Symbiogenesis refers to the merging of two separate organisms to form a single new organism. The idea originated with Konstantin Mereschkowsky in his 1926 book Symbiogenesis and the Origin of Species, who proposed that chloroplasts originated from cyanobacteria captured by a protozoan. Today both chloroplasts and mitochondria are believed to have such an origin; see endosymbiotic hypothesis.

In Acquiring Genomes: A Theory of the Origins of Species, Lynn Margulis argued that symbiogenesis was the primary force in evolution. According to her theory, acquisition and accumulation of random mutations are not sufficient to explain how inherited variations occur, and new organelles, bodies, organs, and species arise from symbiogenesis. Whereas Darwin emphasized competition as the main force behind evolution, Margulis emphasizes cooperation.

However, this idea has little support from other evolutionary biologists. While symbiogenesis has had a major impact on eukaryotic life, little of its diversification can be attributed to it. For instance, the origin of mitochondria made the development of animals possible, but there is no indication of other symbiogeneses that separated the different phyla.