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From seabirds to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Shared Microbial Evidence

It is not the first time experts have suggested Neanderthals and Homo sapiens were closely connected. Among previous studies, scientists have discovered humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," she said, adding that the concept aligned with research that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Interpretation

"This offers a different perspective on ancient interactions," the lead researcher commented.

Writing in the journal a scientific periodical, Brindle and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's largely human-centric, which implies that essentially other animals do not engage in this. Now we know that they likely engage, it may appear different from what our intimate contact looks like," explained Brindle.

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

As a result the team developed a description of kissing based on friendly interactions involving directed oral interaction with a member of the same species, with some motion of the mouth but no transfer of nutrition.

Study Approach

Brindle said they focused on reports of kissing in primates from the African continent and Asia, including primates, apes and orangutans, and employed digital recordings to verify the reports.

The researchers then combined this information with information on the evolutionary relationships between extant and extinct species of such primates.

Historical Origins

Researchers say the findings indicate kissing developed approximately 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a kiss, the researchers say. But the behavior may not have been confined to their own species.

"The fact that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives probably engaged, indicates that the two [species] are also likely to have engage," the researcher added.

Biological Significance

While the scientific reasoning is debated, Brindle explained kissing could be used in reproductive situations to potentially increase reproductive success or assist in selecting between mates, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates commented that as intimate contact was observed in a broad spectrum of apes it made sense its origins lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a broader range of species might extend its beginnings back further still.

"Behaviors that we think of as characteristics of human life, like intimate contact, are not unique to us if we examine carefully at different species," he said.

Social Elements

Another professor explained that intimate contact had a social component as it was not common to all societies.

"Nonetheless, as people we thrive or fail on the quality of our emotional bonds, and ways of promoting trust and intimacy will have been important for eons," the professor stated. "It might be an concept that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it ought to be no surprise that Neanderthals – and including Neanderthals and our own species together – kissed."