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That work might even help us understand how these neurons evolved in the first place. Allman already has some ideas about where they came from. Our VENs reside in a region of the brain that evolved to integrate taste and smell, so he suggests that many of the traits now associated with the FI evolved from the simple act of deciding whether food is good to eat or likely to make your ill. When reaching that decision, he says, the quicker the "gut" reaction kicks in the better. And if you can detect this process in others, so much the better.
"One of the important functions that seems to reside in the FI has to do with empathy," he says. "My take on this is that empathy arose in the context of shared food - it's very important to observe if members of your social group are becoming ill as a result of eating something." The basic feeding circuity, including the rudimentary VENs, may then have been co-opted by some species to work in other situations that involve a decision, like working out if a person is trustworthy or to be avoided. "So when we have a feeling, whether it be about a foodstuff or situation or another person, I think that engages the circuitry in the fronto-insular cortex and the VENS are one of the outputs of that circuitry," says Allman.
There are also a lot of studies showing links between smell and taste and the feelings of strong emotions. Our physical reaction to something we find morally disgusting, for example, is more or less identical to our reaction to a bitter taste, suggesting they may share common brain wiring (Science, vol 323, p 1222). Other work has shown that judging a morally questionable act, such as theft, while smelling something disgusting leads to harsher moral judgements (Personality and Social Psychology Bulletin, vol 34, p 1096). What's more, Allman points out that our language is loaded with analogies - we might find an experience "delicious", say, or a person "nauseating". This is no accident, he says.