That's disgusting! Make up your own Just-So story about the evolution of an emotion

The evolution of disgust
Everyone seems to be talking about disgust these days, from why it evolved to what parts of our brains light up when we feel it (it's the anterior insular cortex).  There was a story in the NYT about it on Tuesday ("Survival's Ick Factor"), and a review of a new book (one of many) about it in the Sunday NYT Book Review, a conference in Germany, and an issue of the Philosophical Transactions of the Royal Society devoted to the subject.  Darwin included disgust in his list of the 6 basic human emotions, and wrote of seeing it on the faces of his infant children. 

Indeed, it seems that disgust now explains many human characteristics from tribalism, to disease avoidance, to poison critter avoidance, and mate choice.  And, disgust gone haywire explains psychological pathologies from obsessive compulsive disorder to excessive anxiety.

A paper in Perspectives in Biology and Medicine in 2001 lists the basic disgust elicitors. 

We suggest that the objects or events which elicit disgust can be placed in the following five broad categories:

1. Bodily excretions and body parts

2. Decay and spoiled food

3. Particular living creatures

4. Certain categories of "other people"

5. Violations of morality or social norms

Bodily secretions are the most widely reported elicitors of the disgust emotion. Feces appear on all of the lists, while vomit, sweat, spittle, blood, pus, and sexual fluids appear frequently. Body parts, such as nail clippings, cut hair, intestines, and wounds, evoke disgust, as do dead bodies. Certain animals are repeatedly mentioned, in particular pigs, dogs, cats, fish, rats, snakes and worms, lice, cockroaches, maggots, and flies. Spoiled food, especially meat and fish, and other decaying substances, such as rubbish, are disgusting to many respondents. Certain categories of other people are also found disgusting, notably those who are perceived as being either in poor health, of lower social status, contaminated by contact with a disgusting substance, or immoral in their behavior.

And then there are sensory cues, smells, feel, sounds.  A number of writers explain that all these things are disgusting because they remind us of our animal -- unhealthy? -- origins.  Others say it evolved to defend body and soul from pollution (as apparently being reminded of our animal origins pollutes the soul).  

In their exploration of Darwinian medicine, Nesse and Williams (Evolution and Healing, 1995) suggest that an instinctive disgust may motivate the avoidance of feces, vomit, and people who may be contagious, and that disgust is one of the mechanisms crafted by natural selection to help us keep our distance from contagion. Pinker (How the mind works,1998) proposes that disgust is "intuitive microbiology," and that this explains our aversion to objects that have been in contact with disgusting substances: "Since germs are transmissible by contact, it is no surprise that something that touches a yucky substance is itself forever yucky." 

It's nice that this emotion is finally getting the attention that it clearly deserves.

But wait a second!
Except -- there had to be an except! -- except that a lot of this starts to sound suspiciously like just another elaborate evolutionary Just-So story.  New parents, nurses, physicians all quickly lose any disgust at bodily excretions, and one person's spoiled food is another's delicacy.  Just think of the rich array of foods that people on this planet eat.  Not to mention dogs, who'll eat just about anything.  Dogs share many of our emotions, and, if essentially all humans feel disgust, our sense of disgust had to have evolved earlier than we did, so shouldn't other lineages who share our disgust-feeling common ancestor, such as dogs, also share our supposedly instinctual disgust with eating, say, rotten meat, or vomit?

Dead Zambian shrew, not Holly's shrew

Which may explain why Holly reports holding up a dead shrew to her two dogs and finding that they wouldn't touch it.  She says her dogs would happily tear apart a dead squirrel, but not the shrew.  She thinks maybe it died of pesticide poisoning, though she couldn't smell anything.  Were they disgusted (by at least this one thing!), thus saving themselves from pesticide poisoning?  Or is it that they have learned to tear apart squirrels and not shrews?  Who knows?

But then, why is it disgusting to some people to eat insects, while others thrive on them (roasted, chocolate covered, etc.)?  Or why did Americans once disdain disgusting lobster....and now drop big bucks for a nice, juicy claw?  European Americans recoil at the thought of eating horse meat, while to many of their Old World brethren it's a delicacy. Or what about latakia pipe tobacco and lapsong suchong tea, 'cured' as one might say, over dung fires?  The list could go on and on and on, but what it means is that there's an obvious learned component.

But, let's agree for the sake of argument that disgust as an emotional reaction in fact evolved as a specific trait.  And even that disgust might have its uses (though, too much of it can be a problem).  All this means is that, as other successful traits that have stood the test of evolutionary time, disgust itself is adaptable.  That is, yes, we may all feel disgust, but what disgusts us at any given time is culturally determined, not innate.  Otherwise, how could we learn that Twinkies were disgusting?  (Or not.  It turns out that if you search in Google Images for Twinkies, you'll find a photo of Twinkies Fondue; Twinkies, circus peanuts, caramel Ho-Hos, marshmallows, and candied orange slices on a skewer, waiting to be dipped into molten chocolate.  Or Scottish deep-fried Mars bars!  Who thinks these things up?)

Our better idea!
And any of us can think of alternative hypotheses as to what disgust is 'for'.  Here's ours -- how about that it's part of our repertoire of communication, rather than an innate ability to save ourselves from decaying meat?  Why would we need a facial expression that communicates disgust if the emotion itself were the survival tactic, alerting us not to eat that rotting wildebeest?  Surely we could teach our children that even bunny rabbits were disgusting, if we started them young enough.  So, in adaptive terms, it's communicating that we're disgusted that's important, not what we're disgusted by.  Why?  Because it elicits caretaking, a survival tactic if there ever was one. And of course survival is very directly tied to evolutionary fitness.

But all this hoopla about disgust is a bit disgusting itself.  Are we really desperate to have specialties so that someone can be called by the NY Times "a pioneer of modern disgust research"? It's one thing to specialize, even to this extent, and perfectly legitimate to identify 'disgust' and try to understand its neurophysiology and physiological triggers -- if there really is an 'it'.  But it's quite another big step to attempt to Darwinize something so vague, and the fact that Darwin mentioned it doesn't change that.  Evolutionary scenarios are hard to pin down, even with well-defined traits.  The evidence by and large suggests that most of the human versions of this emotion, if it is a particular emotion, are learned and experiential and culture-specific -- adaptable. 

Obviously the inherent aspects, the 'adaptive' aspects of our disgusting behavior are unclear, hard to identify, harder to prove, and in any case it is not obvious that we have any such adaptations that were not in place eons before a human ever stepped on a wildebeest patty (barefoot--UGH!).