History › Earth & early life › Great Oxidation Event
Oxygen-making photosynthesis (cyanobacteria)
by c. 2.7–3 billion years ago· Event· Life & early humans
DebatedHistorians or scientists disagree about what happened, when, or why.
Cyanobacteria, tiny blue-green microbes, learned to split water using sunlight and released oxygen as waste.
See it on the History map →What happened
Earlier microbes already used sunlight, but without making oxygen. Cyanobacteria found a way to pull hydrogen from water, giving off oxygen gas, which gave them a nearly unlimited raw material. Plants later borrowed the trick: their chloroplasts descend from captured cyanobacteria. When it began is debated, with estimates from before 3 billion years ago to just before the Great Oxidation Event.
Why it matters
It underpins almost every food chain today and is the source of nearly all the oxygen in the air.
How we know
Chemical traces of oxygen in rocks older than 2.5 billion years, fossil microbial mats, and genetic family trees of cyanobacteria. Each line of evidence gives different dates.
Still debated
If oxygen-making began hundreds of millions of years before oxygen built up in the air, why the long delay? That is debated.
Connections (2)
▸First life gave rise to Oxygen-making photosynthesis (cyanobacteria)Mostly agreed
The connection
Early microbes fed on chemicals or used sunlight without releasing oxygen. Over hundreds of millions of years, the ancestors of cyanobacteria combined two light-capturing systems into one that could split water. That gave them an almost unlimited fuel supply, with oxygen released as waste.
When and how
In oceans and shallow waters, by about 2.7–3 billion years ago, possibly earlier.
How sure are we?
Photosynthesis genes show family links between cyanobacteria and other light-using bacteria; ancient rocks contain chemical signs of early oxygen.
Debates and open questions
How and when the two light-capturing systems came together are actively debated.
▸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.