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Among seabirds to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, researchers suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Oral Clues

It is not the first time scientists have proposed ancient relatives and early modern humans were closely connected. Among earlier research, scientists have found modern people and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the concept chimed with studies that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, revealing interbreeding was at play.

Intimate Interpretation

"This offers a different spin on ancient interactions," Brindle commented.

Writing in the journal Evolution and Human Behavior, the researcher and her team detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not restricted by how humans smooch.

Defining Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's very much been focused on humans, which implies that basically other animals do not engage in this. Now we know that they probably do, it may appear different from what human kissing looks like," said Brindle.

Nonetheless, she said some behaviors that looked like kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as certain marine animals.

As a result the research group came up with a description of intimate contact based on friendly interactions involving directed oral interaction with a individual of the identical group, with some movement of the oral area but no transfer of food.

Research Approach

The lead researcher said they concentrated on reports of kissing in primates from Africa and Asia, including primates, chimpanzees and orangutans, and employed digital recordings to verify the observations.

Scientists then integrated this information with information on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Origins

The team say the findings indicate intimate contact developed approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of ancient hominins on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the scientists conclude. But the behavior might not have been limited to their own species.

"The fact that modern people kiss, the reality that we now have demonstrated that Neanderthals probably engaged, suggests that the two [species] are also likely to have kissed," Brindle noted.

Biological Significance

While the scientific reasoning is discussed, Brindle said kissing could be used in sexual contexts to possibly enhance mating outcomes or assist in selecting between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the activities of primates commented that as intimate contact was seen in a broad spectrum of primates it was logical its roots lie deep in our evolutionary past, and an analysis of different forms of kissing among a wider variety of animals might push its origins back further still.

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

Social Aspects

Another professor said that intimate contact had a social component as it was not common to all societies.

"However, as humans we thrive or fail on the quality of our relationships, and ways of encouraging confidence and intimacy will have been important for millions of years," the professor stated. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that Neanderthals – and even them and our human ancestors together – kissed."