“SYMBIOGENESIS or endosymbiotic theory, is an evolutionary theory which explains the origin of eukaryotic cells from prokaryotes. It states that several key organelles of eukaryotes originated as symbiosis between separate single-celled organisms. According to this theory, mitochondria and plastids (e.g. chloroplasts), and possibly other organelles, represent formerly free-living bacteria that were taken inside another cell as an endosymbiont, around 1.5 billion years ago.”
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