"Without their threatening presence, there is no need for gene recombination or sexual (versus asexual) reproduction. We wouldn’t have to keep up with the Joneses, or in this case the Escherichia coli or the Entamoeba histolytica, which are constantly mutating to get better at attacking us so they can reproduce and survive. Disgust is the emotion that protects purity. Haidt suggests that the emotion of disgust arose when hominids became meat eaters. It appears to be a uniquely human emotion."