Why are mitochondria and chloroplasts believed to have entered early eukaryotic cells by endosymbiosis of bacteria?

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Mitochondria and chloroplasts are believed to have entered early eukaryotic cells by endosymbiosis of bacteria because they possess all of the characteristics of bacteria. They are the same size as bacteria and contain DNA. Additionally, they are not derived from the endomembrane system as other organelles in eukaryotic cells are. This supports the theory that early eukaryotic cells engulfed free-living bacteria, eventually forming a symbiotic relationship that led to the development of mitochondria and chloroplasts as we know them today.

Why Are Mitochondria and Chloroplasts Believed to Have Entered Early