The endosymbiotic theory suggests that a prokaryotic cell ate another prokaryote, giving rise to eukaryotes.

Mitochrondria of the eukaryotic cells.
As many researchers hypothesize that the old single-celled organism or the origin of the complex-celled organisms came from the endosymbiosis of the mitochrondrion organism and the prokaryotic cell. It has been said that mitochondria was an independent organism which then to have been evovled itself after planting itself inside a prokaryotic cell which aided cellular respiration and production of ATP (Adenosine Triphosphate). This then aided the prokaryotic cell to be more sophisticated and caused another change from having without a true nucleus to a eukaryotic cell with a nucleus and embedded DNA.