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From Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species appear to kiss. Currently, researchers suggest that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were intimately acquainted. In earlier research, scientists have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Probably they were kissing," she said, adding that the idea aligned with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genome, revealing genetic mixing was at play.

Romantic Interpretation

"This offers a more romantic perspective on human-Neanderthal relations," Brindle said.

Writing in the publication a scientific periodical, Brindle and her team detail how, to investigate the evolutionary origins of kissing, they first had to develop a definition that was not limited to how humans kiss.

Describing Intimate Contact

"Previously there were some efforts to define a kiss, but it's largely human-centric, which implies that basically non-human species do not engage in this. Now we understand that they probably do, it might just not look from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", seen in fish called French grunts.

Consequently the research group came up with a definition of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the identical group, with some motion of the oral area but absence of food.

Study Approach

Brindle said they focused on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and orangutans, and used digital recordings to confirm the reports.

The researchers then integrated this information with details on the evolutionary relationships between extant and ancient types of such primates.

Historical Origins

The team propose the results indicate intimate contact developed somewhere between 21.5m and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a intimate act, the scientists conclude. But the behavior might not have been limited to their own species.

"Reality that humans engage intimately, the fact that we currently have shown that ancient relatives probably engaged, suggests that the two [species] are also likely to have kissed," the researcher noted.

Biological Significance

Although the evolutionary explanation is debated, the expert explained intimate contact could be employed in sexual contexts to possibly increase mating outcomes or help choose between mates, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior 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 various types of kissing among a broader range of animals might extend its origins back even earlier still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not unique to us if we look closely at different species," the expert noted.

Cultural Elements

Another professor explained that intimate contact had a cultural element as it was not universal to all human groups.

"Nonetheless, as humans we thrive or fail on the quality of our emotional bonds, and ways of promoting confidence and closeness will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it ought to be no surprise that ancient hominins – and including Neanderthals and our human ancestors together – engaged intimately."