Lynn Margulis, who developed the endosymbiotic theory for the origins of some cellular organelles, has died, according to a note on the Web page of the University of Massachusetts, Amherst's geosciences department where she was a professor. Margulis was 73. She was also known for her contributions to James Lovelock's Gaia hypothesis that the living and non-living components of the Earth together comprise a self-regulating system.