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Among seabirds to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and might even have locked lips with modern humans.

Common Oral Clues

This isn't the initial instance experts have proposed ancient relatives and early modern humans were closely connected. In previous studies, scientists have found modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the idea aligned with research that has revealed humans of non-African ancestry contain ancient genetic material in their genome, demonstrating genetic mixing was at play.

Intimate Interpretation

"This offers a more romantic perspective on ancient interactions," the lead researcher said.

Publishing in the journal a scientific periodical, the researcher and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people kiss.

Defining Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's very much been focused on humans, which implies that essentially non-human species do not engage in this. Currently we know that they likely engage, it may appear different from what our intimate contact resembles," said the evolutionary biologist.

Nonetheless, she noted some behaviors that resembled intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species known as certain marine animals.

Consequently the research group came up with a description of kissing based on friendly interactions involving intentional oral interaction with a member of the same species, with some motion of the mouth but absence of food.

Research Methods

Brindle explained they concentrated on reports of intimate behavior in primates from the African continent and Asia, including primates, apes and great apes, and used online videos to confirm the reports.

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

Historical Timeline

The team propose the findings suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree means it is probable they, too, engaged in a intimate act, the scientists say. But the behavior may not have been confined to their own species.

"The fact that humans kiss, the fact that we now have demonstrated that Neanderthals probably engaged, indicates that the both groups are probably did engage," the researcher 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 assist in selecting between partners, while it could assist reinforce bonding when used in a platonic way.

Another expert in the activities of great apes commented that as intimate contact was observed in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an examination of different forms of kissing among a wider variety of animals might extend its beginnings back even earlier still.

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

Cultural Elements

An archaeology expert said that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as humans we succeed or struggle on the quality of our relationships, and methods of encouraging trust and closeness will have been significant for eons," she said. "It might be an concept that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be no surprise that Neanderthals – and even them and our own species collectively – kissed."