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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Evidence

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the concept aligned with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Romantic Interpretation

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

Publishing in the journal a scientific periodical, Brindle and colleagues report how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not limited to how humans kiss.

Describing Kissing

"Previously there were some efforts to define a intimate act, but it's largely human-centric, which implies that essentially non-human species don't kiss. Currently we know that they probably do, it might just not look from what our intimate contact resembles," explained Brindle.

Nonetheless, she said some actions that resembled intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", seen in aquatic species known as certain marine animals.

As a result the research group developed a description of intimate contact centered around friendly interactions involving directed oral interaction with a member of the same species, with some movement of the oral area but absence of nutrition.

Research Methods

Brindle said they focused on reports of kissing in non-human species from the African continent and Asia, including bonobos, chimpanzees and great apes, and employed online videos to confirm the reports.

Scientists then combined this data with details on the evolutionary relationships between living and ancient species of such primates.

Historical Timeline

Researchers say the results suggest kissing developed approximately 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of Neanderthals on this evolutionary lineage means it is likely they, too, indulged in a kiss, the researchers conclude. But the activity might not have been limited to their specific group.

"Reality that modern people kiss, the reality that we now have demonstrated that ancient relatives very likely kissed, indicates that the two [species] are also likely to have kissed," the researcher added.

Biological Importance

While the evolutionary explanation is debated, Brindle said intimate contact could be used in sexual contexts to potentially increase mating outcomes or help choose between mates, while it might help strengthen connections when used in a platonic way.

Another expert in the activities of great apes said that as intimate contact was observed in a broad spectrum of primates it made sense its roots extend far into our evolutionary past, and an examination of various types of intimate behavior among a wider variety of animals might push its origins back even earlier still.

"Things that we think of as signatures of human life, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Cultural Aspects

An archaeology expert said that kissing had a social component as it was not universal to all human groups.

"However, as humans we succeed or struggle on the strength of our relationships, and methods of promoting confidence and intimacy will have been significant for millions of years," the professor stated. "It might be an concept that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be expected that ancient hominins – and including them and our human ancestors together – engaged intimately."