Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals did it too – and possibly exchanged kisses with modern humans.
It is not the first time experts have suggested ancient relatives and early modern humans were intimately acquainted. In earlier research, researchers have found humans and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.
"Probably they were kissing," she said, explaining that the concept aligned with studies that has revealed people of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.
"This offers a more romantic spin on ancient interactions," Brindle commented.
Publishing in the publication Evolution and Human Behavior, the researcher and colleagues report how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how people smooch.
"Previously there were some efforts to describe a intimate act, 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," explained the evolutionary biologist.
Nonetheless, she noted some behaviors that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in aquatic species known as French grunts.
As a result the research group developed a definition of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but absence of nutrition.
Brindle explained they focused on accounts of intimate behavior in non-human species from the African continent and Asian regions, including primates, chimpanzees and great apes, and employed online videos to confirm the reports.
Scientists then combined this data with information on the genetic connections between extant and ancient species of such animals.
The team propose the findings suggest kissing developed approximately 21.5m and 16.9m years ago in the predecessors of the great primates.
Placement of Neanderthals on this evolutionary lineage means it is probable they, too, indulged in a kiss, the researchers say. But the behavior may not have been confined to their own species.
"Reality that humans engage intimately, the fact that we now have demonstrated that Neanderthals probably kissed, indicates that the two [species] are probably did engage," the researcher added.
While the scientific reasoning is discussed, Brindle explained kissing could be used in reproductive situations to potentially enhance mating outcomes or help choose between partners, while it could assist reinforce bonding when used in a non-sexual manner.
Another expert in the activities of great apes said that as kissing behavior was observed in a wide range of primates it made sense its origins extend far into our evolutionary past, and an examination of various types of kissing among a broader range of species might extend its origins back further still.
"Behaviors that we think of as characteristics of human life, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.
Another professor explained that intimate contact had a social component as it was not universal to all societies.
"However, as humans we thrive or fail on the strength of our relationships, and ways of encouraging trust and closeness will have been important for eons," she said. "This could represent an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be expected that Neanderthals – and including Neanderthals and our own species collectively – kissed."
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