Endosymbiotic theory (or Symbiogenesis) is an evolutionary theory that was initially proposed more than 100 years ago to explain the origins of eukaryotic cells from the prokaryotic ones. This theory postulated that several key organelles of eukaryotes could have been originated as a symbiosis between separate organisms. Later in the 1920s, this idea was extended to explain the origins of mitochondria and, some 30 years ago, it was definitively accepted. According to this theory, a pro-bacteria was hosted by an eukaryotic cell and evolved within it, providing the host with all the ATP that is needed (therefore acting as a cellular “power house”). Some other studies were confirming this hypothesis by assessing that substantial gene additions were impossible without the energetic boost provided by the colonizing mitochondrion in the eukaryotic lineage.
More recently this idea was questioned by a work in which the authors analyzed the cellular energetics and genomics data from a wide variety of species obtaining an indication that, relatively to the ATP requirements of a cell throughout its life, the costs of a gene at the DNA, RNA, and protein levels decline with cell volume in both bacteria and eukaryotes. These costs are usually sufficiently large to be perceived by natural selection in bacterial populations, but not in eukaryotes experiencing high levels of random genetic drift.
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