Endosymbiont theory is well-accepted for one evolutionary process but controversial for explaining another. The two processes are horizontal gene transfer via gene fusion and eukaryotic nucleus development.
Endosymbiotic theory, or serial endosymbiotic theory, is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms.
Moreover, the endosymbiotic theory states that some of the organelles in eukaryotic cells were once prokaryotic microbes. Mitochondria and chloroplasts are the same size as prokaryotic cells and divide by binary fission. Mitochondria and chloroplasts have their own DNA which is circular, not linear.
Hence, the Endosymbiotic Theory states that the mitochondria and chloroplast in eukaryotic cells were once aerobic bacteria (prokaryote) that were ingested by a large anaerobic bacteria (prokaryote).
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