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From seabirds to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins did it too – and possibly locked lips with modern humans.

Common Microbial Evidence

It is not the first time experts have proposed ancient relatives and early modern humans were closely connected. In earlier research, scientists have discovered humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with studies that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Spin

"This offers a different perspective on ancient interactions," the lead researcher commented.

Publishing in the journal Evolution and Human Behavior, 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 humans kiss.

Defining Kissing

"Previously there were some previous attempts to define a intimate act, but it's largely human-centric, which implies that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing resembles," explained Brindle.

Nonetheless, she noted some behaviors that looked like intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species known as French grunts.

As a result the team came up with a definition of kissing based on social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Study Methods

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

Scientists then combined this information with information on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Timeline

Researchers say the results suggest intimate contact developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the large apes.

The position of Neanderthals on this evolutionary lineage means it is likely they, too, indulged in a kiss, the researchers conclude. But the behavior might not have been confined to their own species.

"The fact that modern people engage intimately, the reality that we now have shown that Neanderthals very likely kissed, suggests that the two [species] are probably did kissed," the researcher added.

Evolutionary Importance

While the scientific reasoning is discussed, Brindle explained kissing could be employed in sexual contexts to potentially increase mating outcomes or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of apes it was logical its roots lie deep in our ancient history, and an examination of various types of kissing among a wider variety of species might push its origins back even earlier still.

"Things that we consider as characteristics of our species, like kissing, are not unique to us if we look closely at different species," the expert noted.

Cultural Elements

An archaeology expert explained that kissing had a cultural element as it was not common to all societies.

"However, as humans we succeed or struggle on the quality of our relationships, and ways of promoting confidence and intimacy will have been significant for eons," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but actually it ought to be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."