1905 ~ Mereschkowsky proposed a reticulated tree of endosymbiosis for the origin of algal plastids.
1920s ~ Ivan Wallin suggested a bacterial origin for mitochondria.
1959 ~ Stocking and Gifford discovered DNA in the plastids of Spirogyra, a green algae.
1960s ~ Lynn Margulis argued the case for endosymbiotic origins of mitochondria and plastids.
1970 ~ Margulis published her argument for the endosymbiotic origin of eukaryotes in The Origin of Eukaryotic Cells.
1977~ Carl Woese declared the case for prokaryotic endosymbiosis “clear cut” and “proven”. Other biologists subsequently declared the endosymbiotic theory demonstrated beyond a reasonable doubt.
1981 ~ In Symbiosis in Cell Evolution, Margulis argued that eukaryotic cells originated as communities of interacting entities. She extended the argument to including endosymbiotic incorporation of spirochaetes that developed into eukaryotic undulopodia -- flagella and cilia. (This proposal has not gained wide acceptance because flagella lack DNA and do not show ultrastructural similarities to prokaryotes.)