Era· The columns of time
Neoarchean
2.8 Ga → 2.5 Ga
Three hundred million years of oxygen made and swallowed at once: cyanobacteria are already splitting water, and the ocean takes back everything they release.
What the evidence shows
Oxygenic photosynthesis is the single most consequential invention in the history of life, and it belongs to the Archean, to this era or the one before, roughly 3.2 to 2.8 billion years ago. Its effect is delayed by hundreds of millions of years: dissolved iron in the oceans takes up the oxygen and precipitates, laying down the banded iron formations we still mine for iron today. Free oxygen never accumulates in the air, and the era closes with the atmosphere unchanged.
Bounds and conventions
A long gap separates the invention from its consequence, and neither end of that gap is sharply dated. What earns the era a place here is a process that began before it and finished after it, so its edges mark nothing in the story they contain.
Sources
- Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
- Fournier et al. (2021), Proceedings of the Royal Society B · The Archean origin of oxygenic photosynthesis and extant cyanobacterial lineages
- Bekker et al. (2010), Economic Geology · Iron formation: the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes