Endosymbiosis
Endosymbiont theory is the idea that eukaryote cells arose in evolution by the fusion of previously free-living protists (prokaryotes).p143 The theory was first suggested by the Russian botanist Konstantin Mereshkovsky in 1905. The  nature of lichens had been worked out; mitochondria and chloroplasts had been seen in cells, and their similarity to bacteria noted. The theory was initially dismissed, but was re-examined when more was known about cell biology, by Lynn Margulis