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From Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins 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 ancient relatives and early modern humans were closely connected. In previous studies, scientists have discovered modern people and their thick-browed cousins possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, adding that the idea aligned with research that has revealed people of non-African ancestry contain ancient genetic material in their genome, demonstrating interbreeding was occurring.

Romantic Spin

"This offers a more romantic perspective on ancient interactions," Brindle said.

Publishing in the publication a scientific periodical, the researcher and her team report how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not limited to how humans smooch.

Defining Intimate Contact

"There have been some efforts to define a kiss, but it's largely human-centric, which means that basically non-human species do not engage in this. Now we know that they probably do, it may appear different from what human kissing resembles," said Brindle.

Nonetheless, she said some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in fish called certain marine animals.

As a result the research group developed a description of kissing based on friendly interactions involving intentional oral interaction with a member of the identical group, with some movement of the oral area but no transfer of food.

Research Methods

Brindle said they concentrated on accounts of kissing in non-human species from the African continent and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to verify the reports.

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

Evolutionary Timeline

Researchers propose the results suggest intimate contact developed somewhere between 21.5m and 16.9m years ago in the ancestors of the large apes.

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

"Reality that humans engage intimately, the fact that we now have demonstrated that Neanderthals very likely kissed, suggests that the two [species] are also likely to have kissed," the researcher noted.

Evolutionary Importance

Although the scientific reasoning is discussed, Brindle said intimate contact could be employed in sexual contexts to potentially increase mating outcomes or help choose between mates, while it could assist strengthen connections when used in a platonic way.

A separate researcher in the behavior of great apes commented that as kissing behavior was observed in a wide range of primates it was logical its origins lie deep in our evolutionary past, and an examination of different forms of kissing among a broader range of species might push its origins back further still.

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

Social Elements

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all human groups.

"However, as people we thrive or fail on the strength of our relationships, and methods of encouraging trust and closeness will have been important for millions of years," the professor stated. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it should be no surprise that ancient hominins – and even them and our own species collectively – kissed."