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Endosymbiotic hypothesis

The endosymbiotic hypothesis is a hypothesis about the origins of mitochondria and chloroplasts, which are organelles of eukaryotic cells. According to this, these originated as prokaryotic endosymbionts, which came to live inside eukaryotic cells. The hypothesis postulates that the mitochondria evolved from aerobic bacteria (probably proteobacteria, related to the rickettsias), that the chloroplast evolved from endosymbiotic cyanobacteria (autotrophic prokaryotes). The evidence for this theory is compelling as a whole, and it is now generally accepted.

The idea that the eukaryotic cell is a group of microorganisms was first suggested in the 1920s by the American biologist Ivan Wallin. The endosymbiont theory of mitochondria and chloroplasts was proposed by Lynn Margulis of the University of Massachusetts Amherst. In 1981, Margulis published Symbiosis in Cell Evolution in which she proposed that the eukaryotic cells originated as communities of interacting entities that joined together in a specific order. The procaryote elements could have entered a host cell, perhaps as an indigested prey or as a parasite. Over time, the elements and the host could have developed a mutually beneficial interaction, later evolving in an obligatory symbiosis.

Dr. Margulis has also proposed that eukaryotic flagella and cilia may have arisen from endosymbiotic spirochetes, but these organelles do not contain DNA and do not show any ultrastructural similarities to any prokaryotes, and as a result this idea does not have wide support. Margulis claims that symbiotic relationships are a major driving force behind evolution. According to Margulis and Sagan (1996), "Life did not take over the globe by combat, but by networking" (i.e., by cooperation, interaction, and mutual dependence between living organisms). She considers Darwin's notion of evolution driven by competition to be incomplete.

Table of contents
1 Scientific evidence for the theory
2 Related articles
3 References

Scientific evidence for the theory

Evidence that mitochondria and chloroplasts arose via an ancient endosymbiosis of a bacteria is as follows:

Related articles