The Great Oxidation Event
Microbes fill the air with oxygen and remake the planet
Quick facts
- When
- About 2.4 to 2.1 billion years ago
- Cause
- Oxygen from photosynthesizing cyanobacteria
- Before it
- Almost no free oxygen in the air
- Consequence
- Toxic to much existing life; enabled complex life later
What happened
For roughly the first half of Earth's history the air held almost no free oxygen, and the planet's chemistry was completely different from today's. Then cyanobacteria, microbes that make energy from sunlight through photosynthesis, released oxygen as a waste product, and over hundreds of millions of years it built up faster than the rocks and oceans could absorb it. The American Society for Microbiology places this Great Oxidation Event sometime between about 2.4 and 2.1 billion years ago, when oxygen finally accumulated in the atmosphere.
Why it matters
This was one of the largest changes in the planet's chemistry ever, and it was driven entirely by life. Oxygen was toxic to much of the life that existed at the time, so it reshaped which organisms could survive, and it eventually made possible the oxygen-breathing, complex life, including animals and us, that came much later. A microbe rewrote the atmosphere of a whole world.
How we know
The American Society for Microbiology describes the Great Oxidation Event and dates it to roughly 2.4 to 2.1 billion years ago, explaining that oxygen-producing cyanobacteria were the cause and that Earth's atmosphere before it was very different.
Sources
- American Society for Microbiology. The Great Oxidation Event: how cyanobacteria changed life (American Society for Microbiology) (2022) · General sourceasm.org · Cited as a "reference" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
- Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils (Palaeontology, via PubMed) (2016) · Peer-reviewed (author-declared)pubmed.ncbi.nlm.nih.gov · Cited as a "journal" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
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