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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time scientists have suggested Neanderthals and Homo sapiens were closely connected. In previous studies, scientists have discovered modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, explaining that the concept aligned with research that has found humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Intimate Spin

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

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how humans smooch.

Defining Intimate Contact

"There have been some efforts to define a kiss, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Now we understand that they likely engage, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

Nonetheless, she noted some actions that looked like kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species called certain marine animals.

As a result the research group came up with a description of intimate contact based on friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but no transfer of nutrition.

Research Approach

Brindle explained they focused on accounts of intimate behavior in non-human species from the African continent and Asia, including primates, apes and orangutans, and employed online videos to verify the observations.

Scientists then combined this data with details on the evolutionary relationships between living and ancient types of such animals.

Historical Origins

The team say the results indicate intimate contact evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the researchers conclude. But the behavior might not have been limited to their own species.

"Reality that modern people engage intimately, the reality that we currently have demonstrated that Neanderthals probably engaged, indicates that the both groups are also likely to have kissed," Brindle noted.

Evolutionary Importance

Although the scientific reasoning is debated, the expert explained kissing could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between partners, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the activities of great apes said that as intimate contact was seen in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an analysis of various types of kissing among a wider variety of animals might push its origins back even earlier still.

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

Social Elements

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and methods of encouraging trust and closeness will have been important for eons," she said. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and even them and our own species collectively – kissed."