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Among seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Now, scientists suggest that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Shared Microbial Clues

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, scientists have discovered modern people and their Neanderthal relatives possessed the same mouth microbe for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with studies that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Romantic Spin

"This offers a different perspective on ancient interactions," Brindle commented.

Publishing in the publication a scientific periodical, the researcher and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Defining 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 don't kiss. Now we know that they probably do, it may appear different from what our intimate contact looks like," said the evolutionary biologist.

However, she noted some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", observed in fish known as French grunts.

Consequently the team came up with a description of kissing based on friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of nutrition.

Study Approach

The lead researcher explained they focused on accounts of kissing in non-human species from Africa and Asia, including bonobos, chimpanzees and great apes, and used online videos to confirm the reports.

The researchers then combined this information with information on the genetic connections between extant and extinct types of such animals.

Historical Origins

The team propose the results indicate kissing evolved approximately 21.5 million 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 confined to their own species.

"Reality that modern people kiss, the fact that we now have demonstrated that Neanderthals very likely engaged, suggests that the two [species] are probably did engage," Brindle noted.

Evolutionary Significance

While the scientific reasoning is debated, Brindle said intimate contact could be used in reproductive situations to possibly enhance reproductive success or assist in selecting between mates, while it could assist strengthen connections when used in a non-sexual manner.

A separate researcher in the activities of great apes commented that as kissing behavior was seen in a wide range of apes it was logical its origins extend far into our evolutionary past, and an examination of different forms of kissing among a wider variety of species might extend its beginnings back even earlier still.

"Things that we consider as characteristics of our species, like kissing, are not unique to us if we look closely at other animals," the expert noted.

Cultural Elements

An archaeology expert explained that kissing had a social component as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and ways of encouraging trust and intimacy will have been significant for millions of years," she said. "This could represent an concept that appears a bit incongruous 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 human ancestors together – engaged intimately."