Another possibility is that the evolution of mating types was shaped by a more basic kind of asymmetry, dating back to the early days of cell fusion. Back then, if some cells carried viruses or endosymbionts and others didn't, natural selection may have favored fusion between infected cells and those that were still uninfected, says Rolf Hoekstra, professor of genetics at Wageningen University in the Netherlands. Later on, the machinery for asymmetric cell fusion may have been co-opted for sex. That would mean that in one sense, viral infection helped pave the way for baby-making—and sex had a binary difference (infected vs. uninfected) built in to its core. This idea is plausible, though entirely speculative, as Hoekstra acknowledges.