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Among seabirds to polar bears, chimpanzees to great apes, certain species appear to kiss. Now, researchers propose that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time experts have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, scientists have discovered modern people and their Neanderthal relatives shared 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," she said, explaining that the concept chimed with studies that has revealed humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Romantic Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher said.

Writing in the journal Evolution and Human Behavior, the researcher and her team report how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how humans smooch.

Describing Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been focused on humans, which implies that essentially other animals do not engage in this. Currently we understand that they likely engage, it may appear different from what human kissing resembles," explained the evolutionary biologist.

However, she said some behaviors that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish known as French grunts.

As a result the research group developed a description of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but no transfer of food.

Research Approach

Brindle said they concentrated on accounts of kissing in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and used online videos to verify the observations.

The researchers then integrated this data with information on the evolutionary relationships between extant and extinct species of such primates.

Historical Timeline

The team say the findings suggest kissing developed somewhere between 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of ancient hominins on this family tree means it is likely they, too, engaged in a kiss, the researchers conclude. But the behavior might not have been limited to their own species.

"The fact that modern people kiss, the fact that we now have demonstrated that ancient relatives probably kissed, indicates that the two [species] are probably did engage," Brindle noted.

Evolutionary Importance

While the evolutionary explanation is discussed, Brindle said kissing could be used in sexual contexts to possibly increase mating outcomes or help choose between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the activities of great apes said that as kissing behavior was seen in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of animals might extend its beginnings back even earlier still.

"Things that we think of as signatures of our species, like kissing, are not unique to us if we examine carefully at other animals," the expert noted.

Social Aspects

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

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and methods of promoting trust and intimacy will have been significant for millions of years," the professor stated. "It might be an image that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be expected that Neanderthals – and including them and our human ancestors collectively – kissed."