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From Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. Among previous studies, scientists have found modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the concept aligned with studies that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Intimate Spin

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Writing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the evolutionary origins of kissing, they first had to develop a description that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's very much been focused on humans, which means that basically other animals don't kiss. Currently we know that they probably do, it may appear different from what human kissing resembles," explained the evolutionary biologist.

However, she said some behaviors that looked like kissing were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in fish known as French grunts.

Consequently the research group developed a definition 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.

Study Methods

The lead researcher explained they concentrated on reports of kissing in non-human species from the African continent and Asia, including primates, chimpanzees and orangutans, and used digital recordings to verify the reports.

Scientists then combined this information with information on the evolutionary relationships between extant and extinct species of such primates.

Historical Timeline

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

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the researchers say. But the behavior 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 probably kissed, suggests that the two [species] are also likely to have engage," the researcher added.

Biological Significance

While the scientific reasoning is debated, the expert explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or help choose between mates, while it might help reinforce bonding when used in a platonic way.

Another expert in the behavior of primates commented that as intimate contact was seen in a broad spectrum of primates it made sense its origins lie deep in our evolutionary past, and an analysis of various types of kissing among a wider variety of animals might push its beginnings back even earlier still.

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

Cultural Aspects

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

"However, as humans we succeed or struggle on the strength of our relationships, and methods of encouraging confidence and intimacy will have been significant for eons," she said. "It might be an image that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be no surprise that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."