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Among Galápagos albatrosses to polar bears, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Oral Clues

It is not the first time experts have proposed ancient relatives and early modern humans were closely connected. Among previous studies, scientists have found humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea chimed with studies that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing genetic mixing was at play.

Intimate Interpretation

"It certainly puts a more romantic spin on ancient interactions," the lead researcher said.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Defining Kissing

"Previously there were some previous attempts to describe a intimate act, but it's very much been human-centric, which implies that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what our intimate contact looks like," said Brindle.

Nonetheless, she noted some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in aquatic species known as French grunts.

Consequently the team came up with a definition of kissing centered around social behaviors involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but no transfer of nutrition.

Study Approach

The lead researcher said they focused on accounts of intimate behavior in primates from Africa and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the reports.

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

Historical Timeline

Researchers propose the findings indicate intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the scientists say. But the behavior may not have been limited to their specific group.

"Reality that humans engage intimately, the fact that we currently have demonstrated that ancient relatives probably kissed, suggests that the both groups are also likely to have kissed," the researcher added.

Biological Significance

Although the evolutionary explanation is discussed, the expert said kissing could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a non-sexual manner.

Another expert in the behavior of great apes commented that as kissing behavior was seen in a wide range of primates it was logical its roots lie deep in our ancient history, and an analysis of different forms of intimate behavior among a broader range of animals might extend its origins back further still.

"Behaviors that we consider as characteristics of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Cultural Aspects

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

"However, as humans we thrive or fail on the quality of our emotional bonds, and methods of encouraging trust and closeness will have been important for millions of years," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be expected that Neanderthals – and including them and our human ancestors together – engaged intimately."