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From Galápagos albatrosses to Arctic mammals, primates to orangutans, various animals appear to kiss. Now, scientists propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Microbial Clues

It is not the first time experts have suggested ancient relatives and early modern humans were closely connected. Among previous studies, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," 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.

Intimate Interpretation

"It certainly puts a different spin on ancient interactions," Brindle commented.

Publishing 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 limited to how humans kiss.

Describing Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's very much been human-centric, which means that essentially other animals don't kiss. Now we understand that they probably do, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

Nonetheless, she said some behaviors that looked like kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species known as French grunts.

Consequently the team 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 movement of the oral area but absence of food.

Study Methods

Brindle said they concentrated on reports of intimate behavior in non-human species from Africa and Asia, including primates, chimpanzees and orangutans, and used digital recordings to confirm the reports.

Scientists then combined this information with details on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Origins

The team propose the results suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

Placement of Neanderthals on this family tree suggests it is probable they, too, engaged in a kiss, the scientists say. But the activity might not have been limited to their own species.

"Reality that humans engage intimately, the fact that we now have shown that ancient relatives very likely kissed, indicates that the both groups are probably did engage," the researcher added.

Biological Importance

While the scientific reasoning is discussed, Brindle said intimate contact could be employed in reproductive situations to possibly enhance mating outcomes or assist in selecting between mates, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of primates commented that as kissing behavior was observed in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a wider variety of animals might extend its beginnings back even earlier still.

"Things that we consider as signatures of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," the expert noted.

Cultural Elements

Another professor said that intimate contact had a cultural element as it was not universal to all societies.

"However, as people we thrive or fail on the strength of our relationships, and ways of encouraging trust and intimacy will have been important for eons," she said. "It might be an image that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be no surprise that Neanderthals – and even Neanderthals and our own species together – kissed."