Which biologist proposed the endosymbiotic theory of how complex cells evolved?

Lynn Margulis proposed the modern endosymbiotic theory of how complex cells evolved.

Margulis argued that mitochondria and chloroplasts began as free-living bacteria that were engulfed by ancestral cells and eventually formed permanent partnerships. These organelles retain several bacterial features, including circular DNA, double membranes, and division processes resembling bacterial fission.

She presented the idea in a 1967 paper originally titled “On the Origin of Mitosing Cells.” The proposal was initially controversial because it challenged strictly gradual models of cell evolution. Evidence accumulated over time from molecular biology, cell biology, and comparative genetics.

Margulis did not originate every earlier version of the idea, but she developed and forcefully defended the theory that became central to modern evolutionary biology. Endosymbiosis explains major transitions in cell complexity, not the origin of life itself.