Hypothesized Scenario for the Evolution of the Eukaryotic Cell (SET or Serial Endosymbiont Theory)
Some of the anaerobic fermenting bacteria retreat to anaerobic environment.
Others enclosed the chromosomes into a nucleus and established a symbiotic partnership with the aerobic heterotrophic bacteria forming the first eukaryotic cells. The heterotrophic bacteria then become the mitochondria.
One group of these eukaryotic cells establishes a partnership with photosynthetic cyanobacteria. These cyanobacteria eventually become chloroplasts.