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From Galápagos albatrosses to polar bears, primates to orangutans, various animals appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In previous studies, researchers have discovered humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with research that has found humans of non-African ancestry contain Neanderthal DNA in their genome, revealing genetic mixing was occurring.

Romantic Spin

"This offers a more romantic perspective on human-Neanderthal relations," the lead researcher commented.

Publishing in the publication a scientific periodical, the researcher and her team detail how, to explore the historical roots of kissing, they first had to develop a description that was not limited to how humans smooch.

Defining Kissing

"There have been some efforts to define a intimate act, but it's largely focused on humans, which means that essentially non-human species don't kiss. Now we know that they probably do, it may appear different from what our intimate contact looks like," said Brindle.

However, she noted some actions that resembled intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in fish known as French grunts.

As a result the team came up with a definition of kissing centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but no transfer of nutrition.

Study Approach

The lead researcher said they focused on accounts of kissing in non-human species from Africa and Asian regions, including primates, chimpanzees and orangutans, and used digital recordings to verify the reports.

The researchers then integrated this information with details on the evolutionary relationships between extant and ancient types of such primates.

Evolutionary Timeline

Researchers propose the results suggest intimate contact evolved somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

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

"Reality that modern people engage intimately, the reality that we now have demonstrated that Neanderthals very likely kissed, indicates that the both groups are probably did kissed," the researcher added.

Biological Significance

Although the scientific reasoning is discussed, Brindle said kissing could be employed in reproductive situations to possibly enhance reproductive success or assist in selecting between mates, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as intimate contact was seen in a wide range of primates it made sense its origins extend far into our ancient history, and an analysis of different forms of intimate behavior among a wider variety of species might push its origins back even earlier still.

"Behaviors that we think of as characteristics 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 human groups.

"However, as humans we succeed or struggle on the strength of our emotional bonds, and ways of promoting trust and closeness will have been important for millions of years," the professor stated. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it ought to be expected that ancient hominins – and even them and our human ancestors together – kissed."