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Among seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Currently, researchers suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Oral Clues

It is not the first time experts have suggested Neanderthals and early modern humans were intimately acquainted. In previous studies, scientists have discovered humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with research that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing interbreeding was at play.

Romantic Interpretation

"It certainly puts a more romantic spin on human-Neanderthal relations," the lead researcher said.

Publishing in the publication a scientific periodical, the researcher and her team detail how, to investigate the historical roots of kissing, they first had to develop a definition that was not restricted by how people kiss.

Describing Kissing

"Previously there were some efforts to define a intimate act, but it's largely human-centric, which implies that basically non-human species do not engage in this. Now we understand that they likely engage, it might just not look from what our intimate contact looks like," explained Brindle.

Nonetheless, she noted some actions that looked like kissing were distinct activities – such as the chewing and transfer of food, or "mouth contact", observed in fish known as certain marine animals.

Consequently the team came up with a definition of kissing based on social behaviors involving intentional oral interaction with a member of the same species, with some motion of the oral area but absence of food.

Research Approach

The lead researcher said they focused on accounts of intimate behavior in non-human species from Africa and Asian regions, including primates, chimpanzees and orangutans, and used digital recordings to confirm the observations.

The researchers then integrated this information with information on the genetic connections between extant and ancient species of such primates.

Evolutionary Timeline

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

The position of ancient hominins on this evolutionary lineage 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 modern people engage intimately, the fact that we now have shown that Neanderthals probably engaged, suggests that the two [species] are also likely to have engage," the researcher added.

Evolutionary Significance

While the evolutionary explanation is discussed, Brindle said kissing could be used in reproductive situations to potentially enhance reproductive success 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 said that as intimate contact was seen in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an analysis of various types of intimate behavior among a broader range of animals might push its beginnings back further still.

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

Social Aspects

An archaeology expert said that intimate contact had a cultural element as it was not universal to all human groups.

"However, as people we thrive or fail on the strength of our relationships, and methods of encouraging confidence and intimacy will have been significant for millions of years," she said. "It might be an image that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be expected that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."