Endosymbiosis.

An ancient prokaryote developed an endomembrane system by infolding of its plasma membrane.

Infolding also allowed it to engulf smaller cells by endocytosis.

The smaller cell may escape digestion and develop a symbiotic relationship:


  • An aerobic endosymbiont may evolve into a mitochondrion and allow its anaerobic host to use oxygen as an energy source.
  • Another endosymbiotic event may allow a photosynthetic prokaryotes to become a plastid (a pigment-bearing organelle such as a chloroplast ).