What is the name of the process by which eukaryotic cells acquired mitochondria?

Endosymbiosis is the process by which eukaryotic cells acquired mitochondria from formerly free-living bacteria.

The endosymbiotic theory proposes that an ancestral host cell engulfed an aerobic bacterium but did not digest it. Instead, the two organisms formed a lasting partnership: the bacterium supplied efficient energy production, while the host provided protection and nutrients. Over evolutionary time, the bacterium became an organelle—the mitochondrion.

Mitochondria retain several clues to this history. They have their own DNA, usually arranged in a circular chromosome, and their ribosomes resemble bacterial ribosomes. They also reproduce by division, much like bacteria. Most of their ancestral genes, however, were either lost or transferred to the host cell’s nucleus.

The broader theory is also called symbiogenesis or the endosymbiotic theory. Lynn Margulis helped bring it scientific acceptance with work published in 1967, although earlier versions of the idea date to the nineteenth and early twentieth centuries. Phagocytosis describes engulfing material, but it is not the name of the evolutionary process that produced mitochondria.