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Among seabirds to polar bears, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists propose that ancient hominins did it too – and possibly locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have suggested Neanderthals and early modern humans were intimately acquainted. Among previous studies, scientists have discovered modern people and their thick-browed cousins shared the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the concept aligned with research that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating interbreeding was at play.

Intimate Interpretation

"This offers a more romantic spin on human-Neanderthal relations," Brindle commented.

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

Describing Intimate Contact

"There have been some efforts to define a kiss, but it's very much been human-centric, which implies that essentially other animals do not engage in this. Now we understand that they probably do, it might just not look from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she said some behaviors that looked like kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species called certain marine animals.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving intentional oral interaction with a member of the identical group, with some motion of the mouth but absence of nutrition.

Study Approach

Brindle said they focused on reports of intimate behavior in primates from Africa and Asia, including primates, apes and orangutans, and employed online videos to verify the reports.

The researchers then integrated this information with information on the evolutionary relationships between living and extinct types of such animals.

Historical Timeline

Researchers propose the findings indicate intimate contact evolved approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the scientists conclude. But the activity might not have been confined to their specific group.

"Reality that modern people engage intimately, the reality that we now have shown that ancient relatives very likely kissed, indicates that the two [species] are also likely to have kissed," Brindle added.

Biological Significance

While the scientific reasoning is debated, Brindle explained kissing could be employed in reproductive situations to possibly enhance mating outcomes or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the activities of great apes commented that as intimate contact 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 intimate behavior among a wider variety of animals might extend its beginnings back further still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not unique to us if we look closely at different species," the expert noted.

Social Elements

Another professor said that kissing had a social component as it was not universal to all societies.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and methods of promoting confidence and intimacy will have been important for eons," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and including Neanderthals and our own species collectively – engaged intimately."