Eon· The columns of time

Proterozoic

2.5 Ga → 539 Ma

The Proterozoic, from 2.5 billion to 539 million years ago, is the longest eon of the time scale: oxygen settled in the atmosphere, the eukaryotic cell appeared and the planet froze over several times.

Longest eon of the geological time scale, covering two billion years during which oxygen establishes itself in the atmosphere, the eukaryotic cell appears and the planet undergoes several global glaciations.

What the evidence shows

Its first episode is the Great Oxidation, dated between 2.46 and 2.43 billion years, when free oxygen accumulates in the atmosphere for the first time: the largest chemical change in the history of the planet, and a poison to the organisms that had evolved without it. The rise is not steady and appears to have oscillated for a further two hundred million years before stabilising. Complex cells appear much later, and with no direct causal link: genomic and fossil data place modern eukaryotes after 1.84 billion years, more than half a billion years after the atmosphere changed. Animals may appear towards the end of the eon, in reefs some 890 million years old whose structures may be sponge tissue. The eon closes immediately before mineralised hard parts become general.

Bounds and conventions

The younger limit, 538.8 million years, is among the best defined on the chart: a golden spike at Fortune Head, in Newfoundland, at the first appearance of the trace fossil Treptichnus pedum. The older limit is the round figure of 2.5 billion years inherited from the preceding eon, and the oxidation that opens the eon's history begins forty to seventy million years after it.