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Among seabirds to polar bears, primates to great apes, various animals appear to kiss. Now, scientists propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with modern humans.

Shared Microbial Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. Among previous studies, scientists have found humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," she said, explaining that the idea aligned with research that has revealed humans of non-African ancestry contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Romantic Interpretation

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

Publishing in the journal a scientific periodical, Brindle and her team report how, to investigate the evolutionary origins of kissing, they first had to come up with a description that was not limited to how people smooch.

Describing Kissing

"Previously there were some previous attempts to define a kiss, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing resembles," explained Brindle.

However, she said some behaviors that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in aquatic species called certain marine animals.

As a result the team developed a description of intimate contact centered around social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but absence of food.

Study Methods

Brindle said they focused on accounts of intimate behavior in primates from Africa and Asia, including bonobos, apes and orangutans, and used online videos to confirm the observations.

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

Historical Timeline

Researchers propose the findings indicate kissing evolved approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this family tree suggests it is likely they, too, indulged in a kiss, the scientists say. But the activity might not have been limited to their specific group.

"The fact that modern people engage intimately, the fact that we currently have shown that Neanderthals probably kissed, indicates that the two [species] are probably did engage," the researcher added.

Evolutionary Significance

Although the evolutionary explanation is debated, the expert explained intimate contact could be used in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it might help strengthen connections when used in a platonic way.

Another expert in the activities of great apes said that as intimate contact was seen in a wide range of apes it made sense its origins lie deep in our ancient history, and an examination of different forms of kissing among a wider variety of species might extend its beginnings back further still.

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

Cultural Aspects

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

"However, as people we succeed or struggle on the strength of our relationships, and methods of encouraging trust and closeness will have been significant for eons," the professor stated. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and including Neanderthals and our own species together – engaged intimately."