This scientific paper by Lynn Sagan, published in 1967, proposes a theory on the origin of eukaryotic cells, the "higher" cells that divide by classical mitosis. The theory argues that three fundamental organelles, mitochondria, photosynthetic plastids, and the basal bodies (9+2) of flagella, were originally free-living prokaryotic cells. Sagan describes the evolution of photosynthesis under anaerobic conditions in the early atmosphere, giving rise to anaerobic bacteria, photosynthetic bacteria, and finally to cyanophyte algae (and protoplasts). Subsequently, the evolution of aerobic metabolism in prokaryotes would have given rise to aerobic bacteria (protoflagella and promitochondria), presumably during the transition to an oxidizing atmosphere.
Classical mitosis would have evolved in protozoan-like cells millions of years after the evolution of photosynthesis. Sagan presents a plausible scheme for the origin of classical mitosis in early amoeboflagellates, and how during the evolution of mitosis, some of these protozoans acquired photosynthetic plastids (derived from prokaryotes) symbiotically, giving rise to eukaryotic algae and green plants. The paper provides cytological, biochemical, and paleontological evidence for this theory, together with suggestions for its possible experimental verification, and discusses the implications of this theory for the systematics of lower organisms.

Sagan, L. (1967). On the origin of mitosing cells. Journal of Theoretical Biology, 14, 225–274.