Ancient cell mergers: the idea that powers all life—and was almost dismissed

Fifteen journals rejected Lynn Margulis’s 1966 proposal that all complex life arose from ancient mergers of cells; her eventual 1967 paper On the Origin of Mitosing Cells argued mitochondria and chloroplasts are descended from free-living bacteria absorbed by early cells. The theory gained support as evidence accumulated—mitochondrial DNA resembles bacterial genomes, they reproduce by binary fission, and their ribosomes are bacterial-like—leading to the acceptance of endosymbiosis and, later, recognition of secondary endosymbiosis and extensive horizontal gene transfer. The result is a view of life as a web of cooperative mergers, not a simple tree.