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...ltaneously, but happened in what scientist’s term as serial endosymbiosis. This simple explains that the larger heterotrophic cell engulfed the smaller cells at separate times in the stage of its life. The two prokaryotes probably gained entry into the larger cell as an internal parasite or undigested prey. Early endomembrane and cytoskeleton evolution made it possible for the larger cell to engulf the smaller ones and wrap them in vesicles. The closest relative to what the larger cell could have engulfed, according to researchers, are alpha proteobacteria, and the closest relative to photosynthetic bacteria is cyanobacteria. These to bacteria have also evolved but they have in common the fact that they both share a common ancestry to the bacteria that gave start to chloroplast and mitochondria. Secondary symbiosis would better explain why each organelle has a double membrane. For exam...