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Among Galápagos albatrosses to polar bears, primates to great apes, various animals appear to kiss. Currently, scientists suggest that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, scientists have found humans and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," she said, explaining that the idea chimed with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing genetic mixing was at play.

Intimate Interpretation

"It certainly puts a different spin on human-Neanderthal relations," the lead researcher commented.

Writing in the journal a scientific periodical, Brindle and colleagues report how, to investigate the evolutionary origins of kissing, they first had to come up with a description that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some efforts to define a kiss, but it's largely focused on humans, which implies that essentially other animals do not engage in this. Now we know that they likely engage, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

However, she said some actions that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in fish called certain marine animals.

As a result the research group developed a definition of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some motion of the oral area but no transfer of nutrition.

Study Methods

The lead researcher explained they focused on accounts of kissing in primates from the African continent and Asia, including primates, chimpanzees and orangutans, and employed digital recordings to verify the reports.

The researchers then combined this data with details on the genetic connections between extant and ancient types of such primates.

Historical Timeline

Researchers say the findings indicate intimate contact evolved approximately 21.5m and 16.9m years ago in the ancestors of the large apes.

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the scientists say. But the activity might not have been limited to their specific group.

"Reality that humans engage intimately, the reality that we currently have shown that Neanderthals very likely engaged, suggests that the two [species] are probably did kissed," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is debated, Brindle said intimate contact could be used in reproductive situations to possibly enhance mating outcomes or help choose between partners, while it might help reinforce bonding when used in a non-sexual manner.

Another expert in the activities of great apes said that as intimate contact was observed in a broad spectrum of primates it was logical its origins lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of animals might push its beginnings back even earlier still.

"Behaviors that we consider as signatures of human life, like kissing, are not exclusive to us if we look closely at other animals," he said.

Social Aspects

Another professor explained that kissing had a social component as it was not universal to all human groups.

"However, as people we succeed or struggle on the quality of our relationships, and ways 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 misplaced ideas of a rather ruthless and ancient history, but really it ought to be no surprise that Neanderthals – and including them and our human ancestors together – engaged intimately."