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Among Galápagos albatrosses to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Clues

It is not the first time scientists have suggested Neanderthals and Homo sapiens were closely connected. In earlier research, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the idea aligned with research that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing interbreeding was at play.

Intimate Interpretation

"This offers a more romantic spin on human-Neanderthal relations," the lead researcher commented.

Writing in the journal a scientific periodical, Brindle and colleagues report how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how people smooch.

Defining Kissing

"Previously there were some efforts to define a kiss, but it's very much been focused on humans, which means that essentially non-human species do not engage in this. Now we know that they probably do, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

Nonetheless, she noted some behaviors that resembled kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in fish known as French grunts.

Consequently the team came up with a description of kissing centered around social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but no transfer of food.

Research Methods

The lead researcher explained they concentrated on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and orangutans, and used digital recordings to confirm the observations.

The researchers then integrated this data with information on the genetic connections between extant and extinct species of such animals.

Evolutionary Origins

The team propose the findings suggest kissing evolved approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

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

"Reality that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives probably engaged, suggests that the both groups are also likely to have kissed," Brindle noted.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle explained kissing could be used in reproductive situations to potentially increase mating outcomes or assist in selecting between partners, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of primates it made sense 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 beginnings back further still.

"Things that we think of as signatures of human life, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Social Aspects

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

"However, as humans we thrive or fail on the strength of our relationships, and methods of promoting confidence and closeness will have been important for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and including them and our own species together – kissed."