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From seabirds to Arctic mammals, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Common Oral Clues

It is not the first time experts have suggested Neanderthals and early modern humans were intimately acquainted. Among previous studies, scientists have found modern people and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with studies that has revealed humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Interpretation

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

Writing in the publication a scientific periodical, Brindle and colleagues detail how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans kiss.

Defining Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's very much been human-centric, which implies that essentially non-human species do not engage in this. Now we understand that they likely engage, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

Nonetheless, she said some actions that resembled kissing were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in fish known as certain marine animals.

Consequently the research group came up with a description of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some movement of the oral area but absence of nutrition.

Study Methods

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

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

Evolutionary Timeline

The team say the findings indicate kissing developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the great primates.

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

"The fact that humans kiss, the reality that we currently have shown that Neanderthals very likely kissed, indicates that the both groups are probably did kissed," the researcher noted.

Evolutionary Significance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be used in sexual contexts to possibly increase mating outcomes or assist in selecting between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of primates commented that as intimate contact was seen in a wide range 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 species might push its beginnings back even earlier still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not unique to us if we examine carefully at other animals," he said.

Cultural Aspects

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

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and methods of encouraging trust and intimacy will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but actually it ought to be no surprise that ancient hominins – and including Neanderthals and our human ancestors together – kissed."