Endosymbiosis.
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   An ancient prokaryote developed an endomembrane system by infolding of
   its plasma membrane.
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   Infolding also allowed it to engulf smaller cells by endocytosis.
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   The  smaller  cell  may  escape  digestion  and  develop  a  symbiotic
   relationship:
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<BR><LI> An  aerobic endosymbiont may evolve into a mitochondrion and allow
its anaerobic host to use oxygen as an energy source.
<BR><LI> Another endosymbiotic event may allow a photosynthetic prokaryotes
to  become  a  plastid  (a  pigment-bearing  organelle  such  as a
chloroplast ).
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