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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time experts have suggested ancient relatives and early modern humans were closely connected. Among earlier research, scientists have found modern people and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the concept aligned with research that has revealed people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was at play.

Romantic Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and her team detail how, to investigate the historical roots of kissing, they first had to come up with 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 very much been focused on humans, which implies that basically non-human species do not engage in this. Currently we understand that they likely engage, it may appear different from what human kissing resembles," said Brindle.

However, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

As a result the research group came up with a definition of kissing centered around social behaviors involving intentional oral interaction with a member of the same species, with some motion of the oral area but no transfer of nutrition.

Study Methods

The lead researcher explained they focused on reports of intimate behavior in non-human species from Africa and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to verify the observations.

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

Evolutionary Origins

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

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the researchers say. But the behavior might not have been confined to their own species.

"Reality that modern people engage intimately, the reality that we currently have shown that ancient relatives very likely engaged, suggests that the both groups are also likely to have engage," Brindle noted.

Evolutionary Importance

While the scientific reasoning is discussed, Brindle explained intimate contact could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of primates commented that as kissing behavior was observed in a broad spectrum of primates it made sense its roots extend far into our ancient history, and an examination of different forms of kissing among a wider variety of species might extend its origins back further still.

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

Social Aspects

An archaeology expert said that kissing had a cultural element as it was not universal to all human groups.

"However, as humans we succeed or struggle on the quality of our relationships, and ways of promoting confidence and closeness will have been significant for eons," she said. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be no surprise that ancient hominins – and including them and our own species together – kissed."