For most of our single-celled ancestors, reproduction and sex are entirely distinct. Reproduction involves making more individuals. Most Earth organisms reproduce in single-parent style: by fission, budding, or forming small internal offspring cells. Sex involves fusion of genetic material from at least two individuals. Sex evolves as a survival strategy. In times of extreme stress – colds of winter, drying summer heat – our protoctist ancestors resort to cannibalism to survive. Some do not totally digest their meal; they become doubled beings and, gobbling still others in order to survive, most bloat up and die.
When the environment rebounds, the survivors need to shed their doubleness and tripleness to avoid dying. These problem solvers evolve ways to regularly double every winter (or dry season) through sex, and relieve the doubling every spring.