LIFE × EVOLUTION

Photosynthesis Learns to Split Water

~3.2–2.8 billion years ago

Archean · Mesoarchean

Sunlight is easy to catch; water is hard to break. Microbes ancestral to today's cyanobacteria found a way to tear the water molecule apart and take its electrons, discarding oxygen as waste: oxygenic photosynthesis, the most consequential invention in the history of metabolism, and one that, as far as anyone can tell, happened only once.

Why it matters

Every breath drawn since has come from this reaction. It also primed the crisis to come: an unstoppable source of a gas that was poison to almost everything then alive.

Dating & uncertainty

No fossil records the invention itself. Its age is read from the branching order of cyanobacterial genes and from chemical traces of oxygen in rocks long before there was any in the atmosphere, which together place it in the Mesoarchean, between 3.2 and 2.8 billion years ago. The machinery itself is not seen directly until far later: thylakoids, the stacked sheets where the reaction runs, preserved inside fossil cells 1.75 billion years old.

Sources

1. Fournier et al. (2021), Proceedings of the Royal Society B · The Archean origin of oxygenic photosynthesis and extant cyanobacterial lineages
2. Sánchez-Baracaldo et al. (2022), Trends in Microbiology · Cyanobacteria and biogeochemical cycles through Earth history
3. Lepot (2020), Earth-Science Reviews · Signatures of early microbial life from the Archean (4 to 2.5 Ga) eon
4. Demoulin et al. (2024), Nature · Oldest thylakoids in fossil cells directly evidence oxygenic photosynthesis

Earth history as one day

08:08:2715 h 52 min before midnight

if Earth’s 4.54 billion years were compressed into a single day