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From Galápagos albatrosses to polar bears, primates to great apes, certain species appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were intimately acquainted. Among earlier research, researchers have found modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," the researcher noted, adding that the idea aligned with studies that has found humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Interpretation

"This offers a different spin on ancient interactions," the lead researcher said.

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the historical roots of kissing, they first had to develop a description that was not restricted by how people smooch.

Defining Kissing

"Previously there were some previous attempts to define a intimate act, but it's largely human-centric, which implies that essentially other animals do not engage in this. Currently we know that they probably do, it might just not look from what our intimate contact looks like," said Brindle.

However, she noted some behaviors that looked like intimate contact were something rather different – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species called certain marine animals.

As a result the team came up with a definition of intimate contact centered around social behaviors involving intentional oral interaction with a individual of the same species, with some motion of the oral area but no transfer of nutrition.

Research Approach

Brindle said they focused on accounts of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the observations.

The researchers then combined this information with details on the evolutionary relationships between extant and extinct types of such animals.

Evolutionary Origins

Researchers propose the results suggest kissing evolved somewhere between 21.5m and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this evolutionary lineage suggests it is likely they, too, engaged in a intimate act, the scientists conclude. But the activity might not have been limited to their specific group.

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

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle said kissing could be employed in sexual contexts to potentially enhance reproductive success or help choose between partners, while it might help strengthen connections when practiced in a platonic way.

Another expert in the behavior of primates said that as intimate contact was seen in a broad spectrum of primates it was logical its roots extend far into our ancient history, and an examination of various types of intimate behavior among a wider variety of animals might extend its origins back further still.

"Things that we think of as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at different species," the expert noted.

Social Aspects

Another professor said that intimate contact 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 ways 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 misplaced ideas of a rather ruthless and ancient history, but really it should be expected that ancient hominins – and even Neanderthals and our own species collectively – engaged intimately."