The most likely ancestor for today's mitochondria according to the endosymbiosis hypothesis is a:

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The endosymbiosis hypothesis suggests that mitochondria evolved from free-living bacteria that were engulfed by a host cell. The closest relative of mitochondria is Rickettsia, which is an intracellular bacteria that lives inside the cells of its host, similar to mitochondria. Bacillus, Clostridium, and Escherichia are all bacteria that do not have the same intracellular lifestyle as mitochondria, and cyanobacteria are not likely ancestors of mitochondria.

The Most Likely Ancestor for Today's Mitochondria According to the Endosymbiosis