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Great Oxidation Event
c. 2.4–2.1 billion years ago· Event· Universe & Earth
Well documentedSolid records, physical evidence or scientific consensus.
Between about 2.4 and 2.1 billion years ago, oxygen made by microbes first built up in Earth's air and shallow seas.
See it on the History map →What happened
For a long time, oxygen from cyanobacteria was soaked up by iron dissolved in seawater and by volcanic gases. Once those 'sinks' could not keep up, oxygen accumulated, though still far below today's levels. It rusted iron out of the oceans into vast banded deposits and may have poisoned many microbes. By removing methane, a warming gas, it may also have triggered long ice ages.
Why it matters
Oxygen made energy-hungry complex life possible and created the ozone layer. Much of the world's iron ore comes from banded rocks of this era.
Think of it like
Like a sponge: the oceans' dissolved iron soaked up oxygen until saturated, and only then did oxygen spill into the air.
How we know
Banded iron formations; ancient soils that suddenly show rusting; and the disappearance, about 2.4 billion years ago, of an unusual sulfur signature that forms only in oxygen-free air.
Still debated
Why oxygen rose when it did, after earlier brief 'whiffs', and why levels then stayed low for over a billion years are debated.
Inside this (1)
Connections (3)
▸Oxygen-making photosynthesis (cyanobacteria) caused Great Oxidation EventWell documented
The connection
Cyanobacteria released oxygen for millions of years. At first, iron in seawater and volcanic gases absorbed it. Once these sinks could no longer keep up, oxygen built up in the air and shallow oceans about 2.4 billion years ago, transforming Earth's chemistry.
When and how
Worldwide, about 2.4–2.1 billion years ago; recorded in rocks from South Africa, Australia and North America.
How we know
No other known process could produce so much free oxygen; rock records show oxygen rising after cyanobacteria were established.
Debates and open questions
Why the rise came perhaps hundreds of millions of years after photosynthesis began, possibly due to changes in volcanoes or continents, is debated.
▸Great Oxidation Event paved the way for First complex cells (eukaryotes)Mostly agreed
The connection
Breathing oxygen releases far more energy from food than processes that work without it. Once oxygen was available, cells with oxygen-using mitochondria could power larger, more complex bodies. Complex cells appear in the fossil record a few hundred million years after oxygen first rose.
When and how
In oceans worldwide, between about 2.4 and 1.6 billion years ago.
How sure are we?
The timing fits and mitochondria run on oxygen, but the link is indirect, inferred from the order of events in rocks.
Debates and open questions
Some scientists think early eukaryotes needed little oxygen, so the rise may have helped rather than caused their appearance.
▸Great Oxidation Event built the ozone layer that allowed Life's move onto landMostly agreed
The connection
Oxygen in the air forms ozone high in the atmosphere, which blocks much of the Sun's harmful ultraviolet light. The Great Oxidation began this shield, and a later oxygen rise about 800–540 million years ago strengthened it. Without it, life on bare land would have been badly damaged.
When and how
In the upper atmosphere, building gradually over the two billion years after oxygen first rose about 2.4 billion years ago.
How sure are we?
Basic chemistry shows sunlight turns oxygen into ozone, and models tie ozone strength to oxygen levels; exactly when the shield became strong enough is less certain.
Debates and open questions
How much ozone protection early land life actually needed, and when the shield was strong enough, are debated.