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From seabirds to polar bears, primates to great apes, various animals engage in mouth-to-mouth contact. Now, researchers suggest that Neanderthals did it too – and possibly exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time experts have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, researchers have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, adding that the concept chimed with studies that has revealed people of non-African ancestry contain ancient genetic material in their genome, demonstrating genetic mixing was at play.

Romantic Interpretation

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

Writing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not restricted by how people kiss.

Describing Kissing

"There have been some previous attempts to define a intimate act, but it's very much been human-centric, which means that essentially non-human species do not engage in this. Currently we know that they probably do, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she noted some actions that resembled intimate contact were distinct activities – such as the processing and food sharing, or "mouth contact", seen in aquatic species known as certain marine animals.

As a result the team developed a definition of kissing centered around social behaviors involving directed oral interaction with a member of the same species, with some motion of the oral area but absence of nutrition.

Study Methods

The lead researcher explained they concentrated on reports of kissing in non-human species from the African continent and Asia, including bonobos, apes and great apes, and used digital recordings to verify the reports.

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

Evolutionary Origins

The team propose the results suggest intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the predecessors of the great primates.

Placement of Neanderthals on this family tree suggests it is likely they, too, indulged in a intimate act, the researchers conclude. But the behavior may not have been limited to their specific group.

"Reality that modern people kiss, the fact that we now have demonstrated that Neanderthals probably kissed, suggests that the both groups are probably did kissed," the researcher noted.

Biological Importance

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

Another expert in the activities of great apes said that as intimate contact was observed in a broad spectrum of apes it made sense its origins extend far into our ancient history, and an analysis of various types of kissing among a broader range of species might extend its beginnings back further still.

"Behaviors that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at other animals," he said.

Cultural Aspects

Another professor said that kissing had a cultural element as it was not common to all human groups.

"However, as people we succeed or struggle on the quality of our relationships, and ways of promoting trust and closeness will have been significant for eons," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and even them and our own species collectively – kissed."