The events occurred in the following order:
1. Large and small prokaryotes existed together.
2. Large prokaryotes engulfed smaller prokaryotes
3. Both varieties of prokaryotes began living symbiotically.
4. Mitochondria and chloroplasts evolved from prokaryotes.
The endosymbiotic theory is the theory which explains how eukaryotic cells emerged from prokaryotic cells. The theory holds that some eukaryotic cells such as mitochondria and chloroplast arise from a symbiotci relationships which existed between two prokaryotic cells.