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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance experts have suggested Neanderthals and early modern humans were intimately acquainted. Among earlier research, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were kissing," she said, adding that the idea aligned with research that has found people of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating interbreeding was occurring.

Intimate Interpretation

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

Writing in the publication a scientific periodical, Brindle and her team detail how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not limited to how humans smooch.

Defining Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's largely human-centric, which implies that basically other animals do not engage in this. Currently we know that they likely engage, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she noted some actions that looked like intimate contact were something rather different – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but absence of food.

Research Methods

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

Scientists then combined this data with details on the evolutionary relationships between living and ancient types of such animals.

Historical Timeline

The team propose the findings suggest kissing evolved approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

Placement of Neanderthals on this family tree means it is probable they, too, indulged in a kiss, the scientists conclude. But the activity might not have been confined to their own species.

"The fact that modern people kiss, the fact that we currently have demonstrated that Neanderthals very likely kissed, suggests that the both groups are also likely to have kissed," Brindle added.

Biological Significance

While the evolutionary explanation is debated, the expert explained kissing could be employed in reproductive situations to potentially increase mating outcomes or help choose between mates, while it could assist strengthen connections when used in a platonic way.

A separate researcher in the behavior of great apes commented that as kissing behavior was observed in a wide range of apes it was logical its roots lie deep in our ancient history, and an analysis of various types of kissing among a broader range of animals might extend its origins back further still.

"Behaviors that we think of as characteristics of our species, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Cultural Aspects

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

"Nonetheless, as people we succeed or struggle on the quality of our relationships, and methods of promoting confidence and closeness will have been important for eons," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it should be expected that ancient hominins – and including them and our own species collectively – kissed."