When did the first bacterial endosymbiosis theory emerge?

Although earlier ideas existed, the modern endosymbiotic theory was strongly developed by Lynn Margulis in the 1960s and 1970s. It proposed that mitochondria and chloroplasts originated as free-living bacteria that became permanent partners inside other cells. Evidence from their DNA, double membranes, and bacterial-like ribosomes later supported the theory. This idea reshaped evolutionary biology by showing that major cellular structures could have bacterial origins.