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

Common Microbial Evidence

It is not the first time experts have proposed Neanderthals and Homo sapiens were intimately acquainted. In previous studies, researchers have found humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, explaining that the concept aligned with research that has found humans of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating interbreeding was occurring.

Intimate Interpretation

"This offers a different perspective on human-Neanderthal relations," Brindle said.

Writing in the journal a scientific periodical, the researcher and her team report how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how people smooch.

Defining Intimate Contact

"There have been some efforts to define a kiss, but it's very much been human-centric, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she noted some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "mouth contact", seen in fish called certain marine animals.

Consequently the team developed a description of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the mouth but absence of nutrition.

Research Methods

Brindle said they focused on accounts of kissing in non-human species from the African continent and Asia, including bonobos, chimpanzees and great apes, and used online videos to confirm the reports.

The researchers then integrated this data with information on the genetic connections between living and extinct types of such primates.

Historical Origins

The team say the findings indicate intimate contact developed somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the researchers say. But the behavior may not have been confined to their own species.

"Reality that modern people kiss, the reality that we now have demonstrated that Neanderthals probably engaged, indicates that the two [species] are also likely to have engage," Brindle added.

Biological Significance

While the scientific reasoning is discussed, Brindle said intimate contact could be employed in sexual contexts 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 activities of primates commented that as intimate contact was seen in a broad spectrum of apes it made sense its roots lie deep in our ancient history, and an analysis of various types of intimate behavior among a broader range of animals might push its origins back even earlier still.

"Behaviors that we think of as characteristics of our species, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Cultural Elements

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of promoting confidence and closeness will have been important for millions of years," the professor stated. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be expected that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."