Life › Complex cells & first multicellular life › Eukaryotes: complex cells with a nucleus
Chloroplasts: algae capture cyanobacteria
by c. 1.6 billion years ago? (estimates range 1.9–1.2 billion)· Evolutionary innovation· Plants & fungi
Strong evidenceGood evidence; dates or details still being refined.
The light-harvesting compartments of plant and algal cells, descended from cyanobacteria taken in by an early eukaryote.
See it on the Life map →What happened
A eukaryote that already had mitochondria engulfed a cyanobacterium and kept it, turning it into a chloroplast that runs oxygen-making photosynthesis. Its descendants became three groups: green algae and land plants, red algae, and glaucophytes. Later, other eukaryotes swallowed whole algae, passing chloroplasts on secondhand to kelp, diatoms and others. A separate, much younger capture happened in the amoeba Paulinella, about 100 million years ago. Estimates for the main event range from 1.9 to 1.2 billion years ago.
Why it matters
It put photosynthesis inside eukaryotes, creating algae and eventually plants, the base of most food chains and a major source of today's oxygen.
The evidence (fossils, genes, rocks)
Chloroplasts keep their own DNA, which matches cyanobacteria (closest living relative: Gloeomargarita), plus bacterial-style ribosomes and double membranes; glaucophyte chloroplasts even keep a bacterial cell wall.
Still debated
Exactly when it happened, and why such a useful capture apparently succeeded only twice in Earth's history.
Connections (4)
▸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.
▸Chloroplasts: algae capture cyanobacteria power red algae like Bangiomorpha: the oldest firmly identified red algaWell established
The connection
Red algae, like green algae and plants, descend from the eukaryote that first captured a cyanobacterium. Because Bangiomorpha closely resembles living red algae, it implies the chloroplast had already been acquired by about 1.05 billion years ago, setting a minimum age for that event.
When and how
Somerset Island, Arctic Canada, about 1.05 billion years ago; described by Nicholas Butterfield in 2000.
The evidence
Bangiomorpha's shape and cell-division pattern match living Bangia, which has red-algal chloroplasts; molecular clocks use it as a calibration point.
Debates and open questions
Its chloroplasts are not preserved directly; the link rests on comparison with living red algae. Older possible red algae could push the date back.
▸Lynn Margulis championed the bacterial origin of Chloroplasts: algae capture cyanobacteriaWell established
The connection
Russian botanist Konstantin Mereschkowski suggested in 1905 that chloroplasts, the green bodies that do photosynthesis in plant cells, were captured cyanobacteria. Margulis revived and extended the idea in 1967, arguing that algae and plants gained photosynthesis by engulfing such bacteria. Like mitochondria, chloroplasts have their own DNA and bacteria-like ribosomes.
When and how
Mereschkowski, 1905; Margulis, 1967 and her 1970 book; supported in the 1970s by RNA comparisons, including work by Ford Doolittle's group in Canada.
The evidence
Chloroplast genes sit inside the cyanobacteria branch of the gene tree, and the amoeba Paulinella shows a separate, younger capture within roughly the last 100 million years.
Debates and open questions
Whether all ordinary chloroplasts trace to a single capture, and which living group of cyanobacteria is closest to the original partner.
▸Chloroplasts: algae capture cyanobacteria later powered Plants colonize landWell established
The connection
Every land plant inherited chloroplasts, the light-harvesting compartments that began as cyanobacteria captured by an algal cell roughly a billion years earlier. Land plants descend from green algae carrying these chloroplasts, so the same photosynthesis machinery that evolved in water ran the first plants on land.
When and how
Chloroplasts arose roughly 1.9–1.2 billion years ago in early algae; their green-algal descendants moved onto land about 470 million years ago.
The evidence
Chloroplasts have their own DNA, closely related to that of cyanobacteria, and land-plant chloroplasts closely match those of green algae.
Debates and open questions
When the original capture happened is debated, and how land-plant chloroplasts adapted to stronger, drier sunlight is still studied.