Answer:
Endosymbiosis
Explanation:
Endosymbiosis is a theory developed to explain the origin of eukaryotic cells from a prokaryotic origin. Organelles of the eukaryotic cells such as mitochondria and chloroplast were thought to have developed from this process.
According to this theory, a proto-eukaryote eats up or engulf a proto-mitochondrion or protochloroplast (in form of microbes) to become eukaryotic organelles such as mitochondrion and chloroplast respectively.