Any type of a close and long-term interaction between two different biological organisms, be it mutualistic, commensal, or parasitic. The organisms, each termed a symbiont, may be of the same or of different species.

An interaction between organisms that live closely together. Normally when we think of symbiosis we think of something that benefits both organisms in some way—but in reality, symbiosis can be neutral to one of the partners or can even harm one of them. It all depends on the context.

Lynn Margulis, a celebrated, boundary-pushing biologist, argued that we have to understand symbiosis in order to understand how every living thing on Earth evolved. Organisms in symbiosis—from clownfish and anemones to rhinos and tick birds—have their reasons for sticking together. And inevitably, there are outcomes and consequences. (Evolutionarily or otherwise.) It’s complicated.