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Among seabirds to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists propose that Neanderthals also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, scientists have discovered humans and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea chimed with studies that has found humans of certain genetic backgrounds contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Romantic Spin

"It certainly puts a more romantic spin on ancient interactions," Brindle said.

Writing in the publication Evolution and Human Behavior, Brindle and her team detail how, to investigate the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how humans kiss.

Describing Kissing

"Previously there were some previous attempts to describe a kiss, but it's largely human-centric, which implies that basically other animals do not engage in this. Currently we understand that they likely engage, it may appear different from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she said some behaviors that looked like kissing were something rather different – such as the chewing and food sharing, or "mouth contact", observed in fish called French grunts.

Consequently the team developed a definition of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the oral area but no transfer of nutrition.

Research Methods

Brindle explained they focused on reports of kissing in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and used online videos to verify the reports.

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

Historical Origins

Researchers propose the results suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a intimate act, the scientists conclude. But the behavior might not have been confined to their specific group.

"Reality that humans engage intimately, the fact that we currently have shown that Neanderthals probably kissed, suggests that the two [species] are probably did kissed," the researcher noted.

Evolutionary Significance

Although the scientific reasoning is debated, Brindle said intimate contact could be used in reproductive situations to potentially increase reproductive success or assist in selecting between mates, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the activities of primates commented 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 analysis of different forms of kissing among a wider variety of species might push its beginnings back even earlier still.

"Things that we think of as characteristics of human life, like kissing, are not exclusive to us if we look closely at other animals," he said.

Social Elements

An archaeology expert said that kissing had a cultural element as it was not universal to all societies.

"However, as people we thrive or fail on the strength of our emotional bonds, and methods of encouraging trust and intimacy will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but really it ought to be no surprise that ancient hominins – and even Neanderthals and our human ancestors together – engaged intimately."