Hypothesized Scenario for the Evolution of the Eukaryotic Cell (SET or Serial Endosymbiont Theory)
Earth’s early atmospheric lacking in molecular oxygen.
Bodies of water (perhaps primitive oceans rich in organic nutrients, including ATP).
First cells (primitive prokaryotes) utilized external ATP to process energy-requiring reactions.
External ATP consumed and becomes low in supply.
Cells evolve mechanism to manufacture their own ATP (fermentation) in the anaerobic environment.
Organic molecules needed to carry out fermentation become scarce.