Endosymbiosis
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Endosymbiotic theory holds that mitochondria and chloroplasts descend from free-living bacteria an ancestral host cell engulfed and kept rather than digested.
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Margulis's 1967 argument cited the size, DNA, and ribosomes mitochondria and chloroplasts share with bacteria as evidence for that ancestry.
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This entry teaches endosymbiosis as the field's well-supported account, not one option among equals.