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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, scientists propose that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Evidence

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the concept aligned with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Spin

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

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

Defining Kissing

"Previously there were some efforts to define a intimate act, but it's largely human-centric, which implies that basically non-human species don't kiss. Currently we know that they likely engage, it may appear different from what human kissing resembles," said the evolutionary biologist.

However, she noted some behaviors that resembled intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", observed in aquatic species called certain marine animals.

Consequently the team came up with a definition of kissing based on social behaviors involving intentional oral interaction with a member of the same species, with some movement of the mouth but absence of food.

Study Methods

The lead researcher said they focused on accounts of kissing in non-human species from the African continent and Asian regions, including primates, chimpanzees and great apes, and employed online videos to verify the observations.

Scientists then integrated this information with information on the evolutionary relationships between living and extinct species of such animals.

Historical Origins

Researchers say the findings suggest kissing evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree means it is probable they, too, indulged in a kiss, the scientists conclude. But the activity may not have been limited to their specific group.

"The fact that modern people kiss, the fact that we now have demonstrated that Neanderthals very likely kissed, indicates that the two [species] are also likely to have engage," the researcher added.

Biological Importance

Although the evolutionary explanation is discussed, the expert explained intimate contact could be used in sexual contexts to possibly enhance mating outcomes or assist in selecting between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was observed in a wide range of apes it made sense its roots lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of animals might push its origins back even earlier still.

"Behaviors that we consider as signatures of human life, like intimate contact, are not unique to us if we examine carefully at other animals," he said.

Social Aspects

Another professor said 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 emotional bonds, and ways of encouraging trust and closeness will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be no surprise that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."