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From seabirds to Arctic mammals, primates to great apes, various animals appear to kiss. Currently, researchers propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance experts have proposed Neanderthals and early modern humans were closely connected. In earlier research, researchers have found modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with studies that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genome, demonstrating genetic mixing was occurring.

Intimate Spin

"It certainly puts a different spin on ancient interactions," the lead researcher commented.

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 restricted by how people kiss.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's largely human-centric, which means that basically non-human species do not engage in this. Now we understand that they probably do, it may appear different from what our intimate contact resembles," said Brindle.

However, she noted some behaviors that looked like intimate contact were something rather different – such as the processing and transfer of food, or "mouth contact", observed in fish known as French grunts.

Consequently the team developed a description of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but no transfer of nutrition.

Research Approach

Brindle explained they concentrated on reports of kissing in primates from Africa and Asia, including primates, apes and great apes, and used online videos to confirm the reports.

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

Historical Origins

Researchers propose the findings suggest kissing developed approximately 21.5 million and 16.9 million years ago in the predecessors of the large apes.

The position of ancient hominins on this family tree means it is likely they, too, indulged in a intimate act, the researchers conclude. But the activity may not have been confined to their specific group.

"The fact that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives very likely kissed, suggests that the two [species] are also likely to have kissed," the researcher noted.

Evolutionary Significance

While the scientific reasoning is debated, the expert explained kissing could be used in reproductive situations to potentially enhance mating outcomes or assist in selecting between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes commented that as kissing behavior was observed in a wide range of primates it made sense its roots extend far into our ancient history, and an analysis of different forms of intimate behavior among a broader range of animals might push its origins back even earlier still.

"Things that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," the expert noted.

Social Aspects

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

"However, as people we succeed or struggle on the strength of our relationships, and ways of encouraging confidence and closeness will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and even them and our human ancestors collectively – engaged intimately."