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From seabirds to Arctic mammals, chimpanzees to great apes, various animals appear to kiss. Currently, scientists propose that ancient hominins did it too – and possibly locked lips with early Homo sapiens.

Common Microbial Evidence

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were closely connected. In previous studies, scientists have found humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the idea chimed with studies that has revealed humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing genetic mixing was at play.

Romantic Spin

"It certainly puts a more romantic spin on human-Neanderthal relations," Brindle commented.

Writing in the publication a scientific periodical, Brindle and her team report 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

"Previously there were some efforts to define a kiss, but it's very much been human-centric, which implies that basically non-human species don't kiss. Now we know that they probably do, it might just not look from what human kissing looks like," said Brindle.

Nonetheless, she said some behaviors that resembled kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in fish called French grunts.

Consequently the research group developed a description of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but no transfer of food.

Study Methods

The lead researcher explained they concentrated on reports of intimate behavior in primates from Africa and Asian regions, including primates, apes and orangutans, and employed online videos to verify the reports.

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

Evolutionary Origins

Researchers propose the results suggest intimate contact developed approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, engaged in a kiss, the scientists conclude. But the behavior might not have been limited to their own species.

"Reality that humans kiss, the reality that we now have shown that Neanderthals probably kissed, suggests that the both groups are also likely to have kissed," the researcher added.

Biological Significance

Although the evolutionary explanation is discussed, Brindle explained kissing could be used in reproductive situations to possibly enhance reproductive success or assist in selecting between mates, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the behavior of great apes commented that as kissing behavior was seen in a wide range of apes it was logical its origins lie deep in our ancient history, and an analysis of different forms of intimate behavior among a broader range of animals might push its origins back even earlier still.

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

Social Elements

An archaeology expert said that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and ways of encouraging confidence and intimacy will have been important for eons," she said. "This could represent an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it should be no surprise that Neanderthals – and even them and our human ancestors together – engaged intimately."