Serial Endosymbiotic Theory

Mitochondria (the energy-producing structures in nearly all complex cells) and chloroplasts (in plant cells) have their own DNA, separate from the cell's main genome, and that DNA resembles bacterial DNA far more than it resembles the host cell's. Margulis argued this wasn't coincidence — these structures were once free-living bacteria that an ancestral cell engulfed, and rather than being digested, the engulfed bacteria survived and the relationship became mutually beneficial (the host provided protection and resources, the engulfed bacteria provided efficient energy production), eventually becoming permanent and obligatory for both. This meant complex ('eukaryotic') cells didn't arise through gradual internal mutation alone, but through a merger between separate organisms — cooperation, not just competition, was a major creative force in the history of life. The theory was fiercely resisted for years before genetic evidence (the bacterial-like DNA itself) confirmed it.