Symbiogenesis

Symbiogenesis, or endosymbiotic theory, is an evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms, first articulated in 1910 by the Russian botanist Konstantin Mereschkowski, and advanced and substantiated with microbiological evidence by Lynn Margulis in 1967. It holds that the organelles distinguishing eukaryote cells evolved through symbiosis of individual single-celled prokaryotes (bacteria and archaea).

The theory holds that , plastids such as chloroplasts, and possibly other organelles of eukaryotic cells represent formerly free-living prokaryotes taken one inside the other in endosymbiosis, around 1.5 billion years ago. In more detail, mitochondria appear to be related to Rickettsiales proteobacteria, and chloroplasts to nitrogen-fixing filamentous cyanobacteria.

Among the many lines of evidence supporting symbiogenesis are that new mitochondria and plastids are formed only through binary fission, and that cells cannot create new ones otherwise; that the transport proteins called porins are found in the outer membranes of mitochondria, chloroplasts and bacterial cell membranes; that cardiolipin is found only in the inner mitochondrial membrane and bacterial cell membranes; and that some mitochondria and plastids contain single circular DNA molecules similar to the DNA of bacteria.

The theory of symbiogenesis (Greek: σύν syn "together", βίωσις biosis "living", and γένεσις genesis "origin or birth") was first articulated by the Russian botanist Konstantin Mereschkowski in his work in 1910, The Theory of Two Plasms as the Basis of Symbiogenesis, a New Study or the Origins of Organisms, although the fundamentals of the idea already had appeared in his earlier 1905 work, The nature and origins of chromatophores in the plant kingdom. Mereschkowski was familiar with work by botanist Andreas Schimper, who had observed in 1883 that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria, and who had himself tentatively proposed (in a footnote) that green plants had arisen from a symbiotic union of two organisms. In 1918 the French scientist Paul Portier published Les Symbiotes in which he claimed that the mitochondria originated from a symbiosis process.Ivan Wallin extended the idea of an endosymbiotic origin to in the 1920s. The Russian botanist Boris Kozo-Polyansky was the first to explain the theory in terms of Darwinian evolution. In his 1924 book Symbiogenesis: A New Principle of Evolution he wrote, "The theory of symbiogenesis is a theory of selection relying on the phenomenon of symbiosis." These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example studies by Hans Ris published in 1961), combined with the discovery that plastids and mitochondria contain their own DNA (which by that stage was recognized to be the hereditary material of organisms) led to a resurrection of the idea in the 1960s.

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