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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Shared Microbial Clues

It is not the first time experts have suggested Neanderthals and Homo sapiens were intimately acquainted. Among previous studies, scientists have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the concept chimed with research that has revealed people of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Spin

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

Writing in the journal Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of kissing, they first had to come up with a description that was not restricted by how people kiss.

Describing Intimate Contact

"There have been some previous attempts to describe a kiss, but it's very much been focused on humans, which implies that essentially non-human species don't kiss. Currently we know that they probably do, it may appear different from what our intimate contact looks like," explained Brindle.

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 known as certain marine animals.

Consequently the research group came up with a definition of kissing centered around social behaviors involving intentional oral interaction with a member of the same species, with some movement of the oral area but no transfer of food.

Study Approach

Brindle said they concentrated on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and great apes, and used digital recordings to confirm the observations.

Scientists then integrated this information with details on the evolutionary relationships between extant and extinct types of such animals.

Evolutionary Origins

Researchers say the findings indicate kissing developed approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

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

"Reality that humans engage intimately, the fact that we now have shown that ancient relatives very likely engaged, suggests that the two [species] are probably did kissed," the researcher noted.

Biological Importance

Although the evolutionary explanation is discussed, Brindle said kissing could be employed in sexual contexts to potentially increase mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of primates commented that as kissing behavior was observed in a broad spectrum of apes it was logical its origins lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of species might push its origins back even earlier still.

"Things that we consider as signatures of human life, like intimate contact, are not unique to us if we look closely at different species," he said.

Social Elements

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

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and ways of promoting trust and closeness will have been important for eons," the professor stated. "It might be an concept that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but really it should be expected that Neanderthals – and including them and our own species collectively – engaged intimately."