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Among Galápagos albatrosses to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins did it too – and might even have locked lips with modern humans.

Shared Oral Clues

This isn't the initial instance experts have proposed Neanderthals and early modern humans were closely connected. Among previous studies, scientists have discovered modern people and their thick-browed cousins 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, adding that the concept aligned with studies that has revealed people of non-African ancestry have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

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

Publishing in the publication a scientific periodical, Brindle and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how people kiss.

Defining Kissing

"There have been some previous attempts to define a kiss, but it's largely human-centric, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what human kissing looks like," explained the evolutionary biologist.

However, she noted some behaviors that looked like kissing were distinct activities – such as the processing and food sharing, or "mouth contact", seen in aquatic species known as certain marine animals.

Consequently the team developed a description of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the oral area but no transfer of nutrition.

Study Methods

The lead researcher said they concentrated on reports of kissing in primates from Africa and Asian regions, including primates, apes and great apes, and employed digital recordings to verify the observations.

Scientists then combined this data with details on the evolutionary relationships between living and extinct types of such primates.

Evolutionary Timeline

Researchers propose the results indicate kissing developed approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a kiss, the researchers say. But the activity might not have been limited to their specific group.

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

Evolutionary Significance

Although the scientific reasoning is discussed, the expert explained kissing could be employed in reproductive situations to potentially increase mating outcomes or help choose between mates, while it might help strengthen connections when used in a platonic way.

Another expert in the behavior of great apes commented that as intimate contact was seen in a wide range of apes it was logical its roots extend far into our evolutionary past, and an examination of different forms of kissing among a wider variety of species might push its beginnings back even earlier still.

"Things that we consider as signatures of human life, like intimate contact, are not unique to us if we examine carefully at different species," he said.

Social Aspects

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

"However, as people we thrive or fail on the strength of our relationships, and ways of promoting trust and intimacy will have been important for eons," she said. "This could represent an concept that appears a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be no surprise that ancient hominins – and including them and our own species collectively – kissed."