The Serial Endosymbiont Theory
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The serial endosymbiont theory proposes that mitochondrion and plastids such as chloroplasts of the modern day eukaryotic cell evolved from different types of bacteria that were engulfed by prokaryotes through endophagocytosis. An endosymbiont is any organism that lives within another cell or organism.
It is believed that the mitochondrion of todays organisms was first an aerobic form of bacteria that was ingested by an anaerobic prokaryote but the aerobic bacteria wasnt digested. Inside the anaerobic cell the aerobic bacterium would have been able to flourish in the cytoplasm of the host cell feeding on the partially digested food particles. The aerobic bacterium would have been able build up huge stores of energy using oxygen to breakdown the particles. With the abundance of energy inside the aerobic bacterium it is believed that some of it might have leaked from the bacterium into the anaerobic prokaryote in the form of ATP. This would have allowed the anaerobic cell to break down food using the aerobic pathway with the help of the bacterium. Eventually the aerobic bacterium wouldnt be able to sustain life outside the anaerobic prokaryote and would have become a mitochondrion.
Plastids such as chloroplasts were developed in much the same way as mitochondrion. The one exception is chloroplasts started from cyanobactrium, a type of blue green algae. The host cell and the bacterium formed a symbiotic relationship and the cyanobactrium evolved into the first chloroplast.
This theory was first proposed by Andreas Schimper in 1883. However it didnt become widely accepted until the 1980s when it was popularized by Lynn Margulis in her book the Origin of Eukaryotic Cells.
Evidence for this theory can be found within a mitochondrion or plastid. The most striking is that both of these contain a small store of DNA that is different from the cells nucleus and is similar to that of bacteria, its circular in shape. Both also contain two or more membranes the innermost resembling that of a bacterial membrane.
This theory is mainly trying to explain how eukaryotic cells came into being and how they were formed from anaerobic prokaryotes and bacteria.