That's accepted as scientific orthodoxy now, but it wasn't always. In the 1960's, evolutionary biologist Lyn Margulis proposed the theory that cell organelles such as mitochondria and chloroplasts were once independent bacteria, which combined through symbiotic mergers of bacteria to evolve into eukaryotic cells. Ernst Mayr called this "perhaps the most important and dramatic event in the history of life".
Throughout her career, Margulis' work could arouse intense objection. One grant application elicited the response, "Your research is crap, do not bother to apply again", and her formative paper, "On the Origin of Mitosing Cells", appeared in 1967 after being rejected by about fifteen journals.
In 2002, Discover magazine recognized Margulis as one of the 50 most important women in science.
In 1995, Richard Dawkins said, "I greatly admire Lynn Margulis's sheer courage and stamina in sticking by the endosymbiosis theory, and carrying it through from being an unorthodoxy to an orthodoxy. I'm referring to the theory that the eukaryotic cell is a symbiotic union of primitive prokaryotic cells. This is one of the great achievements of twentieth-century evolutionary biology, and I greatly admire her for it."