Period· The columns of time

Orosirian

2.05 Ga → 1.8 Ga

Mountains rise on every continent, the first supercontinent that can be mapped takes shape, and a cell swallows a bacterium and keeps it.

What the evidence shows

This is the period of great collisions, and its name says so, from the Greek for mountain range: continental blocks weld together along mountain belts and gather nearly all the world's land into a single mass, Columbia, the first assembly that has left enough rock to be drawn as a coherent map. Two of the largest impacts known also fall here, Vredefort at 2.02 billion years, whose crater simulations now put near 250 kilometres wide, and Sudbury at 1.85 billion. Around that same date the main global period of banded iron deposition ends: iron, sulphur and carbon measured in Canadian rocks of that age show a deep ocean still without oxygen but newly rich in sulphide, and it was the sulphide that stripped the dissolved iron out. And a bacterium ends up inside another cell and stays, becoming the mitochondrion that burns oxygen in complex cells; molecular estimates place that merger anywhere between 2.05 and 1.21 billion years, which is to say it may belong to this period or to any of several that follow.

Bounds and conventions

The bounds are conventional. The events the period is known for are dated by their rocks and not by it, and the most consequential of them, the appearance of the eukaryotic cell, is placed between 2.0 and 1.8 billion years by the fossils but a good deal later by several molecular clocks. The order in which the nucleus and the mitochondrion arrived is still argued.

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. Pisarevsky et al. (2014), Precambrian Research · Mesoproterozoic paleogeography: supercontinent and beyond
3. Allen et al. (2022), Journal of Geophysical Research: Planets · A revision of the formation conditions of the Vredefort crater
4. Poulton et al. (2004), Nature · The transition to a sulphidic ocean approximately 1.84 billion years ago
5. Betts et al. (2018), Nature Ecology & Evolution · Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin