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Among seabirds to Arctic mammals, primates to great apes, various animals appear to kiss. Currently, scientists propose that Neanderthals did it too – and possibly exchanged kisses with modern humans.

Common Oral Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were closely connected. Among previous studies, researchers have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with studies that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Intimate Spin

"This offers a different spin on human-Neanderthal relations," Brindle said.

Publishing in the journal a scientific periodical, Brindle and colleagues report how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some previous attempts to describe a kiss, but it's largely focused on humans, which implies that essentially other animals do not engage in this. Now we understand that they likely engage, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

Nonetheless, she noted some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "mouth contact", seen in fish known as French grunts.

As a result the team came up with a definition of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the mouth but no transfer of food.

Research Approach

Brindle explained they concentrated on reports of intimate behavior in primates from Africa and Asia, including primates, apes and orangutans, and employed digital recordings to verify the observations.

The researchers then integrated this information with details on the genetic connections between extant and extinct types of such primates.

Evolutionary Timeline

The team say the findings indicate kissing evolved somewhere between 21.5 million and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the scientists say. But the behavior might not have been confined to their own species.

"The fact that modern people engage intimately, the reality that we now have shown that ancient relatives probably kissed, indicates that the two [species] are probably did kissed," the researcher added.

Evolutionary Importance

While the scientific reasoning is debated, the expert explained intimate contact could be used in sexual contexts to possibly increase reproductive success or assist in selecting between mates, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of great apes said that as intimate contact was seen in a wide range of apes it made sense its roots extend far into our ancient history, and an examination of different forms of kissing among a broader range of species might push its beginnings back even earlier still.

"Things that we consider as signatures of our species, like kissing, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Aspects

An archaeology expert explained that intimate contact had a cultural element as it was not common to all human groups.

"However, as humans we thrive or fail on the strength of our relationships, 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 contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be expected that Neanderthals – and even Neanderthals and our own species collectively – engaged intimately."