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From seabirds to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Currently, researchers suggest that Neanderthals also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Clues

This isn't the initial instance experts have proposed Neanderthals and early modern humans were intimately acquainted. Among earlier research, researchers have discovered humans and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, adding that the concept aligned with research that has found people of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

"It certainly puts a different spin on human-Neanderthal relations," Brindle said.

Writing in the publication Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to come up with a definition that was not restricted by how people kiss.

Defining Intimate Contact

"There have been some previous attempts to describe a intimate act, but it's largely focused on humans, which implies that basically other animals do not engage in this. Currently we understand that they probably do, it might just not look from what human kissing looks like," said Brindle.

Nonetheless, she said some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species known as French grunts.

As a result the research group came up with a definition of kissing centered around social behaviors involving directed oral interaction with a member of the same species, with some movement of the mouth but absence of nutrition.

Study Methods

Brindle explained they focused on reports of kissing in primates from the African continent and Asia, including primates, chimpanzees and great apes, and employed online videos to confirm the observations.

Scientists then combined this data with details on the genetic connections between extant and extinct species of such animals.

Historical Timeline

Researchers propose the results suggest intimate contact developed approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this family tree means it is likely they, too, engaged in a kiss, the researchers say. But the behavior might not have been limited to their own species.

"The fact that humans kiss, the fact that we currently have shown that Neanderthals probably engaged, indicates that the two [species] are probably did engage," Brindle noted.

Biological Significance

While the evolutionary explanation is discussed, the expert said intimate contact could be employed in reproductive situations to potentially increase reproductive success or help choose between mates, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of great apes said that as intimate contact was seen in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an examination of various types of intimate behavior among a wider variety of animals might push its origins back even earlier still.

"Behaviors that we think of as signatures of our species, like kissing, are not unique to us if we look closely at other animals," he said.

Cultural Aspects

Another professor explained that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and methods of promoting confidence and intimacy will have been significant for eons," the professor stated. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be expected that Neanderthals – and even them and our human ancestors collectively – kissed."