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From Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Now, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Evidence

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, scientists have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, adding that the concept aligned with studies that has found people of non-African ancestry contain Neanderthal DNA in their genome, demonstrating genetic mixing was occurring.

Romantic Spin

"It certainly puts a more romantic spin on ancient interactions," Brindle said.

Writing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of kissing, they first had to develop a definition that was not restricted by how people smooch.

Defining Intimate Contact

"There have been some efforts to define a intimate act, but it's largely human-centric, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact looks like," explained Brindle.

However, she said some behaviors that looked like kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in fish called French grunts.

Consequently the research group developed a definition of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but absence of nutrition.

Study Methods

The lead researcher explained they focused on accounts of kissing in primates from Africa and Asia, including bonobos, apes and great apes, and employed digital recordings to confirm the reports.

The researchers then integrated this data with information on the genetic connections between extant and ancient types of such animals.

Historical Timeline

Researchers say the findings suggest kissing evolved somewhere between 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree means it is likely they, too, indulged in a kiss, the scientists conclude. But the activity might not have been limited to their specific group.

"Reality that modern people engage intimately, the fact that we currently have demonstrated that Neanderthals probably kissed, suggests that the both groups are probably did engage," Brindle noted.

Biological Significance

While the scientific reasoning is discussed, the expert said intimate contact could be used in reproductive situations to possibly increase reproductive success or assist in selecting between mates, while it could assist strengthen connections when used in a platonic way.

Another expert in the behavior of great apes said that as kissing behavior was observed in a wide range of primates it made sense its roots lie deep in our ancient history, and an examination of various types of intimate behavior among a wider variety of species might extend its beginnings back even earlier still.

"Things that we consider as signatures of human life, like kissing, are not unique to us if we examine carefully at other animals," the expert noted.

Social Aspects

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

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and methods of encouraging trust and intimacy will have been significant for millions of years," the professor stated. "It might be an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be no surprise that ancient hominins – and even Neanderthals and our human ancestors collectively – kissed."