A model of the origin of eukaryotes through serial endosymbiosis.

An ancient prokaryote developed an endomembrane system that allowed it to engulf smaller cells and package them within vesicles.

The smaller cell may escape digestion and develop a symbiotic relationship: an aerobic endosymbiont may allow its anaerobic host to use the increasing oxygen in the environment, and evolve into a mitochondrion.

Another endosymbiotic event may allow a photosynthetic endosymbiont to become a plastid such as a chloroplast.