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From seabirds to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time experts have suggested ancient relatives and early modern humans were closely connected. 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 evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept aligned with research that has found people of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Intimate Spin

"This offers a different spin on human-Neanderthal relations," Brindle said.

Publishing in the journal a scientific periodical, Brindle and colleagues report how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not restricted by how people kiss.

Defining Intimate Contact

"There have been some efforts to define a kiss, but it's very much been focused on humans, which means that basically other animals do not engage in this. Currently we know that they probably do, it may appear different from what our intimate contact looks like," explained 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 called certain marine animals.

Consequently the research group came up with a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some motion of the oral area but no transfer of nutrition.

Research Methods

Brindle explained they focused on reports of intimate behavior in non-human species from the African continent and Asia, including bonobos, apes and great apes, and used digital recordings to verify the observations.

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

Historical Origins

The team propose the findings suggest kissing evolved somewhere between 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the researchers conclude. But the behavior may not have been limited to their own species.

"The fact that modern people kiss, the fact that we currently have shown that Neanderthals probably engaged, suggests that the both groups are probably did kissed," the researcher noted.

Evolutionary Significance

Although the scientific reasoning is debated, the expert explained kissing could be employed in sexual contexts 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 activities of great apes said that as kissing behavior was observed in a wide range of primates it made sense its roots lie deep in our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might push its beginnings back further still.

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

Social Aspects

An archaeology expert explained that intimate contact had a cultural element as it was not common to all human groups.

"However, as people we thrive or fail on the strength of our relationships, and methods of promoting trust and intimacy will have been important for millions of years," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and even them and our own species collectively – engaged intimately."