Russian botanist Constantin Mereschkowsky (1855-1921) was first to argue for an endosymbiotic origin of the chloroplast and nucleus in a 1905 paper. He based his argument for the chloroplast on the observed fact of symbiosis and on prior work that showed the organelles reproduce themselves even when separated from the nucleus. Although his idea was taken seriously for the first two decades of the 20th century, an influential textbook writer dismissed Mereschkowsky's proposal for the nucleus after the Russian's death, plunging endosymbiotic theorizing for any part of the cell into a half-century dark age.
In the figure below, Mereschkowsky depicted life on Earth as emerging twice. First came bacteria, some of which became the nuclei of other cells, later the chloroplasts and finally spawned fungi. Second were amoeba-like cells that took in the bacterial endosymbionts and proliferated into plants and animals. This division is tantalizingly close to the split between prokaryotes and eukaryotes recognized today, botanist Bill Martin points out in his translation of Mereschkowsky's 1905 paper.JRM