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From Galápagos albatrosses to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, researchers have discovered modern people and their Neanderthal relatives possessed 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," the researcher noted, adding that the concept aligned with studies that has revealed people of non-African ancestry contain ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Romantic Spin

"It certainly puts a different spin on human-Neanderthal relations," the lead researcher commented.

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how humans kiss.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's largely focused on humans, which implies that basically other animals don't kiss. Now we know that they likely engage, it may appear different from what our intimate contact resembles," said Brindle.

Nonetheless, she said some actions that resembled intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in fish known as French grunts.

As a result the team came up with a definition of kissing centered around friendly interactions involving directed oral interaction with a individual of the same species, with some movement of the mouth but no transfer of food.

Research Methods

The lead researcher explained they focused on accounts of kissing in non-human species from the African continent and Asia, including bonobos, apes and orangutans, and used online videos to confirm the observations.

The researchers then combined this data with information on the genetic connections between living and ancient species of such primates.

Evolutionary Origins

Researchers propose the findings indicate kissing evolved approximately 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins 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.

"Reality that humans kiss, the fact that we now have demonstrated that ancient relatives very likely engaged, suggests that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is discussed, the expert said intimate contact could be employed in sexual contexts to potentially increase mating outcomes or assist in selecting between partners, while it might help strengthen connections when practiced in a non-sexual manner.

A separate researcher in the activities of great apes said that as kissing behavior was observed in a wide range of apes it made sense its origins extend far into our ancient history, and an examination of different forms of kissing among a broader range of animals might push its beginnings back further still.

"Behaviors that we consider as characteristics of human life, like kissing, are not exclusive to us if we examine carefully at different species," he said.

Cultural Elements

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

"However, as humans we succeed or struggle on the strength of our relationships, and methods of encouraging trust and closeness will have been important for eons," she said. "It might be an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it should be no surprise that Neanderthals – and including them and our human ancestors collectively – engaged intimately."