συμβίωσις

The Greek words syn (συν, 'together') and bios (βίος, 'life') combine to make symbiosis (συμβίωσις)—'living together.' In 1879, Heinrich Anton de Bary, a German botanist, used the term to describe the intimate relationship between fungi and algae that forms lichen. They lived not just near each other but merged into a single organism that was neither fungus nor algae alone.

De Bary was studying how algae survived inside fungal tissues. The relationship didn't fit existing categories. It wasn't parasitism (where one organism damages the host). It wasn't predation. It was two organisms so merged that they'd lost individual identity. He needed a word for this. He created one: symbiosis.

The discovery exploded in scope. Mycorrhizal fungi live inside plant roots, trading nutrients for sugars. Mitochondria—the powerhouses of your cells—are actually enslaved bacteria that your ancestors absorbed two billion years ago. Coral animals are partnerships with zooxanthellae algae. Cows and termites depend entirely on bacteria in their guts to digest food. Life is soaked in symbiosis.

De Bary's word spread because it named something true and common that biologists hadn't recognized before. Every major evolutionary step seems to involve organisms merging: the first eukaryotic cell was a symbiosis of bacteria. You are a walking colony of symbiotic partnerships. The word 'symbiosis' is so ordinary now that we forget how radical it was: the idea that life is fundamentally about living together.