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Among seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Now, scientists suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Clues

It is not the first time experts have proposed ancient relatives and early modern humans were closely connected. In previous studies, scientists have discovered humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the concept aligned with studies that has found people of certain genetic backgrounds contain ancient genetic material in their genome, revealing genetic mixing was at play.

Intimate Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how people smooch.

Defining Kissing

"There have been some efforts to describe a kiss, but it's largely human-centric, which means that basically non-human species don't kiss. Now we know that they likely engage, it may appear different from what our intimate contact looks like," 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", seen in fish known as French grunts.

Consequently the research group developed a definition of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but no transfer of food.

Research Methods

The lead researcher said they concentrated on accounts of intimate behavior in primates from the African continent and Asian regions, including primates, apes and orangutans, and employed online videos to confirm the observations.

The researchers then integrated this data with details on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Timeline

Researchers say the results indicate kissing evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

The position of Neanderthals on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the scientists say. But the activity might not have been confined to their specific group.

"The fact that humans engage intimately, the fact that we now have demonstrated that ancient relatives very likely kissed, suggests that the two [species] are probably did kissed," Brindle noted.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle said intimate contact could be used in reproductive situations to potentially enhance mating outcomes or assist in selecting between mates, while it could assist strengthen connections when practiced in a platonic way.

Another expert in the behavior of primates said that as kissing behavior was seen in a wide range of primates it made sense its roots extend far into our ancient history, and an analysis of various types of kissing among a wider variety of species might push its origins back even earlier still.

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

Social Elements

Another professor said that intimate contact had a cultural element as it was not universal to all societies.

"However, as people we succeed or struggle on the strength of our relationships, and ways of encouraging confidence and intimacy will have been significant for millions of years," the professor stated. "This could represent an image that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but really it should be no surprise that ancient hominins – and including Neanderthals and our human ancestors together – engaged intimately."