Endosymbiotic Acquisition of Mitochondria

An alpha-proteobacterium was engulfed by an ancestral archaeal host cell and, rather than being digested, persisted as an endosymbiont that became the mitochondrion. This singular event provided eukaryotic cells with aerobic respiration capability, exploiting oxygen accumulating after the Great Oxidation Event. Evidence includes mitochondrial double membranes, circular DNA resembling bacterial genomes, and phylogenomic analyses linking mitochondrial genes to the Rickettsiales order of alpha-proteobacteria.