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Among seabirds to Arctic mammals, primates to great apes, various animals appear to kiss. Now, scientists suggest that Neanderthals did it too – and might even have locked lips with modern humans.

Common Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, researchers have found modern people and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

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

Romantic Interpretation

"It certainly puts a different perspective on ancient interactions," Brindle commented.

Publishing in the publication Evolution and Human Behavior, the researcher and colleagues report how, to investigate the evolutionary origins of kissing, they first had to develop a description that was not limited to how people smooch.

Defining Intimate Contact

"There have been some previous attempts to define a intimate act, but it's largely human-centric, which means that essentially other animals don't kiss. Currently we understand that they probably do, it may appear different from what human kissing looks like," explained Brindle.

However, she said some behaviors that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as French grunts.

As a result the research group developed a description of kissing centered around social behaviors involving directed oral interaction with a member of the same species, with some motion of the mouth but no transfer of food.

Study Methods

The lead researcher said they concentrated on accounts of kissing in primates from the African continent and Asian regions, including bonobos, chimpanzees and great apes, and used online videos to confirm the reports.

The researchers then combined this data with details on the evolutionary relationships between living and ancient types of such primates.

Evolutionary Timeline

Researchers say the findings suggest intimate contact developed approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, engaged in a kiss, the researchers conclude. But the activity might not have been limited to their specific group.

"The fact that humans 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," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, Brindle said kissing could be used in sexual contexts to potentially enhance reproductive success or assist in selecting between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of primates said that as intimate contact was observed in a broad spectrum of apes it was logical its roots lie deep in our evolutionary past, and an analysis of different forms of kissing among a wider variety of species might extend its beginnings back even earlier still.

"Behaviors that we think of as signatures of human life, like kissing, are not exclusive to us if we look closely at different species," the expert noted.

Cultural Aspects

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

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and ways of promoting trust and intimacy will have been significant for eons," she said. "This could represent an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be no surprise that Neanderthals – and including them and our own species collectively – engaged intimately."