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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists suggest that Neanderthals also engaged in this behavior – and might even have locked lips with modern humans.

Common Oral Evidence

It is not the first time experts have proposed ancient relatives and early modern humans were closely connected. In previous studies, researchers have found humans and their thick-browed cousins shared the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Probably they were kissing," she said, adding that the idea chimed with research that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a more romantic spin on ancient interactions," the lead researcher said.

Publishing in the publication a scientific periodical, the researcher and her team detail how, to explore the historical roots of kissing, they first had to come up with a definition that was not limited to how humans kiss.

Describing Kissing

"There have been some efforts to describe a kiss, but it's largely human-centric, which implies that essentially other animals don't kiss. Currently we know that they probably do, it may appear different from what our intimate contact resembles," said Brindle.

However, she said some actions that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as certain marine animals.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving directed oral interaction with a member of the same species, with some movement of the mouth but absence of food.

Study Methods

The lead researcher said they focused on reports of intimate behavior in non-human species from the African continent and Asia, including bonobos, apes and great apes, and used digital recordings to confirm the observations.

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

Historical Origins

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

Placement of Neanderthals on this family tree suggests it is likely they, too, indulged in a kiss, the researchers conclude. But the behavior might not have been confined to their own species.

"The fact that humans engage intimately, the reality that we currently have shown that ancient relatives probably engaged, suggests that the two [species] are also likely to have kissed," Brindle added.

Evolutionary Importance

While the scientific reasoning is discussed, the expert said intimate contact could be used in reproductive situations to possibly increase mating outcomes or help choose between mates, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the behavior of great apes said that as intimate contact was seen in a wide range of apes it was logical its origins lie deep in our ancient history, and an analysis of different forms of kissing among a wider variety of animals might extend its beginnings back even earlier still.

"Things that we consider as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Cultural Elements

An archaeology expert explained that kissing 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 ways of encouraging trust and closeness will have been significant for millions of years," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be expected that Neanderthals – and including Neanderthals and our human ancestors together – kissed."