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From seabirds to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins did it too – and possibly locked lips with modern humans.

Shared Oral Evidence

It is not the first time experts have suggested Neanderthals and Homo sapiens were intimately acquainted. Among earlier research, scientists have found humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were engaging in intimate contact," she said, adding that the concept chimed with research that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing interbreeding was at play.

Intimate Spin

"This offers a different spin on ancient interactions," the lead researcher said.

Writing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how humans kiss.

Describing Intimate Contact

"Previously there were some efforts to define a kiss, but it's very much been human-centric, which means that essentially non-human species don't kiss. Currently we understand that they likely engage, it might just not look from what our intimate contact resembles," explained Brindle.

However, she noted some actions that resembled intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

Consequently the research group developed a definition of kissing centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the identical group, with some motion of the mouth but no transfer of food.

Study Approach

The lead researcher explained they concentrated on reports of intimate behavior in primates from Africa and Asian regions, including primates, chimpanzees and orangutans, and employed digital recordings to confirm the reports.

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

Historical Origins

Researchers propose the findings indicate kissing developed somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the scientists conclude. But the activity might not have been confined to their specific group.

"Reality that modern people engage intimately, the reality that we currently have shown that ancient relatives very likely kissed, indicates that the two [species] are probably did engage," the researcher added.

Evolutionary Significance

Although the evolutionary explanation is debated, Brindle said kissing could be employed in sexual contexts to possibly enhance reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was seen in a wide range of apes it was logical its roots lie deep in our evolutionary past, and an examination of different forms of kissing among a broader range of animals might push its beginnings back even earlier still.

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

Social Aspects

An archaeology expert said that intimate contact had a social component as it was not universal to all societies.

"However, as humans we succeed or struggle on the quality of our relationships, and ways of encouraging confidence and closeness will have been important for millions of years," the professor stated. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and even them and our own species together – kissed."