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Among seabirds to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists propose that ancient hominins did it too – and might even have locked lips with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were closely connected. In previous studies, scientists have discovered humans and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, explaining that the idea chimed with studies that has found humans of non-African ancestry have bits of Neanderthal DNA in their genome, revealing interbreeding was at play.

Intimate Interpretation

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

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Describing Kissing

"There have been some previous attempts to define a kiss, but it's very much been human-centric, which implies that essentially non-human species do not engage in this. Currently we know that they probably do, it might just not look from what human kissing resembles," said the evolutionary biologist.

However, she said some behaviors that looked like intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species called French grunts.

Consequently the research group developed a definition of kissing based on social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but absence of nutrition.

Research Approach

Brindle explained they concentrated on reports of kissing in non-human species from the African continent and Asia, including primates, apes and orangutans, and used digital recordings to confirm the reports.

The researchers then combined this data with details on the genetic connections between living and ancient types of such primates.

Historical Origins

Researchers say the findings indicate intimate contact evolved approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this family tree suggests it is probable they, too, indulged in a kiss, the researchers say. But the activity may not have been confined to their own species.

"The fact that humans kiss, the fact that we currently have demonstrated that ancient relatives very likely kissed, indicates that the both groups are also likely to have kissed," the researcher added.

Biological Significance

Although the scientific reasoning is debated, the expert said intimate contact could be used in reproductive situations to potentially enhance reproductive success or assist in selecting between mates, while it might help reinforce bonding when used in a non-sexual manner.

A separate researcher in the activities of primates said that as kissing behavior was seen in a wide range of apes it made sense its roots extend far into our ancient history, and an analysis of different forms of kissing among a broader range of animals might extend its origins back further still.

"Things that we consider as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at different species," he said.

Social Elements

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all societies.

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging trust and closeness will have been important for millions of years," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be no surprise that Neanderthals – and including them and our human ancestors collectively – engaged intimately."