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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Evidence

It is not the first time scientists have suggested Neanderthals and Homo sapiens were closely connected. In earlier research, scientists have found humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," she said, explaining that the idea chimed with research that has found humans of non-African ancestry have bits of Neanderthal DNA in their genome, demonstrating genetic mixing was occurring.

Intimate Spin

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

Publishing in the publication Evolution and Human Behavior, Brindle and her team detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not restricted by how humans kiss.

Describing Intimate Contact

"Previously there were some efforts to define a kiss, but it's largely human-centric, which means that basically other animals do not engage in this. Currently we know that they likely engage, it may appear different from what our intimate contact resembles," said Brindle.

However, she said some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", observed in fish known as French grunts.

Consequently the team developed a definition of intimate contact based on social behaviors involving intentional oral interaction with a individual of the identical group, with some movement of the oral area but no transfer of food.

Study Approach

The lead researcher said they concentrated on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and great apes, and used digital recordings to verify the reports.

The researchers then combined this information with details on the genetic connections between extant and ancient types of such animals.

Historical Origins

The team propose the results suggest kissing evolved approximately 21.5m and 16.9 million years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree suggests it is probable they, too, engaged in a intimate act, the scientists conclude. But the behavior may not have been confined to their specific group.

"Reality that humans kiss, the reality that we now have shown that ancient relatives very likely engaged, suggests that the two [species] are probably did kissed," Brindle added.

Evolutionary Significance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be used in reproductive situations to potentially enhance mating outcomes or assist in selecting between partners, while it could assist strengthen connections when practiced in a platonic way.

Another expert in the activities of great apes commented that as intimate contact was seen in a broad spectrum of primates it was logical its origins lie deep in our ancient history, and an analysis of various types of kissing among a broader range of animals might push its beginnings back even earlier still.

"Things that we consider as characteristics of our species, like kissing, are not exclusive to us if we look closely at other animals," 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 succeed or struggle on the strength of our relationships, and ways of promoting confidence and intimacy will have been significant for millions of years," the professor stated. "It might be an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be expected that ancient hominins – and including Neanderthals and our human ancestors together – kissed."