The best explanation about the eukaryotic cell evolved is this: CHLOROPLAST AND MITOCHONDRIA WERE ONCE PROKARYOTES THAT BEGAN TO LIVE WITHIN LARGER CELLS.
Endosymbiosis theory holds that some cell organelles are developed from prokaryotes which once has a symbiosis relationship with eukaryotes but which later become trapped within the cells of these eukaryotes. Examples of these type of cell organelles are chloroplast and mitochondria.