According to the endosymbiont theory, mitochondria and chloroplasts originated as bacteria which were engulfed by larger cells. How many times is it estimated that this occurred in the past? Are there any examples of this process being observed directly?
Answer
Well, it seems quite obvious that it was not a single I-eat-you-but-you-survived act but rather a convergence of endosymbiotic and host species into a greater and greater cooperation. Of course this leaves a question if there was one or more species of endosymbionts involved.
Mitochondria are a very primeval story forced by the oxygen catastrophe, so it is hard to say, although great majority of mitochondria seems to have a single origin.
Plastids are much more divergent, however it seems that they did originated from a single source, diverged into chloroplasts, cyanelles and rhodoplasts and were later mixed up by numerous acts of secondary and even tertiary endosymbiosis (plus a further evolution); this variety can be especially seen within Euglenas, and they are the main group investigated in this manner.
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