What does endosymbiosis mean?
Endosymbiosis is a biological phenomenon in which one organism lives inside the body — or more specifically the cells — of another, with the two typically forming a mutually beneficial partnership. The word is most closely associated with the endosymbiotic theory, which holds that complex eukaryotic cells evolved when ancient host cells engulfed bacteria; rather than digesting them, they kept them, and these internal partners gradually became the organelles we now call mitochondria and chloroplasts. Beyond this famous case, endosymbiosis describes many living partnerships, such as the algae dwelling within coral polyps that power reef ecosystems. Used mainly in scientific contexts, the term carries connotations of cooperation and deep evolutionary history, illustrating how two distinct organisms can become so integrated that they function as a single unit. Its plural is 'endosymbioses', and the adjective form is 'endosymbiotic'.
nounA symbiotic relationship in which one organism lives inside the cells or body of another, typically to the mutual benefit of both. The term is especially associated with the theory that eukaryotic cells arose when ancestral host cells engulfed bacteria that became mitochondria and chloroplasts.
- A mutually beneficial relationship in which one organism lives inside the body or cells of another.
- Specifically, the evolutionary process by which eukaryotic organelles such as mitochondria and chloroplasts originated from engulfed prokaryotes.
"According to the theory of endosymbiosis, the mitochondria in our cells were once free-living bacteria."
"Lynn Margulis championed the idea that endosymbiosis explains the origin of mitochondria and chloroplasts."
"The coral reef depends on endosymbiosis: tiny algae live inside coral polyps, photosynthesizing and providing them with energy."
Standard plural following the Greek-derived '-sis' → '-ses' pattern (as in 'symbiosis/symbioses').
"Different endosymbioses have arisen repeatedly throughout evolutionary history."
Every breath you take is thanks to endosymbiosis — billions of years ago, one cell swallowed another and never let go, and that swallowed bacterium became your mitochondria.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of endosymbiosis
Endosymbiosis derives from Greek roots: 'endo-' meaning 'within' or 'inside', 'sym-' meaning 'together', and '-biosis' meaning 'way of living', ultimately from the Greek word 'bios' (life). The broader term 'symbiosis' was popularized in biology in the late nineteenth century by the German mycologist Anton de Bary, and 'endosymbiosis' emerged shortly afterward as biologists distinguished partners living inside the host from those living outside. Cognates sharing the same root include 'biology', 'antibiotic', and 'amphibious', all built from Greek elements relating to life.
Related word forms
How endosymbiosis is actually used
A technical term used chiefly in biology, ecology, and discussions of evolution. In everyday writing it appears most often in reference to the endosymbiotic theory of cellular origins. Plural: 'endosymbioses'.
Easily confused with endosymbiosis
Endosymbiosis refers to a symbiont living inside its host's body or cells, whereas exosymbiosis refers to a symbiont living on the exterior surface of its host.