Symbiogenesis theory is an evolutionary theory that explains the origins of eukaryotic cells from prokaryotes. It states that several key organelles of eukaryotes originated as a symbiosis between separate singlecelled organisms. According to this theory, mitochondria, plastids, and possibly other organelles representing formerly freeliving bacteria were taken inside another cell as an endosymbiont around 1.5 billion years ago. Molecular and biochemical evidence suggest that mitochondria developed from proteobacteria and chloroplasts from cyanobacteria.
