Eukaryotic cells contain specialized subunits called “organelles,” which is the main characteristic which differentiates eukaryotic and prokaryotic cells, because prokaryotic cells do not have organelles.
The endosymbiotic theory suggests an origin of organelles. The theory suggests that organelles came about through endocytosis between prokaryotic cells.
Endocytosis is the process of one cell engulfing another.
The endocymbiotic theory suggests that one prokaryotic cell engulfed another, and instead of being digested, the engulfed cell remained intact and functional. The two cells then formed a symbiotic (mutually beneficial) relationship.
By proposing an explanation for the origin of organelles, the endosymbiotic theory also gives an explanation of the origin of the first eukaryotic cells, based on the definition of an eukaryotic cell given above.