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From seabirds to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Currently, researchers suggest that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Common Oral Evidence

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have discovered modern people and their thick-browed cousins possessed the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the concept aligned with research that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Intimate Interpretation

"It certainly puts a different perspective on human-Neanderthal relations," Brindle said.

Publishing in the journal Evolution and Human Behavior, Brindle and her team detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's largely focused on humans, which implies that essentially non-human species don't kiss. Currently we understand that they probably do, it may appear different from what our intimate contact resembles," said Brindle.

However, she said some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species called certain marine animals.

Consequently the research group came up with a definition of kissing centered around social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but no transfer of food.

Research Methods

The lead researcher said they concentrated on reports of intimate behavior in non-human species from Africa and Asia, including primates, apes and orangutans, and employed digital recordings to verify the observations.

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

Evolutionary Origins

The team say the findings indicate intimate contact evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

The position of ancient hominins on this family tree means it is likely they, too, engaged in a intimate act, the scientists conclude. But the behavior might not have been confined to their own species.

"The fact that humans kiss, the reality that we currently have shown that Neanderthals very likely engaged, indicates that the both groups are probably did engage," the researcher added.

Evolutionary Importance

While the scientific reasoning is debated, the expert said kissing could be employed in sexual contexts to potentially increase reproductive success or assist in selecting between mates, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates commented that as intimate contact was observed in a broad spectrum of apes it was logical its origins extend far into our ancient history, and an examination of different forms of kissing among a wider variety of animals might extend its origins back even earlier still.

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

Cultural Elements

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

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been significant for eons," she said. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it should be expected that ancient hominins – and even Neanderthals and our human ancestors together – kissed."