What is endosymbiotic theory?

Endosymbiotic theory explains the origin of eukaryotic cells. Eukaryotes (cells that contain membrane bound organelle) are believed to have evolved from prokaryotic cells (unicellular organism that lacks membrane bound organelles).Endosymbiotic theory hypothesises that prokaryotic cells engulfed other prokaryotic cells, some of which remained undigested. This undigested prokaryote added functionality to the cell it was engulfed by. An example of this is respiration. Over generations, the engulfed cell would lose it's independent functionality and become a supporting organelle.