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Among seabirds to polar bears, primates to great apes, certain species appear to kiss. Now, researchers propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Microbial Clues

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. Among earlier research, scientists have found humans and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the concept aligned with studies that has found people of certain genetic backgrounds have bits of ancient genetic material in their genome, demonstrating interbreeding was at play.

Intimate Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle commented.

Writing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how people smooch.

Defining Kissing

"Previously there were some previous attempts to describe a intimate act, but it's largely focused on humans, which means that basically non-human species do not engage in this. Currently we understand that they probably do, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

However, she said some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", observed in aquatic species called French grunts.

As a result the team developed a description of intimate contact centered around social behaviors involving intentional oral interaction with a individual of the same species, with some movement of the mouth but absence of food.

Research Approach

Brindle said they concentrated on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and orangutans, and used digital recordings to verify the observations.

Scientists then integrated this data with information on the evolutionary relationships between extant and ancient types of such animals.

Historical Origins

Researchers say the findings indicate kissing developed approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

The position of ancient hominins on this family tree suggests it is likely they, too, engaged in a kiss, the scientists say. But the behavior might not have been limited to their own species.

"Reality that modern people engage intimately, the reality that we now have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Importance

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

Another expert in the behavior of great apes commented that as kissing behavior was observed in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an analysis of various types of intimate behavior among a wider variety of animals might push its origins back even earlier still.

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

Cultural Aspects

Another professor 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 emotional bonds, and ways of encouraging confidence and closeness will have been important for eons," she said. "This could represent an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but really it should be expected that Neanderthals – and including Neanderthals and our own species collectively – engaged intimately."