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Mitochondria and plastids contain their own ribosomes; these are more similar to those of bacteria (70S) than those of eukaryotes. [74] Proteins created by mitochondria and chloroplasts use N-formylmethionine as the initiating amino acid, as do proteins created by bacteria but not proteins created by eukaryotic nuclear genes or archaea. [75] [76]
Endosymbiosis played key roles in the development of eukaryotes and plants. Roughly 2.2 billion years ago an archaeon absorbed a bacterium through phagocytosis , that eventually became the mitochondria that provide energy to almost all living eukaryotic cells.
The endosymbiosis theory of organogenesis became widely accepted in the early 1980s, after the genetic material of mitochondria and chloroplasts had been found to be significantly different from that of the symbiont's nuclear DNA. [24] In 1995, English evolutionary biologist Richard Dawkins had this to say about Lynn Margulis and her work:
The viral eukaryogenesis hypothesis posits that eukaryotes are composed of three ancestral elements: a viral component that became the modern nucleus; a prokaryotic cell (an archaeon according to the eocyte hypothesis) which donated the cytoplasm and cell membrane of modern cells; and another prokaryotic cell (here bacterium) that, by endocytosis, became the modern mitochondrion or chloroplast.
Currently there are three types of plastids; primary, secondary and tertiary. Endosymbiosis is reputed to have led to the evolution of eukaryotic organisms today, although the timeline is highly debated. [1] Possible cladogram of chloroplast evolution [2] [3] Circles represent endosymbiotic events.
Margulis did strongly promote and publish versions of this hypothesis until the end of her life. [ 5 ] One primary point in favor of the symbiotic hypothesis was that there are eukaryotes that use symbiotic spirochetes as their motility organelles (some parabasalids inside termite guts, such as Mixotricha and Trichonympha ).
The theory of endosymbiosis, as known as symbiogenesis, provides an explanation for the evolution of eukaryotic organisms. According to the theory of endosymbiosis for the origin of eukaryotic cells, scientists believe that eukaryotes originated from the relationship between two or more prokaryotic cells approximately 2.7 billion years ago.
Evidence for this includes the fact that mitochondria and chloroplasts divide independently of the cell, and that these organelles have their own genome. [73] The biologist Lynn Margulis, famous for her work on endosymbiosis, contended that symbiosis is a major driving force behind evolution.