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Among seabirds to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists propose that Neanderthals did it too – and possibly locked lips with early Homo sapiens.

Shared Oral Evidence

It is not the first time scientists have suggested Neanderthals and Homo sapiens were intimately acquainted. Among earlier research, researchers have found humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the concept aligned with studies that has revealed people of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Spin

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

Writing in the publication Evolution and Human Behavior, Brindle and her team detail how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how humans kiss.

Describing Kissing

"There have been some efforts to define a kiss, but it's largely focused on humans, which means that basically other animals don't kiss. Currently we understand that they probably do, it might just not look from what our intimate contact looks like," said Brindle.

However, she noted some actions that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species called certain marine animals.

Consequently the team came up with a definition of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Research Methods

Brindle explained they concentrated on accounts of intimate behavior in non-human species from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to confirm the observations.

The researchers then combined this information with details on the evolutionary relationships between extant and ancient types of such animals.

Evolutionary Timeline

The team say the findings indicate kissing developed somewhere between 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree suggests it is probable they, too, indulged in a intimate act, the researchers say. But the behavior may not have been limited to their specific group.

"Reality that humans engage intimately, the reality that we now have shown that ancient relatives probably kissed, suggests that the both groups are also likely to have engage," Brindle noted.

Biological Significance

While the scientific reasoning is debated, the expert explained intimate contact could be employed in reproductive situations to potentially increase mating outcomes or assist in selecting between mates, while it might help reinforce bonding when used in a platonic way.

Another expert in the activities of great apes said that as intimate contact was observed in a broad spectrum of primates it made sense its origins extend far into our ancient history, and an analysis of various types of kissing among a broader range of animals might extend its origins back even earlier still.

"Things that we consider as signatures of human life, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Social Aspects

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

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been important for eons," the professor stated. "It might be an concept that appears a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be no surprise that Neanderthals – and even Neanderthals and our human ancestors together – kissed."