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Among seabirds to Arctic mammals, chimpanzees to great apes, various animals appear to kiss. Currently, researchers propose that Neanderthals did it too – and possibly exchanged kisses with early Homo sapiens.

Common Oral Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were intimately acquainted. Among previous studies, researchers have found modern people and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, adding that the concept aligned with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing genetic mixing was at play.

Intimate Spin

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

Writing in the publication a scientific periodical, Brindle and colleagues report how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not limited to how people kiss.

Describing Intimate Contact

"Previously there were some efforts to define a intimate act, but it's very much been human-centric, which means that essentially non-human species don't kiss. Now we know that they likely engage, it may appear different from what our intimate contact resembles," said the evolutionary biologist.

However, she said some actions that looked like intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", observed in fish called French grunts.

Consequently the team came up with a description of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the mouth but no transfer of food.

Study Approach

Brindle said they focused on reports of kissing in non-human species from Africa and Asian regions, including primates, apes and great apes, and employed online videos to confirm the observations.

Scientists then integrated this information with information on the evolutionary relationships between extant 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 predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, indulged in a kiss, the scientists conclude. But the activity may not have been confined to their own species.

"The fact that modern people kiss, the reality that we now have demonstrated that ancient relatives very likely engaged, indicates that the both groups are also likely to have kissed," Brindle added.

Biological Significance

Although the scientific reasoning is debated, the expert explained intimate contact could be employed in sexual contexts to potentially enhance reproductive success or assist in selecting between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the activities of primates commented that as kissing behavior was observed in a broad spectrum of primates it made sense its roots lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a broader range of species might push its beginnings back further still.

"Things that we think of as characteristics of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Social Aspects

An archaeology expert explained that kissing had a social component as it was not common to all human groups.

"However, as humans we succeed or struggle on the quality of our relationships, and ways of promoting confidence and intimacy will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be expected that ancient hominins – and including Neanderthals and our human ancestors together – kissed."