What is the name of the theory that proposes mitochondria originated from a symbiotic bacterium?

The theory that mitochondria originated from a symbiotic bacterium is the endosymbiotic theory. It proposes that an ancestral eukaryotic cell engulfed a free-living bacterium related to modern alphaproteobacteria, and that the partnership eventually became permanent. The former bacterium evolved into the mitochondrion, an organelle that produces much of a cell’s ATP.

Several features of mitochondria support this history. They retain their own DNA, usually as a circular chromosome, and reproduce by dividing in a way that resembles bacterial fission. They also have bacterial-like ribosomes and are surrounded by two membranes, consistent with an engulfment event. Over evolutionary time, many genes from the ancestral symbiont moved into the host cell’s nuclear genome.

The idea has a long history. Konstantin Mereschkowski articulated an early version in 1905 and 1910, while Lynn Margulis revived and supported the theory with microbiological evidence in a landmark 1967 paper. The same framework explains chloroplasts as descendants of captured cyanobacteria. A common mix-up is confusing endosymbiosis with ordinary cooperation: mitochondria are now integrated organelles, not independent bacteria living temporarily inside cells.