Endosymbiotic theory posits that various key cell organelle originated as symbiotic relationships between uni-cellular organisms.

This relationship is known as endosymbiosis, where an endosymbiont is absorbed into and lives inside another organism.


Evidence that key organelles arose from bacteria:

  • Similarities in inner membrane structures/functions
  • Division is similar in these organelles and some prokaryotes
  • Their ribosomes are closer to prokaryotes than to eukarya

Endosymbiont -> Organelle

The way to tell when an organism stops being its own bacteria and becomes an organelle is the reduced genome size; over time most of the endosymbiont's genes are transferred over to the hosts genome.

The host organism would then become eukaryotic, now that it is more than uni-cellular.



  • Ancient bacteria
  • Energy factory of cells
  • Does signaliing, cell cycle, growth and death
  • 1-10 micrometers

Evidence suggests that the mitochondrion developed from proteobacteria


  • Ancient Bacteria
  • Does photosynthesis

Evidence suggests that the chloroplast developed from cyanobacteria.