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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

It is not the first time scientists have proposed Neanderthals and early modern humans were closely connected. Among previous studies, researchers have discovered modern people and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, explaining that the concept chimed with research that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genome, revealing interbreeding was occurring.

Romantic Interpretation

"This offers a different perspective on ancient interactions," the lead researcher said.

Publishing in the publication a scientific periodical, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Describing Kissing

"Previously there were some efforts to define a kiss, but it's largely human-centric, which means that essentially non-human species do not engage in this. Now we understand that they probably do, it may appear different from what human kissing resembles," explained the evolutionary biologist.

Nonetheless, she noted some behaviors that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

Consequently the research group developed a definition of intimate contact centered around social behaviors involving intentional oral interaction with a individual of the identical group, with some movement of the oral area but no transfer of nutrition.

Study Methods

The lead researcher explained they concentrated on reports of intimate behavior in primates from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and used digital recordings to verify the observations.

The researchers then combined this data with details on the evolutionary relationships between extant and extinct species of such primates.

Historical Timeline

Researchers say the results suggest kissing evolved somewhere between 21.5 million and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree means it is likely they, too, engaged in a kiss, the researchers say. But the activity may not have been limited to their own species.

"Reality that modern people kiss, the fact that we currently have shown that ancient relatives probably kissed, indicates that the both groups are also likely to have kissed," the researcher noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle said intimate contact could be employed in reproductive situations to potentially enhance reproductive success or help choose between partners, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the activities of primates commented that as kissing behavior was seen in a wide range of primates it made sense its roots lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of animals might extend its origins back even earlier still.

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

Cultural Aspects

An archaeology expert said that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as people we succeed or struggle on the quality of our relationships, and ways of promoting trust and closeness will have been important for millions of years," the professor stated. "This could represent an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be no surprise that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."