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Joined 2026-09-21 Message
Incoming links: Endosymbiotic theory
Endosymbiosis 2026-10-13
Endosymbiosis is an ecological association in which one organism, the endosymbiont, lives inside another. The definition specifies location, not that both partners benefit, nor that the internal partner has become an organelle. An intracellular association can remain an association between distinct organisms. Endosymbiotic theory addresses a different question: how particular ancestral bacteria became inherited, functionally integrated organelles during evolution.
The striking consequence is that a eukaryotic biological cell can combine histories: its nuclear lineage and its mitochondria need not have the same ancestry. Mitochondria have bacterial ancestry related to Alphaproteobacteria; the plastids of plants and many algae, including chloroplasts, have ancestry within cyanobacteria. These are ancestral lineages, not a claim that a named living bacterial species is the literal ancestor. Present-day bacteria and organelles have both evolved since their shared ancestors. [ref]
Origin and Evolution of Plastids and Photosynthesis in Eukaryotes by Geoffrey I. McFadden. Opening paragraphs and “The endosymbiont: when and who?”: bacterial origins and uncertainty about closest relatives
From an internal partner to an integrated organelle
. Original conceptual schematic, not an observed historical sequence. Persistence, metabolic exchange, endosymbiotic gene transfer, organelle protein import and coordination of division illustrate integration; they are not dated stages in a compulsory order. The illustrated partner is generic, and the host outline does not specify the ancestral host of mitochondria. The protein-delivery arrow abbreviates nuclear gene expression, biological translation in the cytosol and organelle protein import; it does not place protein synthesis inside the cell nucleus. Details.
Endosymbiotic theory 2026-10-13
Endosymbiotic theory explains the bacterial origins of mitochondria and plastids through enduring internal associations and subsequent integration. Survival inside a biological cell is only part of the problem: exchange through membrane transport, inherited division, gene expression and delivery of required proteins must work together. The elegance is evolutionary reuse of an existing cellular capability, followed by changes that make the partners operate as one system. [ref]
Origin and Evolution of Plastids and Photosynthesis in Eukaryotes by Geoffrey I. McFadden. Abstract and “How to tame your symbiont”: metabolic, genetic and division integration
The historical On the origin of mitosing cells by Lynn Sagan (1967, abstract and Introduction, pp. 225–226) synthesized a symbiotic account of eukaryotic origins and proposed tests using molecular evidence. Its Introduction explicitly acknowledged earlier proposals. It also hypothesized origins for other structures; the modern evidence for mitochondria and plastids should be assessed separately from the rest of that historical scheme.
Plastid 2026-10-13
A plastid is an organelle of the lineage that includes chloroplasts. A chloroplast is a photosynthetic plastid; the terms are not interchangeable because some plastids are nonphotosynthetic. Endosymbiotic theory connects plastid origins to cyanobacteria, while endosymbiotic gene transfer and organelle protein import explain genetic and functional integration with the host.