Your cells' power plants were once free-living bacteria
Symbiosis is not just clownfish and anemones. Biologists think it built the complex cell itself, when ancient bacteria were absorbed and became mitochondria and chloroplasts. Partnerships between species also produced lichens, flower pollination and nitrogen-fixing root nodules, yet scientists argued for 130 years over what the word should even cover.
The term comes from Greek for living together. Albert Bernhard Frank used it in 1877 for the partnership inside lichens, and the German mycologist Heinrich Anton de Bary soon broadened it to the cohabitation of unlike organisms. Since then some scientists have wanted it reserved for lasting mutual benefit, while others include any long-term interaction, parasitism among them, and exclude only brief encounters such as predation. In 1949 Edward Haskell proposed a classification of 'co-actions' that biologists later adopted as interactions.
The categories depend on who gains. Mutualism benefits both partners; commensalism helps one while barely affecting the other, like barnacles riding on baleen whales; parasitism helps one at the other's cost. Amensalism harms one side only, and competition harms both. Competition can even occur among cells within one body, which is part of why old cells are cleared from tissues. Partnerships are obligate if a partner cannot live alone and facultative if it can: in lichens, the fungus is stuck, while the alga or cyanobacterium can usually survive by itself.
Location matters too. Ectosymbionts live on a host's surface, including lice on people. Endosymbionts live inside tissues, such as single-celled algae in reef-building corals, rhizobia in legume root nodules and Frankia in alder roots. Bacterial endosymbionts supply essential nutrients to roughly 10 to 15 percent of insects.
Life inside a host carries a cost for the guest. Endosymbiotic bacteria often shed genes for metabolism and DNA repair. Passed down from insect parent to offspring in small numbers, they cannot restore lost functions by recombination, a trap known as Muller's ratchet, so harmful mutations pile up in their nonessential genes. Symbiosis has still been a powerful engine of evolution, letting species colonise new environments.
Source: Symbiosis
