Endosymbiotic Theory

Endosymbiotic theory holds that mitochondria and chloroplasts began as free-living bacteria that were taken inside larger cells and became permanent partners. The modern form of the idea was championed by the biologist Lynn Margulis in 1967, building on earlier proposals by Konstantin Mereschkowsky in 1905.

Both organelles have their own small, circular genome, as in mitochondrial DNA, and they divide by a process like binary fission. They are surrounded by two membranes, the inner one resembling a bacterial membrane, and they make proteins on ribosomes that resemble bacterial ones. Gene sequences place mitochondria within the alphaproteobacteria and chloroplasts within the cyanobacteria.

The mitochondrial partnership happened once, early in the history of eukaryotes. A later primary endosymbiosis of a cyanobacterium gave rise to the plastids of green plants, red algae and glaucophytes. In secondary endosymbiosis, a eukaryote took in another eukaryotic alga, which is why some algae and parasites such as apicomplexans have plastids with three or four membranes.

Over time most of the symbiont's genes moved to the host nucleus, which is why organelles depend on proteins imported from the cytoplasm. This is a kind of horizontal gene transfer that left only a small gene set behind. How the host cell arose is still debated, with Asgard archaea as the leading candidates, as discussed in eukaryogenesis.

• Mitochondria — one of the organelles involved
• Cyanobacteria — the ancestors of plastids
• Eukaryogenesis — the wider origin of complex cells