Life › The microbial world › Oxygen-making photosynthesis
Cyanobacteria
by c. 2.7–3.0 billion years ago (debated)· Organism or group· Microbes & cells
Strong evidenceGood evidence; dates or details still being refined.
The only bacteria that perform oxygen-making photosynthesis, and the ancestors of the chloroplasts in every plant and alga.
See it on the Life map →What happened
Once called blue-green algae, cyanobacteria live in oceans, lakes, soils and hot springs; some also turn nitrogen from the air into natural fertilizer. One, Prochlorococcus, discovered in 1988, is probably Earth's most abundant photosynthesizer, at an estimated 3 × 10²⁷ cells. Firm fossils date to about 2 billion years ago; chemical traces once thought older were later shown to be contamination. Gene studies place their origin around 3.0–2.7 billion years ago, with wide uncertainty.
Why it matters
They oxygenated the planet and, captured inside other cells as chloroplasts, gave rise to algae and plants; they still produce a large share of ocean oxygen.
The evidence (fossils, genes, rocks)
Microfossils such as those in the 1.88-billion-year-old Gunflint Chert; DNA showing chloroplasts descend from cyanobacteria; molecular clocks.
Still debated
When they first appeared and whether early forms already made oxygen; their non-photosynthetic relatives, found in 2013, complicate the story.
Connections (3)
▸Cyanobacteria built many later StromatolitesWell established
The connection
Once cyanobacteria evolved, they became the main builders of stromatolites, forming sunlit mats in shallow water. Stromatolite variety peaked about 1.25 billion years ago and then declined, perhaps as grazing animals and competing algae appeared. Today's Shark Bay stromatolites are also largely built by cyanobacteria.
When and how
From about 3 billion years ago (debated) to the present; Shark Bay, Western Australia, is the best-known living site.
The evidence
Cyanobacteria dominate living stromatolite mats, and fossil cyanobacteria appear in stromatolite-bearing rocks such as Canada's 1.88-billion-year-old Gunflint Chert.
Debates and open questions
Whether the oldest stromatolites, 3.48 billion years old, were built by cyanobacteria or by earlier microbes using photosynthesis that made no oxygen.
▸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.
▸Cyanobacteria were captured to become Chloroplasts: algae capture cyanobacteriaWell established
The connection
An early eukaryote engulfed a cyanobacterium and, instead of digesting it, kept it as an internal solar plant. The captive handed most of its genes to the host nucleus and became the chloroplast, carrying oxygen-making photosynthesis into algae and later plants.
When and how
Roughly 1.9–1.2 billion years ago, possibly in fresh water; the closest living relative, Gloeomargarita, was identified in 2017.
The evidence
Chloroplast DNA and ribosomes match cyanobacteria. Konstantin Mereschkowski proposed the link in 1905, Lynn Margulis revived it, and gene sequencing confirmed it.
Debates and open questions
The date is uncertain, and it is debated whether genes from other bacterial partners also helped turn the captive into a chloroplast.