Era· The columns of time

Paleoproterozoic

2.5 Ga → 1.6 Ga

The Paleoproterozoic, from 2.5 to 1.6 billion years ago, opened with the most important chemical change in the planet's history: oxygen accumulating in an atmosphere where it was toxic to almost everything alive.

Era of nine hundred million years opened by the most important chemical change in the history of the planet: the accumulation of oxygen in the atmosphere, toxic to almost every organism then present.

What the evidence shows

The Great Oxidation is dated between 2.46 and 2.43 billion years, a second line of evidence placing a rapid rise at 2.33 billion. Free oxygen is a poison to obligate anaerobes, organisms whose metabolism is inhibited by its presence and which had occupied the planet for more than a billion years; no fossil record establishes which of them survived. Complex cells appear well after the upheaval to which their origin has been attributed: genomic and fossil data place modern eukaryotes no earlier than 1.84 billion years, half a billion years after the atmosphere changed, which makes two distinct histories.

Bounds and conventions

Both limits of the era are round numbers, and the first precedes by some tens of millions of years the event that gives it its interest. That transition is dated by two methods that do not exactly agree, and the question remains whether the rise was a single crossing or a succession of oscillations, an alternative distinguishing a threshold passed once from a planet requiring a further two hundred million years to stabilise.