Where did kissing come from? Study introduces the ‘groomer’s final kiss hypothesis’

A team at the University of Warwick is suggesting that human kissing evolved from grooming behaviors observed in ancestral great apes. In a study, “The Evolutionary Origin of Human Kissing,” published in Evolutionary Anthropology, the researchers introduce the “groomer’s final kiss hypothesis.”

A kiss is just a kiss, but depending on where and when that kissing takes place requires some understanding of cultural conventions and historical context. Societies have long dictated the rules surrounding who can kiss, where, when, and how it should be done.

In ancient Rome, different types of kisses had unique names with distinct meanings based on the relationship between individuals. There was the osculum, a cheek on the cheek, the basium, a kiss on the lips without sexual intent, and savium for erotic kissing.

Kissing the ring, hand or feet to show respect, along with ceremonial and religious kissing, all have long histories. There are traditional kisses like the wedding kiss, the birthday kiss, the New Year’s kiss, kissing under the mistletoe or blowing a kiss for good luck.

Kissing customs can vary wildly across cultures. In parts of Europe, two cheek-to-cheek kisses are a standard greeting, whereas in other parts, three kisses are expected. If you travel to Belgium, you only get one.

These kisses are typically across genders or between women, while European men might limit kissing other men to close kin. Travel to Oman and a handshake between gentlemen might be followed by a quick kiss on the nose.

The kiss is a versatile method of showing signs of affection, intimacy or social bonding in a well-regulated way, but how kissing got started is an ongoing debate. Some studies suggest an origin in nurturing care behaviors such as infant nursing or premastication where, in the absence of a food processor, caregivers feed infants with pre-chewed food. Other hypotheses relate kissing to a compatibility test, a subtle sniff or taste sampling of microflora to determine genetic health.

When looking for parallels in the nonhuman animal world, finding behavior that matches the form and function of human kissing is challenging. Many animals engage in some sort of nuzzling, but the only nonhumans known to kiss are our closest evolutionary cousins, chimpanzees and bonobos.

Adriano R. Lameira of the Department of Psychology at the University of Warwick led the comprehensive review of existing hypotheses to explore the evolutionary roots of this intimate behavior, noting that grooming is a primary means of establishing and maintaining social bonds in great ape social structures.

Assuming that ancestral apes engaged in grooming this way, it would be a fundamental part of our early culture. As time goes by and humans evolve to have less body hair, the need for extensive grooming diminishes, leading to ever shorter grooming sessions. While the need to have our fur plucked at may have gone away, the social bonding aspect remains important.

According to the study, the final act of ape grooming involves protruding lips and slight suction to remove debris or parasites, a behavior that persisted even as its hygienic function waned. This vestigial action, the researchers termed the “groomer’s final kiss hypothesis,” mirrors the form, context, and function of modern human kissing.

By comparing grooming behaviors across primate species and human societies, the research supports the notion that kissing serves as a symbolic gesture to signal and reinforce social and kinship bonds. While other non-ape primates engage in social bonding activities, they are quite different. An example is given of capuchin monkeys who demonstrate their social connection by sticking fingers in the eyes and nostrils of those they hold dear.

To test the hypothesis, the researchers recommend future research should focus on detailed descriptions of grooming behaviors in great apes. By examining grooming patterns in populations with varying coat thickness, it could provide further insights into the evolutionary trajectory of kissing from the groomer’s final kiss hypothesis.

Sticking fingers in the eyes and nostrils of a colleague might seem out of place as a human greeting, even if it is quite normal for a capuchin monkey. A 2015 study published in American Anthropologist looked at 168 cultures and found that only 46% of them engaged in romantic kissing. Most indigenous hunter-gatherer cultures do not engage in any kind of kissing. Some even find the act disgusting. Western ethnocentrism may perceive kissing as a universal human behavior, but for most human cultures, kissing is just as strange as sticking a finger up someone else’s nose.

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