‘The origin of sex’ begins with John Maynard Smith who suggested that the advantage of sexual reproduction was that sex shuffles genes more effectively than parthenogenesis, introducing more genetic variability, and hence adaptability, into a population. Carl Woese's universal tree of life and the opposing endosymbiotic and protoeukaryotic host theories for the origin of eukaryotes are discussed. Then the first sources of atmospheric oxygen are considered along with the first eukaryote fossils and how sexual reproduction and multicellularity arose. The oldest multicellular eukaryote fossil, Bangiomorpha, is dated as 1.2 billion years old. Finally, the Neoproterozoic, Snowball Earth, and fossils of the Ediacara Hills are described.