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From seabirds to polar bears, chimpanzees to orangutans, various animals appear to kiss. Now, scientists propose that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Common Oral Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have found humans and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the idea aligned with studies that has revealed humans of non-African ancestry have bits of ancient genetic material in their genome, revealing genetic mixing was occurring.

Romantic Interpretation

"This offers a more romantic perspective on human-Neanderthal relations," the lead researcher said.

Publishing in the journal Evolution and Human Behavior, Brindle and her team report how, to investigate the historical roots of intimate contact, they first had to develop a description that was not limited to how humans kiss.

Defining Intimate Contact

"Previously there were some efforts to define a intimate act, but it's largely focused on humans, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact looks like," explained the evolutionary biologist.

However, she noted some actions that resembled intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species known as French grunts.

As a result the research group developed a description of kissing based on friendly interactions involving directed oral interaction with a member of the same species, with some movement of the mouth but absence of nutrition.

Research Methods

Brindle said they concentrated on accounts of kissing in non-human species from the African continent and Asian regions, including bonobos, apes and orangutans, and employed digital recordings to confirm the reports.

Scientists then integrated this information with details on the evolutionary relationships between extant and extinct types of such primates.

Historical Origins

The team say the results suggest intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree means it is probable they, too, engaged in a kiss, the scientists say. But the activity might not have been limited to their own species.

"Reality that humans engage intimately, the fact that we currently have shown that Neanderthals probably kissed, indicates that the both groups are probably did kissed," the researcher added.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle said kissing could be used in reproductive situations to possibly enhance mating outcomes or help choose between partners, while it could assist strengthen connections when practiced in a platonic way.

A separate researcher in the behavior of primates said that as kissing behavior was observed in a wide range of apes it was logical its origins lie deep in our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might extend its origins back even earlier still.

"Things that we consider as signatures of our species, like kissing, are not unique to us if we examine carefully at other animals," he said.

Social Elements

Another professor explained that kissing had a social component as it was not universal to all societies.

"However, as people we thrive or fail on the quality of our emotional bonds, and ways of encouraging confidence and intimacy will have been significant for millions of years," she said. "It might be an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it ought to be no surprise that ancient hominins – and even them and our own species collectively – kissed."