Page "Endosymbiotic theory" Paragraph 4

Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the 1920s.

These theories were initially dismissed or ignored.

More detailed electron microscopic comparisons between cyanobacteria and chloroplasts ( for example studies by Hans Ris ), combined with the discovery that plastids and mitochondria contain their own DNA ( which by that stage was recognized to be the hereditary material of organisms ) led to a resurrection of the idea in the 1960s.