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From seabirds to polar bears, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Evidence

It is not the first time scientists have suggested Neanderthals and Homo sapiens were intimately acquainted. Among previous studies, researchers have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with research that has revealed people of non-African ancestry contain Neanderthal DNA in their genome, revealing interbreeding was occurring.

Intimate Interpretation

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

Writing in the journal a scientific periodical, Brindle and her team detail how, to explore the evolutionary origins of kissing, they first had to develop a description that was not limited to how people smooch.

Describing Intimate Contact

"There have been some previous attempts to define a kiss, but it's largely focused on humans, which means that essentially other animals do not engage in this. Now we know that they likely engage, it might just not look from what human kissing resembles," said Brindle.

Nonetheless, she noted some actions that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

Consequently the team developed a definition of intimate contact based on friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of nutrition.

Research Methods

Brindle said they concentrated on reports of intimate behavior in primates from Africa and Asian regions, including primates, apes and orangutans, and used digital recordings to verify the observations.

The researchers then combined this data with details on the evolutionary relationships between living and ancient species of such primates.

Historical Timeline

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

The position of Neanderthals on this evolutionary lineage suggests it is likely they, too, indulged in a intimate act, the scientists say. But the behavior might not have been confined to their own species.

"The fact that modern people engage intimately, the reality that we currently have demonstrated that Neanderthals very likely engaged, indicates that the both groups are probably did engage," Brindle added.

Biological Importance

While the evolutionary explanation is debated, the expert explained intimate contact could be employed in reproductive situations to potentially increase reproductive success or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

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

"Things that we think of as characteristics of our species, like kissing, are not unique to us if we examine carefully at different species," the expert noted.

Social Aspects

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

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and ways of encouraging trust and closeness will have been important for millions of years," she said. "It might be an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be no surprise that ancient hominins – and even Neanderthals and our own species collectively – engaged intimately."