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From Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance experts have proposed ancient relatives and early modern humans were closely connected. Among previous studies, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the concept chimed with research that has revealed people of certain genetic backgrounds contain Neanderthal DNA in their genome, demonstrating interbreeding was occurring.

Intimate Spin

"This offers a different spin on ancient interactions," Brindle said.

Writing in the publication Evolution and Human Behavior, the researcher and her team detail how, to investigate the historical roots of intimate contact, they first had to develop a description that was not limited to how people kiss.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's very much been focused on humans, which implies that basically non-human species don't kiss. Now we understand that they probably do, it may appear different from what our intimate contact looks like," said Brindle.

However, she said some actions that looked like kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in aquatic species called certain marine animals.

As a result the research group came up with a definition of kissing centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but absence of nutrition.

Research Approach

Brindle said they concentrated on accounts of kissing in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and employed online videos to confirm the observations.

Scientists then integrated this information with information on the evolutionary relationships between extant and extinct types of such animals.

Evolutionary Origins

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

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the researchers say. But the activity may not have been limited to their own species.

"The fact that humans engage intimately, the reality that we now have demonstrated that ancient relatives very likely kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Evolutionary Significance

While the scientific reasoning is debated, Brindle explained intimate contact could be used in sexual contexts to possibly increase mating outcomes or help choose between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the behavior of primates commented that as kissing behavior was seen in a wide range of apes it made sense 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 signatures of our species, like kissing, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Elements

Another professor explained that kissing had a social component as it was not universal to all societies.

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and methods of encouraging confidence and intimacy will have been significant for eons," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be expected that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."