes that this shift laid the environmental foundation for the later evolution of complex eukaryotic cells and, eventually, multicellular life — the lineage that all animal life, including humans, ultimately descends from.

Before the Great Oxidation Event, Earth’s atmosphere held almost no oxygen at all, with gases like methane and carbon dioxide dominating instead. The review describes the oxygenation that followed as a slow, coupled biological and geochemical process rather than a single switch: free oxygen built up in the atmosphere first, lagged in the oceans, and only reached near-modern levels in both roughly two billion years later.

Cyanobacteria are thought to have evolved oxygen-producing photosynthesis hundreds of millions of years before atmospheric oxygen actually began accumulating, because the gas they released initially reacted with dissolved iron and other reduced compounds in the ocean and crust. Only once those natural sinks became saturated did oxygen begin building up in significant amounts, around 2.4 billion years ago — a shift confirmed by the sudden disappearance of a distinctive sulfur isotope signature in sedimentary rocks of that age.

Researchers are still investigating exactly why oxygen accumulated when it did, with the Nature review pointing to ongoing work on microbial communities, volcanic activity, nutrient availability, and broader geological processes as areas of active study.

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