Origins
The noun 'symbiosis' entered scientific and then general English in the late nineteenth century, coined by the German botanist Heinrich Anton de Bary in 1879 from Greek 'symbiōsis' (a living together, companionship), itself from 'symbioun' (to live together), a compound of 'syn-' (together, with) and 'bios' (life, way of living), the latter from Proto-Indo-European *gʷeyh₃- (to live).
The Greek word 'symbiōsis' was not originally a biological term. In classical Greek, it described human cohabitation or companionship — people living together. Aristotle used the related noun 'symbiōtēs' to describe people who shared meals or living quarters. The application to non-human organisms was entirely de Bary's innovation, though the Greek word provided the perfect etymological foundation: if 'bios' meant life, then 'symbiosis' meant living together — the co-existence of different organisms in intimate association.
De Bary's definition was deliberately broad. He defined symbiosis as 'the living together of unlike organisms,' a category that encompassed three distinct types of relationship: mutualism (both organisms benefit), commensalism (one benefits, the other is unaffected), and parasitism (one benefits at the other's expense). This broad definition remains standard in biology, though popular usage has narrowed 'symbiosis' to mean mutualistic relationships exclusively. When a journalist describes a 'symbiotic relationship' between two companies, they mean both are benefiting — a biologist would require more specificity.
Development
The study of symbiosis has transformed biology. The endosymbiotic theory — proposed by Lynn Margulis in 1967 and now widely accepted — holds that mitochondria and chloroplasts, the organelles that power animal and plant cells respectively, were once free-living bacteria that entered into symbiotic relationships with ancestral cells. The mitochondrion in every cell of your body was, billions of years ago, an independent bacterium. It was engulfed by a larger cell and, instead of being digested, formed a partnership: the bacterium provided energy; the host cell provided protection and nutrients. This symbiosis was so successful that it became permanent — the bacterium's descendants are now integral parts of every animal, plant, and fungal cell on Earth.
If Margulis was right — and the evidence strongly supports her — then symbiosis is not a marginal phenomenon but a driving force of evolution comparable to natural selection. Complex multicellular life did not evolve through competition alone but through cooperation: organisms merging, partnering, and integrating into new composite forms. The eukaryotic cell — the building block of all complex life — is itself a symbiotic chimera.
Examples of symbiosis pervade the natural world. Lichens are symbiotic partnerships between fungi and photosynthetic algae or cyanobacteria: the fungus provides structure and moisture retention; the alga or cyanobacterium provides food through photosynthesis. Coral reefs depend on the symbiosis between coral polyps and zooxanthellae (photosynthetic algae that live inside the coral's tissues), providing up to ninety percent of the coral's energy needs. The breakdown of this symbiosis — 'coral bleaching,' caused by rising ocean temperatures — is one of the most visible consequences of climate change.
Later History
The human body is itself a symbiotic system. The human microbiome — the trillions of bacteria, fungi, and other microorganisms that inhabit the gut, skin, mouth, and other surfaces — constitutes a vast symbiotic community. These organisms aid digestion, produce vitamins, train the immune system, and protect against pathogens. The number of microbial cells in and on a human body roughly equals the number of human cells. In a real sense, a human being is not an individual organism but a symbiotic consortium.
The figurative use of 'symbiosis' in social, economic, and political contexts extends the biological metaphor. A 'symbiotic relationship' between a university and the surrounding community implies mutual benefit: the university provides employment, research, and cultural resources; the community provides housing, services, and political support. The metaphor works because it captures the key feature of mutualistic symbiosis: neither party could thrive alone as well as they thrive together.
The Greek prefix 'syn-' (together, with — becoming 'sym-' before labial consonants like 'b') is extraordinarily productive in English. 'Symphony' (sounding together), 'synagogue' (gathering together), 'synthesis' (placing together), 'sympathy' (feeling together), 'synchronize' (timing together), 'syndrome' (running together) — each describes a different mode of togetherness. 'Symbiosis' adds 'living together' to this family.
Literary History
The root 'bios' (life) generates the '-bio-' family: 'biology' (study of life), 'biography' (writing about a life), 'antibiotic' (against life — against microbial life), 'amphibian' (living both ways — in water and on land), 'aerobic' (requiring air for life), and 'microbe' (small life). 'Symbiosis' sits at the intersection of these two prolific roots, naming the phenomenon where separate lives become intertwined — where 'living together' becomes a form of living that neither organism could achieve alone.