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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Common Oral Evidence

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, scientists have discovered humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the idea chimed with studies that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genome, demonstrating genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a more romantic perspective on ancient interactions," Brindle said.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the historical roots of kissing, they first had to come up with a description that was not restricted by how humans smooch.

Defining Kissing

"There have been some previous attempts to define a kiss, but it's very much been human-centric, which means that essentially non-human species do not engage in this. Now we know that they likely engage, it may appear different from what human kissing resembles," explained the evolutionary biologist.

However, she said some behaviors that resembled intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species called certain marine animals.

As a result the research group developed a definition of kissing based on friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the mouth but no transfer of nutrition.

Research Methods

The lead researcher said they concentrated on reports of kissing in non-human species from Africa and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to confirm the observations.

Scientists then integrated this data with information on the evolutionary relationships between extant and extinct types of such animals.

Historical Timeline

The team propose the results indicate intimate contact evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a intimate act, the scientists conclude. But the activity might not have been limited to their specific group.

"The fact that modern people engage intimately, the reality that we currently have shown that Neanderthals very likely kissed, indicates that the two [species] are probably did engage," the researcher added.

Biological Significance

While the scientific reasoning is debated, the expert said intimate contact could be used in reproductive situations to potentially enhance mating outcomes or assist in selecting between partners, while it might help strengthen connections when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes said that as intimate contact was seen in a broad spectrum of primates it made sense its roots extend far into our evolutionary past, and an analysis of different forms of kissing among a wider variety of species might extend its beginnings back further still.

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

Social Elements

An archaeology expert said that kissing had a social component as it was not common to all human groups.

"However, as people we thrive or fail on the strength of our emotional bonds, and methods of encouraging confidence and intimacy will have been important for millions of years," the professor stated. "It might be an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and even them and our human ancestors collectively – kissed."