Life › The microbial world
Oxygen-making photosynthesis
c. 3.0–2.7 billion years ago (debated)· Evolutionary innovation· Microbes & cells
DebatedScientists disagree about when, how or why.
Using sunlight to split water, building sugar from carbon dioxide and releasing oxygen as waste.
See it on the Life map →What happened
Older forms of photosynthesis took electrons from scarcer sources, such as hydrogen sulfide or dissolved iron, and released no oxygen. The breakthrough was linking two light-capturing systems and adding a manganese-calcium cluster able to pull electrons from water. Water is everywhere, so life's food production was no longer limited by scarce chemicals. Dating is debated: chemical hints of oxygen appear around 3 billion years ago, but some argue it evolved only shortly before 2.4 billion.
Why it matters
It became the base of nearly every food chain and supplied the oxygen in the air, the ozone layer and, eventually, the energy budget of animals.
The evidence (fossils, genes, rocks)
Chemical traces of oxygen in ancient rocks (chromium, molybdenum and sulfur isotopes), gene studies of photosynthesis proteins, and fossils of cyanobacteria.
Still debated
How early it began, and why oxygen then took so long to build up in the air, are major open questions.
Inside this (1)
Connections (3)
▸Cyanobacteria invented Oxygen-making photosynthesisStrong evidence
The connection
Ancestors of cyanobacteria linked two older light-capturing systems into one chain and added a manganese cluster that pulls electrons from water, releasing oxygen as waste. They are the only bacteria known to do this. Plants and algae later gained the ability by absorbing cyanobacteria as chloroplasts.
When and how
Sometime before about 2.4 billion years ago, possibly by 3 billion or earlier; the exact date is debated.
How sure are we?
Gene studies of photosynthesis proteins, non-photosynthetic cousins of cyanobacteria found in 2013, and chemical signals of oxygen before 2.4 billion years ago.
Debates and open questions
Whether earlier, now-extinct microbes made oxygen first, and how the two light systems came together, possibly by gene swapping.
▸Oxygen-making photosynthesis caused Great Oxidation EventWell established
The connection
For perhaps hundreds of millions of years, the oxygen that cyanobacteria made was mopped up by iron in seawater and by volcanic gases. When oxygen production outran these 'sinks', the gas finally accumulated in the air, permanently changing Earth's chemistry.
When and how
Oxygen built up worldwide from about 2.43 billion years ago; the key sulfur-isotope evidence came from James Farquhar and colleagues in 2000.
The evidence
Oxygen-making photosynthesis is the only known source big enough; geological signs of free oxygen follow other evidence for cyanobacteria.
Debates and open questions
Why the delay was so long if oxygen photosynthesis began by 3 billion years ago; some argue it evolved only shortly before the rise.
▸Oxygen-making photosynthesis rusted dissolved iron into Banded iron formationsStrong evidence
The connection
Early oceans held large amounts of dissolved iron from vents. Oxygen released by cyanobacteria reacted with it, forming insoluble iron oxide, essentially rust, that settled to the seafloor. Alternating with silica layers, it built the striped banded iron formations, which soaked up oxygen before it could reach the air.
When and how
Mostly 2.8–1.8 billion years ago, in basins now in Western Australia, Minnesota, Brazil and South Africa; geologist Preston Cloud argued the link in the 1960s–70s.
How sure are we?
The rust layers need an oxidizer, and oxygen is the simplest one. But some bands could have formed through iron-using microbes or ultraviolet light, without oxygen.
Debates and open questions
How much of the iron was rusted by free oxygen versus oxygen-free microbes, and what caused the regular banding.