Period· The columns of time
Ectasian
1.4 Ga → 1.2 Ga
Two hundred million years with almost no landmarks, and the stretch of the chart where sex, arguably the most consequential of inventions, is usually placed.
What the evidence shows
No stretch of the chart is emptier: the period takes its name from sedimentary covers that simply go on spreading, and across its 200 million years it records no glaciation, no extinction, no upheaval that has left a mark. What is happening is happening inside cells. Sex is two steps that undo each other, meiosis halving a genome and fertilisation restoring it, and the genes that run meiosis turn up across the eukaryotes, including lineages long thought to be strictly asexual. That distribution places the machinery no later than their common ancestor, which is to say deep in the Proterozoic, at a moment no method can yet fix. Its consequence is arithmetical: an offspring is a reshuffle of two parents rather than a copy of one, and variation stops depending on mutation alone.
Bounds and conventions
Behind its two round numbers there is nothing at all. The origin of sex has no date either: it is placed here from the spread of meiotic genes among living lineages and from molecular clocks, which give a stretch of time rather than a moment, and the selective advantage that favoured it is still argued over.
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.
- Ramesh et al. (2005), Current Biology · A phylogenomic inventory of meiotic genes: evidence for sex in Giardia and an early eukaryotic origin of meiosis
- Hodgskiss & Sperling (2023), Annual Review of Earth and Planetary Sciences · Deconstructing the Lomagundi-Jatuli carbon isotope excursion
- Mukherjee et al. (2018), Scientific Reports · The Boring Billion, a slingshot for complex life on Earth