Symbiosis is the persistent, intimate association between organisms of different species. The term was coined by Heinrich Anton de Bary in 1879 and encompasses mutualism (both benefit), commensalism (one benefits, the other is unaffected), and parasitism (one benefits at the other’s cost). The common thread is obligate relational entanglement: the organisms’ lives are constituted through their association.
Lynn Margulis’s endosymbiotic theory (1967) demonstrated that the organelles of eukaryotic cells — mitochondria and chloroplasts — originated as free-living bacteria that entered into symbiotic relationships with ancestral cells. The eukaryotic cell is not a unitary organism; it is a symbiotic consortium. The boundary between “self” and “other” at the cellular level is a product of relational history, not an original given.
Fungal symbioses are among the oldest and most ecologically consequential. Mycorrhizal associations — partnerships between fungi and plant roots — date back over 400 million years and likely enabled the colonization of land by plants. Over 90% of vascular plant species form them today. Lichens are composite organisms in which a fungus and a photosynthetic partner generate a shared body (thallus) that neither could produce alone — a case where the symbiotic relation produces the organism, not the other way around. The breadth of fungal symbiosis, spanning mutualism, parasitism, and commensalism often within a single fungal lineage depending on context, demonstrates that symbiosis is not a fixed category but a dynamic relational process.
This has consequences that extend beyond cellular biology. The human body hosts approximately as many microbial cells as human cells. The gut microbiome participates in digestion, immune regulation, and even neurological function. The “individual” human organism is, biologically, a holobiont — a multi-species assemblage held together by metabolic and immunological relations. What counts as the organism is not determined by genetics alone but by the network of symbiotic relations that constitute the functioning whole.
For the relational framework, symbiosis is among the most direct biological demonstrations that relations precede relata. The eukaryotic cell does not exist prior to the endosymbiotic relation; it is the product of that relation. The holobiont does not contain its microbiome; it is partially constituted by it. Symbiosis shows that biological identity is relational identity — not a substance with fixed boundaries but a process of ongoing mutual constitution.
Related terms
- Fungal Symbiosis — the most ancient and ecologically widespread symbiotic associations on Earth
- Holobiont — multi-species assemblages constituted through symbiotic relations
- Homeostasis — maintained across symbiotic boundaries in holobionts
- Autopoiesis — self-production that, in eukaryotes, incorporates symbiotic components
- Mutual aid — the social analog of mutualistic symbiosis
- Niche Construction — how symbiotic partners construct each other’s environments