The endosymbiotic theory explains how certain components of eukaryotic cells originated from bacteria that became permanent residents inside other cells. Its best-established claims concern mitochondria and plastids, including chloroplasts. These organelles descended from bacterial partners that gradually lost their independence while becoming integrated into their hosts. The theory therefore describes a major process in evolution: the formation of new cellular organization through the combination of previously separate organisms. Molecular evidence strongly supports these bacterial origins, although the circumstances of the earliest partnerships remain incompletely understood. (comptes-rendus.academie-sciences.fr)
Historical development
Ideas about the symbiotic origins of organelles developed before their genetic material could be examined. In 1905, the Russian botanist Konstantin Mereschkowsky argued that plastids were descendants of cyanobacteria, then commonly called blue-green algae. Ivan Wallin advanced a bacterial-origin hypothesis for mitochondria during the 1920s. These proposals initially lacked the molecular evidence needed to distinguish them decisively from explanations in which organelles arose within a single ancestral cell. (pmc.ncbi.nlm.nih.gov)
Lynn Margulis, publishing as Lynn Sagan, revived and expanded these ideas in her March 1967 paper “On the origin of mitosing cells.” She proposed that mitochondria, photosynthetic plastids, and flagellar basal bodies had originated as separate prokaryotic organisms. Subsequent research supported the mitochondrial and plastid components, but her proposed bacterial origin of the flagellar apparatus did not become part of the accepted core theory. Modern usage consequently distinguishes well-supported organelle origins from broader historical versions of serial endosymbiosis. (pubmed.ncbi.nlm.nih.gov)
Mitochondrial and plastid origins
Mitochondria descend from a bacterial lineage closely related to Alphaproteobacteria. Identifying its precise position remains difficult: different phylogenetic analyses have placed mitochondria within that group or as its sister lineage. These disagreements concern the identity of the closest bacterial relatives, rather than whether mitochondria have a bacterial origin. (nature.com)
Evidence also connects the host lineage involved in eukaryotic origins with archaea. However, the host’s structure, the mechanism by which the partners became associated, and the order in which mitochondrial acquisition and other eukaryotic characteristics appeared remain research questions. Illustrations showing a fully formed eukaryotic cell simply swallowing a bacterium represent one proposed scenario, not an established reconstruction of the original event. (comptes-rendus.academie-sciences.fr)
The conventional plastids of green algae, red algae, glaucophytes, and land plants trace to a cyanobacterial endosymbiont in the ancestor of Archaeplastida. This acquisition introduced bacterial machinery for photosynthesis into a eukaryotic lineage. Plastids subsequently spread into additional groups through further partnerships between eukaryotic organisms. (annualreviews.org)
Evidence for bacterial ancestry
Several independent observations support endosymbiotic origins. Mitochondria and plastids retain their own DNA, encoding a limited subset of the genes required for their activities. Their genetic systems preserve bacterial affinities, and both contain ribosomes descended from bacterial translation machinery. They proliferate through division of pre-existing organelles rather than appearing independently from unrelated cellular structures. (comptes-rendus.academie-sciences.fr)
Their surrounding membranes provide another line of evidence. The two envelope membranes of mitochondria and primary plastids retain relationships to the membranes of their bacterial ancestors. Nevertheless, membrane number alone cannot establish an endosymbiotic origin: membranes can be lost or remodeled, and other cellular compartments also possess multiple membranes. (molevol.hhu.de)
The most decisive evidence comes from molecular phylogenetics. Comparisons of organellar sequences with bacterial sequences recover mitochondrial relationships with alphaproteobacterial lineages and plastid relationships with cyanobacteria. This evidence tests common ancestry directly, whereas similarities in size, shape, or division provide supporting observations. (comptes-rendus.academie-sciences.fr)
From resident bacterium to organelle
An organism living within another cell is an endosymbiont, but intracellular residence does not automatically make it an organelle. Organelle formation involves increasing dependence, coordinated reproduction, exchange of metabolites, and integration of genetic control. During this transition, bacterial genomes undergo extensive reduction as genes become unnecessary, disappear, or move into the host genome. (pmc.ncbi.nlm.nih.gov)
Endosymbiotic gene transfer relocated many ancestral organellar genes to the host’s nucleus. When transferred genes retained organellar functions, their products needed a route back into the compartment. Modern mitochondria and plastids therefore import most of their proteins through specialized targeting and transport systems. Genetic information and biochemical activity became distributed across host and organelle, making the descendants fundamentally different from independently living bacteria. (pmc.ncbi.nlm.nih.gov)
Integration also reorganized metabolism. Transport across organellar membranes connects processes occurring inside the organelle with those elsewhere in the host. Gene acquisition from additional organisms through horizontal gene transfer has also contributed to organelle evolution; not every bacterial-derived nuclear gene necessarily came from the original endosymbiont. (annualreviews.org)
Repeated endosymbiosis and comparative examples
Primary endosymbiosis involves the incorporation of a prokaryotic partner. In secondary endosymbiosis, a eukaryotic host incorporates another eukaryote already containing a plastid. Secondary plastids commonly have three or four surrounding membranes. Cryptophytes and chlorarachniophytes retain a nucleomorph, a highly reduced remnant of the engulfed alga’s nucleus, providing particularly clear evidence of this nested ancestry. Further transfers through tertiary endosymbiosis have added complexity to plastid histories. (pmc.ncbi.nlm.nih.gov)
Photosynthetic species of Paulinella provide an independent example of a cyanobacterium becoming an organelle, separate from the origin of Archaeplastida plastids. Their photosynthetic compartment, called a chromatophore, exhibits genome reduction, host-directed protein import, and metabolic integration. Comparisons between this system and conventional plastids allow researchers to investigate which features repeatedly accompany the transition from endosymbiont to organelle. (pmc.ncbi.nlm.nih.gov)