Sexual reproduction is by far the most prevalent mode of reproduction among organisms more complex than bacteria. How did sexual reproduction arise? The numerous paradoxes surrounding this question have undermined the few attempts to answer it. Sexual reproduction is an evolutionary kludge that resulted from the coevolution of a parasitic relationship between two now endosymbiotic organisms. In addition to explaining the emergence of eukaryotes, this theory unifies the explanations for the emergence of mitosis, meiosis, and alternation of generations, all resulting from the same phenomenon that originated sexual reproduction.