Symbiosis is an association of two or more different species. Considering the impact on the organisms it includes, symbiosis can be divided into three categories: mutualism, commensalism and parasitism. Symbiotic relationships are common and they cover all domains of organisms (including viruses) - archaea, eubacteria and all kingdoms of eukaryotes. The first idea of symbiosis was presented by the Swiss botanist Simon Schwendener in the second half of the 19th century, based on the study of lichens. Symbiosis was first defined by Heinrich Anton de Bary as a phenomenon of different organisms living together. At the beginning of the 20th century, scientists became aware of the importance of symbiosis in evolution and the influence coevolution had on speciation and coadaptation of different organisms. In symbiosis, different organisms provide each other habitat, ecological niche, nutrients or reproduction and they generate new metabolic functions, morphological characteristics and behavioral traits. The result is evolutionary progress that independent individuals could not achieve. In such a way, mycorrhiza, a symbiosis of fungi with plant roots, enabled the colonization of terrestrial habitat by plants, which is essential for life as we know it. In symbiosis, the features of one species often cover the deficiencies of the other. The best example for that is the life of an unusual tubeworm, which lacks a digestive system, that was enabled in extreme habitat of deep-sea hydrothermal vents due to its symbiosis with sulfur-oxidizing chemosynthetic bacteria.