“Life is a symbiotic and cooperative union that allows those who associate to succeed”. The phrase belongs to the biologist Lynn Margulis, one of the most renowned biologists of the twentieth century for her study of the world’s smallest cells and the discovery that they are of crucial importance in the history of life on planet earth. His point of view is that symbiosis, a mutually beneficial association, would be the key mechanism for the appearance of eukaryotic cells, constituents of every living organism that is not a bacterium or an archaebacterium. With such a statement, Margulis challenged evolutionary thinking that states, through the law of the fittest, that only those species with the best capabilities survive over the years, replacing it with the theory of endosymbiosis: it is cooperation that biologically does. life possible.