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From seabirds to polar bears, primates to great apes, various animals appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Shared Microbial Evidence

It is not the first time scientists have suggested Neanderthals and early modern humans were intimately acquainted. In previous studies, scientists have discovered humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the idea aligned with research that has revealed people of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Intimate Interpretation

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

Writing in the journal a scientific periodical, the researcher and colleagues report how, to investigate the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how people smooch.

Defining Kissing

"There have been some efforts to describe a intimate act, but it's largely human-centric, which means that essentially other animals don't kiss. Now we understand that they likely engage, it may appear different from what our intimate contact resembles," said Brindle.

Nonetheless, she noted some actions that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in fish called French grunts.

Consequently the team came up with a definition of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the oral area but absence of nutrition.

Study Approach

Brindle said they focused on accounts of intimate behavior in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and used online videos to confirm the observations.

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

Historical Timeline

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

The position of ancient hominins on this family tree means it is probable they, too, indulged in a kiss, the scientists conclude. But the behavior might not have been confined to their own species.

"Reality that humans engage intimately, the reality that we now have shown that Neanderthals very likely engaged, indicates that the two [species] are probably did kissed," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, the expert explained kissing could be used in sexual contexts to possibly enhance mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of apes it was logical its roots extend far into our evolutionary past, and an analysis of various types of kissing among a wider variety of species might extend its origins back even earlier still.

"Things that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at other animals," he said.

Cultural Elements

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

"However, as people we succeed or struggle on the strength of our relationships, and ways of promoting confidence and intimacy will have been important for eons," the professor stated. "This could represent an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it should be no surprise that ancient hominins – and including Neanderthals and our own species together – kissed."