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Among seabirds to polar bears, chimpanzees 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.

Common Microbial Evidence

This isn't the initial instance experts have suggested ancient relatives and Homo sapiens were closely connected. Among earlier research, researchers have discovered humans 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," the researcher noted, adding that the idea aligned with research that has found people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Spin

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

Writing in the journal a scientific periodical, the researcher and her team report how, to explore the historical roots of intimate contact, they first had to come up with a definition that was not limited to how humans kiss.

Describing Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's largely human-centric, which implies that basically other animals do not engage in this. Now we understand that they probably do, it may appear different from what our intimate contact resembles," said Brindle.

However, she noted some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in fish called French grunts.

Consequently the team developed a description of kissing centered around friendly interactions involving directed oral interaction with a member of the identical group, with some motion of the mouth but absence of food.

Study Approach

Brindle explained they focused on accounts of kissing in non-human species from the African continent and Asian regions, including primates, apes and orangutans, and used digital recordings to verify the observations.

The researchers then combined this data with information on the evolutionary relationships between extant and ancient types of such primates.

Historical Origins

The team propose the findings suggest kissing evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

The position of ancient hominins on this family tree means it is likely they, too, indulged in a kiss, the researchers say. But the activity might not have been limited to their specific group.

"The fact that humans engage intimately, the reality that we currently have demonstrated that ancient relatives very likely engaged, indicates that the both groups are also likely to have kissed," the researcher noted.

Biological Significance

While the evolutionary explanation is debated, the expert explained intimate contact could be used in sexual contexts to possibly enhance reproductive success or assist in selecting between mates, while it might help strengthen connections when used in a platonic way.

Another expert in the behavior of great apes said that as intimate contact was seen in a wide range of apes it was logical its origins lie deep in 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.

"Behaviors that we consider as signatures of our species, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Cultural Aspects

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

"However, as humans we thrive or fail on the strength of our relationships, and ways of encouraging trust and intimacy will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be expected that Neanderthals – and even Neanderthals and our human ancestors collectively – kissed."