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From seabirds to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. Among earlier research, researchers have found humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the idea aligned with studies that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, revealing interbreeding was occurring.

Intimate Interpretation

"This offers a more romantic perspective on human-Neanderthal relations," the lead researcher commented.

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the historical roots of kissing, they first had to come up with a definition that was not limited to how people kiss.

Describing Kissing

"Previously there were some efforts to define a intimate act, but it's very much been human-centric, which means that essentially non-human species don't kiss. Now we know that they likely engage, it may appear different from what our intimate contact resembles," said the evolutionary biologist.

However, she noted some actions that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", seen in fish called French grunts.

As a result the team came up with a description of kissing based on friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the oral area but absence of food.

Research Approach

Brindle said they concentrated on accounts of kissing in primates from the African continent and Asian regions, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the reports.

Scientists then integrated this information with details on the genetic connections between extant and extinct species of such animals.

Evolutionary Origins

Researchers say the results indicate intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

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

"The fact that humans kiss, the reality that we now have shown that ancient relatives very likely kissed, indicates that the two [species] are also likely to have kissed," Brindle added.

Evolutionary Significance

While the evolutionary explanation is discussed, the expert explained kissing could be used in reproductive situations to possibly increase mating outcomes or assist in selecting between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the activities of primates commented that as kissing behavior was seen in a wide range of apes it made sense its roots extend far into our ancient history, and an analysis of different forms of kissing among a broader range of animals might extend its beginnings back even earlier still.

"Behaviors that we think of as characteristics of human life, like kissing, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Aspects

Another professor explained that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and methods of promoting trust and closeness will have been significant for eons," she said. "It might be an concept that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but really it ought to be expected that Neanderthals – and including them and our human ancestors together – engaged intimately."