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Among seabirds to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Now, researchers suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were closely connected. Among previous studies, researchers have found humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Probably they were kissing," she said, adding that the idea aligned with studies that has revealed humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," the lead researcher commented.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to explore the historical roots of intimate contact, they first had to come up with a description that was not restricted by how humans smooch.

Defining Intimate Contact

"There have been some efforts to describe a intimate act, but it's largely focused on humans, which implies that essentially other animals do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing resembles," explained the evolutionary biologist.

Nonetheless, she said some actions that resembled intimate contact were something rather different – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species known as French grunts.

Consequently the research group developed a description of intimate contact based on friendly interactions involving directed oral interaction with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Study Approach

The lead researcher explained they focused on reports of kissing in primates from Africa and Asian regions, including bonobos, apes and orangutans, and employed digital recordings to confirm the observations.

The researchers then combined this data with details on the genetic connections between living and ancient types of such animals.

Evolutionary Origins

The team propose the results indicate kissing developed approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

The position of ancient hominins on this family tree means it is likely they, too, engaged in a intimate act, the researchers say. But the activity might not have been confined to their own species.

"The fact that modern people engage intimately, the fact that we currently have demonstrated that ancient relatives very likely engaged, suggests that the two [species] are probably did kissed," the researcher added.

Biological Importance

Although the scientific reasoning is discussed, the expert explained intimate contact could be employed in reproductive situations to possibly increase reproductive success or assist in selecting between mates, while it might help strengthen connections when used in a non-sexual manner.

A separate researcher in the behavior of primates said that as intimate contact was seen in a wide range of apes it was logical its roots lie deep in our ancient history, and an analysis of different forms of kissing among a wider variety of animals might extend its origins back further still.

"Behaviors that we think of as signatures of our species, like intimate contact, are not unique to us if we look closely at different species," the expert noted.

Social Elements

Another professor explained that kissing had a cultural element as it was not universal to all societies.

"However, as people we thrive or fail on the quality of our relationships, and ways of promoting confidence and intimacy will have been important for eons," she said. "It might be an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but actually it should be expected that ancient hominins – and even Neanderthals and our own species collectively – engaged intimately."