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Among seabirds to Arctic mammals, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Evidence

It is not the first time experts have proposed Neanderthals and early modern humans were intimately acquainted. In earlier research, scientists have discovered modern people and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the concept chimed with research that has revealed people of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Romantic Interpretation

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

Publishing in the publication a scientific periodical, Brindle and her team detail how, to investigate the historical roots of kissing, they first had to come up with a definition that was not restricted by how people smooch.

Describing Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's very much been focused on humans, which means that basically other animals do not engage in this. Currently we understand that they likely engage, it might just not look from what our intimate contact resembles," said Brindle.

However, she noted some actions that looked like intimate contact were something rather different – such as the processing and food sharing, or "mouth contact", seen in aquatic species known as French grunts.

Consequently the team came up with a definition of intimate contact centered around social behaviors involving directed oral interaction with a individual of the same species, with some motion of the mouth but absence of nutrition.

Study Methods

The lead researcher explained they concentrated on reports of intimate behavior in primates from the African continent and Asian regions, including primates, apes and orangutans, and employed online videos to verify the observations.

Scientists then integrated this data with details on the genetic connections between living and ancient types of such primates.

Evolutionary Timeline

The team say the results indicate kissing developed approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

The position of Neanderthals on this family tree suggests it is probable they, too, engaged in a intimate act, the scientists say. But the activity may not have been confined to their specific group.

"Reality that modern people kiss, the fact that we now have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have engage," the researcher added.

Evolutionary Significance

Although the scientific reasoning is debated, Brindle said intimate contact could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between mates, while it might help strengthen connections when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as intimate contact was observed in a wide range of primates it was logical its origins extend far into 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.

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

Cultural Aspects

An archaeology expert explained that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been significant for millions of years," she said. "This could represent an image that appears a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that ancient hominins – and including them and our human ancestors collectively – engaged intimately."