Cyanobacteria were long treated as a footnote in the history of Earth's atmosphere: single-celled, structurally simple, and easy to overlook beside the fossils of larger organisms. That reading has not survived the isotope record. Beginning roughly 2.4 billion years ago, layers of marine sediment preserve a sharp and sustained rise in oxidised minerals — a signature that no geological process of the period accounts for on its own. The organisms responsible were photosynthetic, and they were abundant.
What the record does not settle is how quickly the change took hold. One group of geochemists reads the transition as a single event completed within a few million years, pointing to the abruptness of the mineral shift. Another reads the same layers as the end of a much longer process, arguing that oxygen was produced locally for hundreds of millions of years before it accumulated globally, and that the apparent abruptness is a feature of how sediment preserves a signal rather than of the signal itself.
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