The theory that eukaryotic cells evolved from bacterial ancestors by a series of symbiotic associations. In its most modern form, it suggests that the mitochondria and microtubule organizing systems of present-day eukaryotes evolved from bacteria and spirochaetes that lived as symbionts in a line of single-celled eukaryotes that were the ancestors of both fungi and animals. A subline of these protoctists subsequently entered an endosymbiosis with cyanobacteria. These evolved into chloroplasts, and the algae and plant lineages developed from this group. See Chronology, 1978, Schwartz and Dayhoff; 1981, Margulis; 1986, Shih et al.; cryptomonads, cyanelles, endosymbiont theory, Pelomyxa, ribosomes of organelles, Rickettsia prowazeki, symbiogenesis.
Subjects: Genetics and Genomics.