Our world is swarming with symbioses. Sea anemones and clownfish, land plants and mycorrhizal fungi, rays and remora cleaner fish, corals and algae. All around us, radically different species team up in unconventional ways, forming long-lasting relationships that benefit both parties. Some species take these partnerships to the extreme, with one organism actually moving into the cells or tissues of another. Known as endosymbiosis, this type of interaction led to the creation of key organelles, including the mitochondrion, and has formed the basis of life as we know it.
Among the more profitable endosymbioses is one that allows the host to derive energy from sunlight. The light-harvesting machines of plants and algae, for example, are the products of an ancient merger between a photosynthetic bacterium and a microbial eukaryote.