A theory, devised principally by US biologist Lynn Margulis (1938–2011), that eukaryotic organisms evolved from symbiotic associations between prokaryotic ancestors. Free-living aerobic bacteria and photosynthetic cyanobacteria (see chloroxybacteria) became incorporated inside larger nucleated prokaryotic cells, where they acted as forerunners of the mitochondria and chloroplasts seen in modern eukaryotes. Such events are held to have occurred on several occasions, producing various lineages of both heterotrophic and phototrophic protists, from which evolved ancestors of animals, plants, and fungi. There is strong evidence for the theory, particularly the finding that mitochondria and chloroplasts have DNA similar in form to that of bacteria, and that they contain prokaryotic-type ribosomes. There is also evidence that the ancestors of some modern groups incorporated eukaryotic cells already containing primary chloroplast-type plastids, through a process called secondary endosymbiosis.