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.