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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Shared Microbial Clues

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were intimately acquainted. In earlier research, researchers have discovered humans and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with studies that has revealed people of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing genetic mixing was at play.

Romantic Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," the lead researcher said.

Writing in the publication a scientific periodical, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to come up with a description that was not restricted by how people kiss.

Defining Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's very much been human-centric, which means that basically other animals don't kiss. Currently we know that they probably do, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as French grunts.

Consequently the research group developed a definition of intimate contact based on friendly interactions involving directed oral interaction with a individual of the same species, with some motion of the oral area but no transfer of food.

Research Approach

Brindle explained they concentrated on accounts of kissing in non-human species from Africa and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the observations.

Scientists then integrated this data with details on the evolutionary relationships between extant and extinct types of such animals.

Evolutionary Timeline

Researchers propose the findings indicate kissing evolved somewhere between 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this evolutionary lineage means it is likely they, too, engaged in a kiss, the scientists conclude. But the activity might not have been limited to their own species.

"The fact that humans engage intimately, the fact that we now have shown that ancient relatives very likely engaged, indicates that the both groups are also likely to have engage," Brindle noted.

Evolutionary Importance

While the scientific reasoning is debated, the expert explained kissing could be used in reproductive situations to potentially increase reproductive success or assist in selecting between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of great apes said that as intimate contact was seen in a broad spectrum of primates it made sense its roots lie deep in our evolutionary past, and an analysis of different forms of kissing among a wider variety of species might extend its beginnings back even earlier still.

"Things that we think of as characteristics of our species, like kissing, are not unique to us if we look closely at other animals," he said.

Social Aspects

Another professor explained that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and methods of encouraging confidence and closeness will have been important for eons," she said. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but really it should be no surprise that ancient hominins – and even Neanderthals and our human ancestors collectively – kissed."