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From Galápagos albatrosses to Arctic mammals, primates to great apes, certain species appear to kiss. Currently, researchers propose that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Shared Microbial Evidence

This isn't the initial instance experts have proposed Neanderthals and Homo sapiens were closely connected. Among earlier research, scientists have discovered humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a different spin on ancient interactions," Brindle commented.

Publishing in the publication a scientific periodical, Brindle and colleagues report how, to investigate the historical roots of kissing, they first had to come up with a description that was not restricted by how people smooch.

Defining Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's very much been human-centric, which means that basically other animals don't kiss. Now we know that they likely engage, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

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

Consequently the research group came up with a definition of kissing centered around friendly interactions involving directed oral interaction with a individual of the same species, with some motion of the oral area but no transfer of nutrition.

Research Approach

Brindle explained they focused on accounts of intimate behavior in primates from the African continent and Asia, including bonobos, apes and orangutans, and used online videos to verify the observations.

The researchers then integrated this information with information on the genetic connections between living and extinct types of such animals.

Historical Origins

The team propose the findings suggest intimate contact evolved somewhere between 21.5 million and 16.9m years ago in the ancestors of the great primates.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the scientists say. But the activity may not have been confined to their specific group.

"The fact that modern people kiss, the fact that we currently have shown that ancient relatives probably kissed, indicates that the both groups are also likely to have kissed," the researcher noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, the expert explained kissing could be used in reproductive situations to possibly increase mating outcomes or help choose between mates, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates said that as kissing behavior was seen in a wide range of apes it was logical its origins extend far into our evolutionary past, and an analysis of various types of kissing among a broader range of animals might push its beginnings back even earlier still.

"Things that we consider as signatures of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Social Aspects

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

"Nonetheless, as people we succeed or struggle on the quality of our relationships, and ways of encouraging trust and intimacy will have been important for millions of years," the professor stated. "This could represent an image 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 own species collectively – kissed."