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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Oral Clues

It is not the first time experts have suggested 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 exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the concept aligned with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genome, revealing interbreeding was at play.

Intimate Spin

"This offers a different perspective on human-Neanderthal relations," Brindle commented.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not limited to how people smooch.

Defining Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been human-centric, which implies that basically non-human species don't kiss. Now we understand that they probably do, it may appear different from what our intimate contact resembles," explained Brindle.

Nonetheless, she noted some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species called French grunts.

As a result the research group developed a definition 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.

Study Methods

The lead researcher said they focused on reports of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and orangutans, and employed online videos to verify the observations.

Scientists then integrated this information with details on the genetic connections between extant and ancient types of such animals.

Historical Timeline

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

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the scientists say. But the behavior might not have been limited to their specific group.

"Reality that modern people engage intimately, the reality that we currently have shown that ancient relatives very likely engaged, suggests that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Importance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in reproductive situations to potentially increase reproductive success or assist in selecting between mates, while it might help strengthen connections when used in a non-sexual manner.

A separate researcher in the activities of primates commented that as kissing behavior was seen in a broad spectrum of primates it made sense its origins extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a wider variety of species might extend its origins back further still.

"Behaviors that we think of as characteristics of human life, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Social Elements

Another professor explained that intimate contact had a social component as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of encouraging confidence and intimacy will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that Neanderthals – and even Neanderthals and our human ancestors together – kissed."