Most humans are grossed out by things like oozing sores or rotten meat, and there might well be an evolutionary basis for staying away from these harmful things. But it might go deeper: that disgust might have created our morality.
Valerie Curtis, a researcher at the London School of Hygiene and Tropical Medicine, argues that disgust evolved for the much same reasons that fear did. While our fright will keep us away from huge, scary predators like lions and bears, disgust keeps us away from tinier threats like parasites and harmful bacteria. And disgust is found in all creatures, even the incredibly simple nematode worm, which can recognize and crawl away from potentially deadly bacteria.
Now, all of that seems fairly reasonable and straightforward, but there's a controversial extension to this line of thinking. Curtis explains the theory that the capacity for disgust was at the foundations of our modern conception of morality:
"If I go around leaving poo in your front lawn or spitting in your cups or making nasty smells in public transport or if I go to church in my pajamas, I'm threatening you with my bodily fluids. These are manners, but they're also the precursor of moral behavior. That's at least one of the ways that morality could have evolved in society: simple rules about not getting other people sick with your emanations. If you sit people in a room with bad smells, they punish more severely. Your sense of disgust for people's bad behavior is tied together with your organic system."
It's an interesting idea, although it should be stressed that this doesn't have to be the only source of morality. After all, a societal aversion to acts like murder or theft could develop quite apart from a sense of disgust. But this could be a part of the puzzle, and Curtis argues there's evidence for it in the way certain societies work today:
"Societies with high pathogen risk tend to be societies that have a greater number of religions, they're more close knit, have more socially conservative rules and are more xenophobic. It might be that if you live in a society where you hear a lot about disease, your disgust sensitivity is going to be tuned up higher and as a result, you find that effect across society. It could also be that there are some group selection effects. If one village was really beset by some serious disease problems, would they tend to evolve towards higher disgust settings?"
Of course, as with a lot of ideas in evolutionary psychology, there's a definite danger of taking it too far. Cornell researcher David Pizarro thinks this new research has merit, but is skeptical about some of its applications:
"Even though I do think that the evolutionary approach is the right one to explain the origins of disgust and how it works in life now, I wonder if it can be applied too broadly. It seems unclear to me that you need an evolutionary approach to explain some of the behaviors [discussed]. For instance, avoiding large groups when you know that there's an outbreak of influenza. It seems people would just sort of notice. I don't know that it has to recruit a special system. There are a lot of things like manners that may or may not have anything to do with the avoidance of disease and seem sort of arbitrary."