The correct answer is 2. suggests that a prokaryote ate another prokaryote to produce eukaryotes.
Lynn Margulis (1960) put forward the endosymbiotic theory. The theory states that aerobic bacteria, a prokaryotes require oxygen were engulfed by anaerobic bacteria, also prokaryotes which are poisoned by oxygen. They live together as symbiotic partners for mutual benefits. The aerobic bacteria take care of the toxic oxygen for the anaerobic bacteria, in return the anaerobic bacteria would ingested food and protected the aerobic bacteria. Thus, a eukaryotic cell with a double-membrane bound organelle is formed. The inner lipid bilayer is of engulfed bacterial cell plasma membrane, and the outer lipid bilayer is of bacterial cell that engulfed it.
