A fascinating new theory that predates human life on Earth

Most of us do it, whether it’s romantic, familial, friendly or formal, but kissing among humans and where and how it originated is still hotly debated among historians and anthropologists. Now, a new theory has been put forward: that our evolutionary loss of body hair underlies the origins of this behavior and how it has changed over the hundreds of thousands of years of human existence.

Adriano R. Lameira of the University of Warwick, a primatologist turned evolutionary psychologist, has put forward a new hypothesis about the origins of kissing, dating back to our ancestral primates and their social bonding behavior. This would make it an act many millions of years old.

Although the name evokes something sinister, the The theory of the ‘groomer’s final kiss’ refers to how the interaction akin to lip-to-skin contact was common among ancient primates, serving not only to remove parasites but also to strengthen bonds in a community with a complex social dynamic. But if Homo sapiens and great apes underwent divergent evolution, moving along separate paths from their common ancestor, this grooming behavior in humans became less and less – until only the sole feature of this social interaction, the kiss, remained.

“The evolution of kissing is best understood through the biology and behavior of great apes, which provide living proxies for humans’ hominid ancestors,” says Lameira. “Among terrestrial non-human primates, including great apes, grooming is the dominant and most common signal of social bonding.

“According to the ‘groom’s last kiss’ hypothesis, mutual mouth-to-mouth kissing is predicted to have emerged in and evolved from social contexts in which ancestral apes originally mutually groomed each other at the same time, although this type of grooming is rare in extant great apes compared with one-way care,” Lameira notes in the newspaper.

A study in 2015 found that of 168 different cultures surveyed, only 46% engaged in romantic kissing. However, it is also estimated that approximately 90% of the world’s modern cultures involve some form of kissing in human contact – the majority of which occur between parents and children, but also between family members, friends and as symbolic gestures in ceremonies or formal exchanges.

“Even though the social rules surrounding kissing vary within and between societies, all kissing etiquettes share the common function of regulating and controlling the apparently strong intimate connotation of kissing,” Lameira noted. “Kissing is reserved for specific relationships in specific social cases. This implies that kissing is imbued with an underlying universal biological meaning that cuts across cultures, suggesting an evolutionary basis that predates cultural conventions themselves.”

The action of kissing—pressing your lips against a surface and applying light suction—is much like what terrestrial primates use to remove parasites from a companion’s fur and skin. This labor-intensive grooming may have become essentially unnecessary as humans evolved toward less and less body hair and exposure to such parasites, but its role as a bonding mechanism may have remained.

“Compared to a typical primate, humans groom 89% less than expected, mainly for hygiene purposes,” the researcher noted. “This is consistent with the loss of fur during human evolution.”

At the same time as hair reduction, Lameira noted, was the development of human vocalization leading to more direct means of communication between individuals, potentially making an elaborate “kissing” routine less important for social cohesion. However, this hypothesis is one that is still hotly debated among researchers.

“The general idea across all schools of thought, however, is that the social function of care has been completely lost and replaced by a diverse set of behaviors,” Lameira wrote. “It has never been considered, nor can it be theoretically excluded, that a rudimentary form of care continued to exist, preserving (at least some of) the ancestral social functions.”

A 2023 study found lip-to-lip kissing ancient Mesopotamia circa 2500 BCE, but its origins remain largely theoretical, based on the inherent difficulty in assessing much ancient human and non-human behavior.

The research was published in the journal Evolutionary anthropology.

Source: The University of Warwick via phys.org