A whole lot of mating garter snakesseeer/Imazins/Getty ImagesBirds do it, a few bees get to do it but how did sexual reproduction evolve in the first place? An evolutionary model suggests that it could have started as a way for two cells to pool their resources when favourable environments turn harsh.The idea is when times get hard, you fuse with another cell, says George Constable at the University of York, UK. Then youve got this big cell which has more chance of surviving.