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Symbiogenesis
Symbiogenesis, or endosymbiotic theory, is an evolutionary theory that explains the origin of eukaryotic cells from prokaryotes. It states that several key organelles of eukaryotes originated as a symbiosis between separate single-celled organisms. According to this theory, mitochondria, plastids (for example chloroplasts), and possibly other organelles representing formerly free-living bacteria (prokaryotes) were taken inside another cell as an endosymbiont around 1.5 billion years ago. Molecular and biochemical evidence suggest that mitochondria developed from proteobacteria (in particular, Rickettsiales, the SAR11 clade, or close relatives) and chloroplasts from cyanobacteria (in particular, nitrogen-fixing filamentous cyanobacteria).
==History==
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 Mereschowsky in 1910, although he described the fundamental elements of the theory in a paper five years earlier.〔(journal URL: ())〕 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. Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the 1920s. A 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.
The theory was advanced and substantiated with microbiological evidence by Lynn Margulis in a 1967 paper, ''On the origin of mitosing cells.'' In her 1981 work ''Symbiosis in Cell Evolution'' she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic spirochaetes that developed into eukaryotic flagella and cilia. This last idea has not received much acceptance, because flagella lack DNA and do not show ultrastructural similarities to bacteria or archaea (see also: Evolution of flagella and Prokaryotic cytoskeleton). According to Margulis and Dorion Sagan, "Life did not take over the globe by combat, but by networking" (i.e., by cooperation). The possibility that peroxisomes may have an endosymbiotic origin has also been considered, although they lack DNA. Christian de Duve proposed that they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earth's atmosphere. However, it now appears that they may be formed ''de novo'', contradicting the idea that they have a symbiotic origin.〔 (Provides evidence that contradicts an endosymbiotic origin of peroxisomes. Instead it is suggested that they evolutionarily originate from the Endoplasmic Reticulum)〕
It is thought that over millennia these endosymbionts transferred some of their own DNA to the host cell's nucleus (called "endosymbiotic gene transfer") during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell. The endosymbiotic theory is considered to be a type of saltational evolution.〔Michael Syvanen, Clarence I. Kado ''Horizontal Gene Transfer'' Academic Press, p. 405 ISBN 978-0126801262〕
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