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Among seabirds to Arctic mammals, chimpanzees to great apes, various animals appear to kiss. Currently, scientists suggest that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were closely connected. Among previous studies, researchers have found humans and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the idea aligned with studies that has found humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, revealing interbreeding was occurring.

Romantic Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how people kiss.

Defining Kissing

"There have been some efforts to describe a kiss, but it's largely focused on humans, which means that basically other animals do not engage in this. Now we understand that they likely engage, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

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

Consequently the research group developed a definition of intimate contact based on social behaviors involving directed oral interaction with a member of the identical group, with some movement of the mouth but absence of nutrition.

Study Approach

The lead researcher said they concentrated on accounts of intimate behavior in primates from the African continent and Asia, including bonobos, apes and great apes, and used online videos to confirm the reports.

The researchers then integrated this information with information on the evolutionary relationships between living and extinct species of such primates.

Evolutionary Timeline

The team propose the findings suggest kissing evolved approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

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

"The fact that modern people kiss, the reality that we currently have shown that ancient relatives probably kissed, suggests that the two [species] are probably did engage," Brindle added.

Biological Importance

Although the scientific reasoning is debated, the expert explained kissing could be employed in sexual contexts to potentially increase reproductive success or help choose between partners, while it could assist strengthen connections when practiced in a non-sexual manner.

A separate researcher in the behavior of primates said that as kissing behavior was seen in a broad spectrum of apes it made sense its roots lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of species might extend its origins back further still.

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

Social Elements

Another professor said that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the strength of our relationships, and methods of promoting trust and intimacy will have been significant for millions of years," she said. "This could represent an image that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it ought to be no surprise that ancient hominins – and even them and our human ancestors together – kissed."