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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and might even have locked lips with modern humans.

Shared Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. In previous studies, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

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

Romantic Spin

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the historical roots of kissing, they first had to develop a description that was not limited to how people kiss.

Defining Intimate Contact

"There have been some previous attempts to define a intimate act, but it's largely human-centric, which means that basically other animals don't kiss. Currently we know that they likely engage, it might just not look from what our intimate contact resembles," explained Brindle.

Nonetheless, she noted some actions that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "mouth contact", seen in fish called French grunts.

Consequently the research group came up with a definition of kissing based on social behaviors involving directed oral interaction with a individual of the identical group, with some movement of the mouth but no transfer of food.

Research Approach

The lead researcher said they focused on reports of intimate behavior in non-human species from the African continent and Asia, including primates, chimpanzees and orangutans, and employed online videos to verify the reports.

Scientists then integrated this information with details on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Origins

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

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

"Reality that modern people kiss, the reality that we currently have demonstrated that ancient relatives very likely kissed, suggests that the both groups are probably did engage," Brindle noted.

Evolutionary Significance

Although the scientific reasoning is debated, the expert said intimate contact could be used in sexual contexts to possibly enhance reproductive success or assist in selecting between partners, while it could assist reinforce bonding when used in a platonic way.

A separate researcher in the activities of great apes said that as intimate contact was observed in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an examination of different forms of kissing among a broader range of species might push its origins back even earlier still.

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

Social Aspects

Another professor said that kissing had a cultural element as it was not universal to all societies.

"However, as humans we succeed or struggle on the strength of our relationships, and methods of promoting trust and intimacy will have been significant for millions of years," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be no surprise that Neanderthals – and even Neanderthals and our own species together – engaged intimately."