Period· The columns of time
Calymmian
1.6 Ga → 1.4 Ga
Life becomes big enough to see with the naked eye, under an air that carries at most a thousandth of today's oxygen.
What the evidence shows
Sediment spread in a quiet cover over stable continental platforms, and that cover gave the period its name; the platforms under it turned out to hold a surprise. In the Gaoyuzhuang Formation of North China, carbon films 1.56 billion years old preserve flat, tongue-shaped bodies up to 30 centimetres long and nearly 8 wide: a billion years before the Cambrian, something in the sea was already as long as a forearm. Elsewhere the record is microscopic and abundant: tough organic cysts called acritarchs, some smooth, some bristling with spines, many bearing the one neat opening through which their occupant left. Above them the air was thin in oxygen, at most a thousandth of the present level by the chromium isotopes, and thin in ozone with it, so ultraviolet reached the surface far more freely than now.
Bounds and conventions
Neither bound marks an event: both come from a decision. The Gaoyuzhuang fossils are firmly dated but not firmly identified: their size and the regularity of their shape argue for multicellular eukaryotes, their place within that group remains open, and a colonial bacterial reading has never been formally excluded. The oxygen figure is a ceiling obtained from a single proxy, and other proxies give higher values.
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.
- Zhu et al. (2016), Nature Communications · Decimetre-scale multicellular eukaryotes from the 1.56-billion-year-old Gaoyuzhuang Formation in North China
- Huntley et al. (2006), Precambrian Research · 1.3 billion years of acritarch history: an empirical morphospace approach
- Planavsky et al. (2014), Science · Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals