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Among seabirds to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Microbial Clues

It is not the first time experts have suggested ancient relatives and early modern humans were intimately acquainted. In earlier research, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the idea aligned with research that has found humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Romantic Interpretation

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

Publishing in the journal a scientific periodical, the researcher and her team detail how, to explore the historical roots of kissing, they first had to develop a description that was not restricted by how people smooch.

Defining Kissing

"Previously there were some efforts to describe a intimate act, but it's largely human-centric, which implies that basically other animals don't kiss. Now we know that they probably do, it may appear different from what human kissing resembles," said the evolutionary biologist.

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

As a result the team came up with a description of kissing based on friendly interactions involving directed oral interaction with a individual of the identical group, with some movement of the oral area but absence of nutrition.

Study Methods

Brindle said they concentrated on accounts of intimate behavior in non-human species from the African continent and Asia, including bonobos, chimpanzees and great apes, and employed online videos to confirm the observations.

The researchers then combined this information with details on the genetic connections between living and extinct species of such primates.

Evolutionary Origins

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

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the scientists say. But the behavior might not have been confined to their specific group.

"Reality that modern people engage intimately, the reality that we now have demonstrated that Neanderthals very likely engaged, suggests that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Importance

While the scientific reasoning is discussed, the expert explained intimate contact could be used in reproductive situations to potentially increase mating outcomes or help choose between mates, while it might help strengthen connections when used in a platonic way.

A separate researcher in the behavior of great apes commented that as kissing behavior was observed in a broad spectrum of primates it made sense its roots lie deep in our evolutionary past, and an examination of various types of intimate behavior among a broader range of animals might extend its origins back further still.

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

Social Aspects

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

"However, as humans we thrive or fail on the strength of our relationships, and ways of promoting trust and intimacy will have been important for millions of years," she said. "It might be an image 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 including them and our own species collectively – kissed."