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Among seabirds to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Common Oral Clues

It is not the first time experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In previous studies, researchers have discovered modern people and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept aligned with research that has found humans of non-African ancestry contain Neanderthal DNA in their genome, demonstrating genetic mixing was at play.

Romantic Spin

"It certainly puts a different perspective on ancient interactions," the lead researcher said.

Publishing in the publication Evolution and Human Behavior, the researcher and her team detail how, to investigate the historical roots of intimate contact, they first had to come up with a description that was not limited to how humans smooch.

Defining Kissing

"There have been some previous attempts to define a intimate act, but it's very much been human-centric, which implies that basically other animals do not engage in this. Now we understand that they likely engage, it may appear different from what human kissing looks like," explained Brindle.

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

Consequently the team came up with a definition of kissing centered around friendly interactions involving directed oral interaction with a member of the identical group, with some movement of the oral area but no transfer of nutrition.

Study Methods

Brindle said they concentrated on accounts of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed online videos to verify the reports.

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

Historical Origins

The team propose the findings suggest intimate contact evolved approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a intimate act, the researchers say. But the activity might not have been limited to their own species.

"Reality that humans engage intimately, the reality that we currently have shown that ancient relatives probably engaged, indicates that the two [species] are probably did kissed," the researcher noted.

Evolutionary Importance

Although the evolutionary explanation is debated, Brindle explained kissing could be used in sexual contexts to potentially increase reproductive success or help choose between partners, 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 seen in a broad spectrum of apes it was logical its roots lie deep in our ancient history, and an examination of different forms of intimate behavior among a wider variety of species might push its beginnings back even earlier still.

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

Social Aspects

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

"However, as people we thrive or fail on the strength of our emotional bonds, and methods of promoting trust and closeness will have been important for eons," the professor stated. "This could represent an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it should be expected that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."