Cyanobacteria ingest carbon dioxide and release oxygen, and billions of years ago they began producing more oxygen than Earth's natural processes consumed, causing oxygen to build up in the atmosphere.
Notes on verification
Well-corroborated by multiple independent, reputable scientific sources (Planetary Society, Scientific American, WHOI, ASM). Minor scientific uncertainty over exact dates does not undermine the core claim.
Sources
- Interesting facts about Earth from Harvard paleontologist (news.harvard.edu)
- https://www.planetary.org/articles/how-did-earth-get-its-oxygen (planetary.org)
- https://asm.org/articles/2022/february/the-great-oxidation-event-how-cyanobacteria-change (asm.org)
- https://www.whoi.edu/oceanus/feature/little-things-matter-a-lot/ (whoi.edu)
- https://www.scientificamerican.com/article/origin-of-oxygen-in-atmosphere/ (scientificamerican.com)