Era· The columns of time

Paleoproterozoic

2.5 Ga → 1.6 Ga

Nine hundred million years, and the most violent change the planet has undergone: the air fills with oxygen, and almost everything alive is poisoned by it.

What the evidence shows

The Great Oxidation Event is dated between 2.46 and 2.43 billion years ago, and a second line of evidence places a rapid rise at 2.33 billion. Free oxygen is a poison to the anaerobic life that had held the planet for more than a billion years, though how much of it came through is beyond any count we can make. Complex cells arrive well after the change once thought to have made room for them: genomes and fossils read together put modern eukaryotes no earlier than 1.84 billion years, half a billion years after the air changed, which leaves the two as separate stories.

Bounds and conventions

Both of the era's bounds were set at round figures, and the first falls a few tens of millions of years before the event that gives the era its interest. That transition is dated in two ways that do not quite agree, and whether the rise was a single step or a run of oscillations is still argued: the difference between a threshold crossed once and a planet that took another two hundred million years to settle.

Sources

1. 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.
2. Gumsley et al. (2017), PNAS · Timing and tempo of the Great Oxidation Event
3. Luo et al. (2016), Science Advances · Rapid oxygenation of Earth's atmosphere 2.33 billion years ago
4. Betts et al. (2018), Nature Ecology & Evolution · Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin