Human evolution is disgusting!

When you think of evolution, you probably think of how animals and plants have developed adaptations to gain a survival or reproductive advantage in their environment. Like ‘Darwin’s Finches’ of the Galapagos islands who evolved different types of beaks to better suit the food sources on each island—promoting their survival.

But have you ever wondered about how humans have evolved?

If you’ve seen a rotting animal carcass and then thought “yuck! Gross! Disgusting!” Felt the need to move away from the animal carcass and felt your face contort—your eyes narrow. Your nose wrinkle. Your mouth close, and your lips clench. Then you have felt the emotion of disgust and unwittingly experienced the power of an evolutionary adaptation firsthand.

Researchers in evolutionary psychology hypothesise that disgust complements our physiological immune system by making us feel disgusted towards sources of infection so that we avoid them—making disgust a ‘behavioural immune system’. This is an obvious evolutionary advantage.

Instinctively avoiding sources of infection due to feelings of disgust would have minimised our human ancestors’ chances of becoming sick or dying of infectious diseases, giving them greater opportunity to reproduce and thus pass on the genetic code for disgust to the next generation.

The disgust response and facial expression is protective against infection from disease sources

Disgust’s “characteristic facial expression that is recognizable across cultures” and the tendency to avoid physical contact with disgusting stimuli by physical distancing or dropping the offending object is hypothesised to be protective against infection.

The narrowing of the eyes is purported to minimise the area of the eye potentially exposed to sprays of pathogen contaminated liquid, while the closing of the mouth and clenching of the lips minimises the chances of accidental oral ingestion of a disgusting object. The desire to avoid physical contact with the source of disgust also minimises the chance of exposure to pathogens.

Arguably, our ancestors who displayed these reactions to sources of disease were less likely to be infected and thus were more successful at surviving and reproducing, leading to the emotion of disgust becoming a widespread human adaptation.

Disgust is universally accentuated towards disease relevant stimuli

In a 2004 study, approximately 40 000 participants from 165 countries were shown 20 randomly ordered images including 7 pairs of images consisting of a “disease-salient stimulus” (eg. gooey liquid that looks like body fluids) and a paired non-disease relevant image as a control (eg. blue gooey liquid). Almost all participants surveyed (98%) rated the disease-relevant image as equally or more disgusting than its non-disease relevant counterpart.

These results suggest disgust is an evolutionary adaptation to protect against infection due to disgust being greater towards disease relevant stimuli compared with non-disease relevant stimuli across cultures.

Pregnant women in the first trimester have greater disgust sensitivity than those in the 2nd and 3rd trimester

University of California researchers used a web-based questionnaire to measure the disgust sensitivity of 496 pregnant women by asking them to rate their level of disgust to different scenarios and found that women in the first trimester displayed “heightened disgust sensitivity” compared to those in the 2^nd and 3^rd trimesters, especially regarding food. These results supported the researcher’s hypothesis that heightened disgust sensitivity in the first trimester—compared to the 2^nd and 3^rd—compensates for the mother and foetus’ increased vulnerability to infection—especially via food-borne illness—due to immunosuppression during the first trimester.

These results suggest that our human ancestors who felt a heightened sense of disgust during the first trimester of pregnancy would supposedly have been better at avoiding infection during a period when they were more susceptible to infection, and thus would have experienced greater chances of survival and reproductive success.

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So, the next time you recoil in disgust from a piece of mouldy fruit, or instinctively move even further away from dog poo in disgust. Remember that what you just felt, is likely to be an evolutionary product forged from eons of your ancestors’ triumphs and tribulations, in an ever-changing and chaotic universe, reaching out from within your genetic code to protect you.

Written by Louis Casey, Honours student and Dalyell Scholar, Bachelor of Science and Advanced Studies, The University of Sydney