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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, researchers propose that Neanderthals did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In previous studies, scientists have found humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the idea chimed with research that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Intimate Spin

"This offers a different spin on ancient interactions," the lead researcher said.

Writing in the publication Evolution and Human Behavior, the researcher and colleagues detail how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how people smooch.

Defining Kissing

"Previously there were some previous attempts to describe a intimate act, but it's very much been focused on humans, which implies that essentially non-human species don't kiss. Currently we understand that they probably do, it might just not look from what human kissing looks like," explained Brindle.

However, she said some actions that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species known as certain marine animals.

Consequently the team developed a description of kissing centered around friendly interactions involving intentional mouth-to-mouth contact 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 accounts of intimate behavior in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and employed online videos to confirm the reports.

The researchers then integrated this information with information on the evolutionary relationships between extant and ancient species of such animals.

Evolutionary Origins

Researchers say the findings indicate kissing evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the large apes.

The position of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a intimate act, the scientists say. But the activity might not have been confined to their own species.

"Reality that humans engage intimately, the reality that we currently have demonstrated that ancient relatives probably engaged, indicates that the both groups are also likely to have engage," Brindle added.

Evolutionary Importance

While the scientific reasoning is debated, Brindle explained kissing could be used in reproductive situations to possibly increase reproductive success or help choose between mates, while it might help strengthen connections when used in a platonic way.

A separate researcher in the behavior of great apes commented that as kissing behavior was seen in a wide range of apes it made sense its origins lie deep in our ancient history, and an analysis of various types of intimate behavior among a wider variety of animals might extend its origins back further still.

"Behaviors that we consider as characteristics of human life, like kissing, are not exclusive to us if we look closely at different species," he said.

Cultural Aspects

An archaeology expert explained that kissing had a social component as it was not universal to all societies.

"However, as humans we succeed or struggle on the strength of our emotional bonds, and methods of promoting trust and closeness will have been important for millions of years," the professor stated. "This could represent an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it should be expected that Neanderthals – and including them and our own species collectively – engaged intimately."