What the species is and where it came from
Cyanobacteria—not a single species but a clade of photosynthetic microbes. They evolved chlorophyll-based water photolysis in the Archean, before the GOE. Their range: global shallow marine and freshwater habitats. Their habitat: sunlit surface waters. They eat light and water. Nothing else at the time released free O₂ as metabolic waste.
How it works, in terms someone would actually use
Cyanobacteria split water using sunlight, releasing oxygen as waste. That oxygen reacted with dissolved iron in shallow seas, forming banded iron formations. Once iron was saturated, oxygen built up in the atmosphere.
What it gets right
It correctly identifies cyanobacteria as the oxygen source. It links mineral evidence—pyrite disappearance, hematite red beds, banded iron formation timing—to atmospheric change. It anchors the shift to ~2.46–2.06 Ga.
What it does not
It did not produce modern oxygen levels. It did not cause immediate eukaryotic radiation. It did not eliminate all anoxic niches—many persisted, and still do.
What it changed
It turned Earth’s atmosphere from reducing to oxidizing. It triggered global glaciation. It ended mass-independent sulfur isotope fractionation. It enabled mitochondrial endosymbiosis—but not eukaryotic diversification, which remains debated.
Who it is for, and who it is not
Geologists, palaeobiologists, and anyone who reads rock layers as history. Not for those seeking narratives of progress, intention, or inevitability.
Is it worth your time
Yes—if you want to understand why Earth’s air is breathable, why rust exists, and why complex life needed a billion years to appear after microbes began photosynthesising.
