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From Galápagos albatrosses to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, scientists have found humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, adding that the concept chimed with studies that has revealed people of certain genetic backgrounds contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Spin

"It certainly puts a different perspective on ancient interactions," Brindle said.

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to investigate the evolutionary origins of kissing, they first had to develop a description that was not limited to how humans smooch.

Defining Kissing

"Previously there were some efforts to define a kiss, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Now we understand that they likely engage, it may appear different from what human kissing resembles," explained the evolutionary biologist.

However, she said some behaviors that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in fish called certain marine animals.

As a result the research group came up with a description of kissing centered around social behaviors involving directed oral interaction with a member of the identical group, with some motion of the oral area but no transfer of food.

Study Approach

The lead researcher explained they concentrated on accounts 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.

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

Evolutionary Timeline

Researchers say the findings suggest 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 family tree suggests it is likely they, too, engaged in a intimate act, the scientists conclude. But the activity may not have been confined to their own species.

"Reality that modern people kiss, the reality that we currently have shown that Neanderthals very likely engaged, suggests that the both groups are probably did kissed," the researcher noted.

Biological Significance

While the evolutionary explanation is debated, Brindle explained kissing could be used in sexual contexts to possibly enhance reproductive success or help choose between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of primates said that as intimate contact was observed in a wide range of apes it was logical its origins lie deep in our ancient history, and an analysis of different forms of kissing among a broader range of animals might extend its beginnings back even earlier 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.

Social Elements

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging trust and intimacy will have been important for eons," she said. "It might be an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be expected that Neanderthals – and even them and our human ancestors collectively – engaged intimately."