Eon· The columns of time
Proterozoic
2.5 Ga → 539 Ma
Two billion years, the longest eon of all, and the one in which oxygen enters the air, the cell becomes complex, and the planet freezes over more than once.
What the evidence shows
Its first act is the Great Oxidation Event, dated between 2.46 and 2.43 billion years ago, when free oxygen builds up in the atmosphere for the first time: the largest chemical change in the planet's history, and a poison to everything that had evolved without it. The rise was not steady, and seems to have swung back and forth for another two hundred million years before it held. Complex cells arrive much later, and not because of it. Genomic and fossil evidence places modern eukaryotes after 1.84 billion years, more than half a billion years after the air changed. Animals may appear late in the eon, in reefs of about 890 million years holding structures that, if they are sponge tissue, would be the oldest animal bodies known. The eon closes just short of the moment when hard parts become common and the fossil record turns easy to read.
Bounds and conventions
Its younger bound, 538.8 million years, is one of the best defined on the chart: a golden spike in Newfoundland at the first appearance of a particular burrow. Its older bound is the round 2.5 billion years inherited from the eon before, and the oxidation that opens the eon's story began forty to seventy million years after it.
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.
- Gumsley et al. (2017), PNAS · Timing and tempo of the Great Oxidation Event
- Betts et al. (2018), Nature Ecology & Evolution · Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin
- Turner (2021), Nature · Possible poriferan body fossils in early Neoproterozoic microbial reefs