Margulis wrote a theoretical paper titled The Origin of Mitosing Eukaryotic Cells in 1967. The paper was rejected by about fifteen scientific journals. Margulis recalled (Brockman 1995). It was finally accepted by The Journal of Theoretical Biology and is considered today a landmark in modern endosymbiotic theory.

She formulated the symbiotic theory of evolution, which deals with the interconnection of prokaryotic and cukaryotic cells explaining the emergence of new species by a mechanism known as symbiogenesis

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