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Among seabirds to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time experts have proposed ancient relatives and early modern humans were intimately acquainted. Among earlier research, scientists have found humans and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," she said, adding that the idea aligned with research that has revealed people of non-African ancestry have bits of ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Romantic Interpretation

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

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the historical roots of intimate contact, they first had to develop a description that was not restricted by how people kiss.

Defining Intimate Contact

"There have been some efforts to describe a kiss, but it's very much been human-centric, which means that essentially non-human species do not engage in this. Currently we understand that they likely engage, it may appear different from what human kissing looks like," said Brindle.

Nonetheless, she noted some behaviors that looked like kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called certain marine animals.

As a result the research group developed a description of intimate contact centered around social behaviors involving intentional oral interaction with a individual of the identical group, with some motion of the oral area but absence of nutrition.

Study Methods

Brindle explained they focused on accounts of kissing in primates from the African continent and Asian regions, including primates, chimpanzees and great apes, and employed online videos to verify the observations.

Scientists then combined this data with details on the evolutionary relationships between extant and ancient types of such primates.

Evolutionary Origins

The team propose the results suggest kissing developed approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree means it is likely they, too, engaged in a intimate act, the researchers conclude. But the behavior might not have been confined to their own species.

"Reality that humans engage intimately, the reality that we now have shown that ancient relatives very likely kissed, suggests that the two [species] are probably did kissed," the researcher noted.

Biological Significance

While the scientific reasoning is debated, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or assist in selecting between mates, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates said that as kissing behavior was observed in a wide range of apes it made sense its roots extend far into our ancient history, and an examination of various types of kissing among a broader range of species might push its origins back further still.

"Things that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at other animals," he said.

Social Aspects

An archaeology expert said that kissing had a cultural element as it was not universal to all human groups.

"However, as people we succeed or struggle on the strength of our emotional bonds, and ways of encouraging trust and closeness will have been important for millions of years," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be no surprise that ancient hominins – and including them and our own species collectively – kissed."