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From seabirds to Arctic mammals, chimpanzees to great apes, various animals appear to kiss. Now, scientists propose that Neanderthals did it too – and possibly locked lips with early Homo sapiens.

Shared Microbial Evidence

It is not the first time scientists have proposed ancient relatives and early modern humans were closely connected. Among previous studies, researchers have discovered humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," she said, adding that the concept chimed with studies that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Romantic Interpretation

"This offers a different spin on ancient interactions," Brindle said.

Publishing in the journal a scientific periodical, Brindle and colleagues detail how, to explore the historical roots of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Defining Kissing

"Previously there were some previous attempts to define a intimate act, but it's very much been focused on humans, which means that basically other animals do not engage in this. Currently we know that they likely engage, it may appear different from what human kissing resembles," said the evolutionary biologist.

However, she said some behaviors that resembled kissing were distinct activities – such as the processing and food sharing, or "mouth contact", observed in aquatic species known as certain marine animals.

Consequently the research group came up with a definition of kissing based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Study Methods

Brindle explained they focused on accounts of kissing in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and employed online videos to confirm the observations.

Scientists then integrated this data with details on the genetic connections between living and extinct species of such animals.

Evolutionary Origins

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

The position of Neanderthals on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the scientists conclude. But the activity may not have been confined to their own species.

"Reality that humans kiss, the fact that we now have shown that ancient relatives probably engaged, indicates that the both groups are probably did engage," Brindle noted.

Biological Importance

Although the scientific reasoning is debated, the expert explained intimate contact could be used in reproductive situations to potentially enhance reproductive success or assist in selecting between mates, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates said that as intimate contact was seen in a broad spectrum of apes it was logical its roots extend far into our evolutionary past, and an analysis of various types of intimate behavior among a broader range of species might push its origins back further still.

"Behaviors that we consider as characteristics of human life, like kissing, are not unique to us if we examine carefully at other animals," he said.

Social Aspects

An archaeology expert said that intimate contact had a social component as it was not universal to all societies.

"However, as humans we thrive or fail on the strength of our emotional bonds, and methods of promoting confidence and intimacy will have been important for eons," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be no surprise that ancient hominins – and even them and our human ancestors together – engaged intimately."