Eon· The columns of time
Archean
4.03 Ga → 2.5 Ga
A billion and a half years in which the planet acquires everything that matters, and almost none of it leaves a mark: continents, oceans, a magnetic field, and life.
What the evidence shows
Life shows up in the record early in the eon, and molecular clocks put its origin earlier still, in the Hadean. Every claim that old has been contested. At 3.43 billion years the case becomes hard to refuse: the Pilbara of Western Australia holds seven distinct forms of stromatolite, the laminated stone that microbial mats leave behind, across a tidal carbonate platform and on a scale nothing else of that age can match. No purely mineral origin proposed for them accounts for the pattern. Everything alive today descends from organisms of this eon, and none of them left a body fossil worth the name: what survives is what they built. The atmosphere holds no free oxygen, and would kill anything that breathes.
Bounds and conventions
Neither bound was placed at an event. The younger one, 2.5 billion years, is a round number, agreed because nothing in the record singles out the passage and a division had to go somewhere.
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.
- Nutman et al. (2016), Nature · Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures
- Allwood et al. (2006), Nature · Stromatolite reef from the Early Archaean era of Australia