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From seabirds to polar bears, primates to great apes, certain species engage in mouth-to-mouth contact. Now, researchers suggest that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Microbial Evidence

It is not the first time scientists have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, researchers have discovered modern people and their Neanderthal relatives 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 concept aligned with studies that has found people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a different perspective on ancient interactions," the lead researcher said.

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 definition that was not restricted by how people kiss.

Defining Intimate Contact

"There have been some previous attempts to describe a kiss, but it's largely human-centric, which implies that essentially other animals do not engage in this. Currently we know that they likely engage, it might just not look from what human kissing resembles," explained Brindle.

Nonetheless, she said some behaviors that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "mouth contact", seen in aquatic species known as French grunts.

Consequently the research group came up with a description of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Research Methods

Brindle said they concentrated on reports of kissing in primates from Africa and Asian regions, including bonobos, apes and great apes, and employed digital recordings to confirm the observations.

Scientists then combined this information with details on the evolutionary relationships between living and extinct species of such primates.

Evolutionary Origins

Researchers say the results suggest intimate contact evolved approximately 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, indulged in a intimate act, the researchers say. But the behavior might not have been confined to their specific group.

"Reality that humans kiss, the reality that we currently have demonstrated that Neanderthals probably engaged, suggests that the two [species] are also likely to have kissed," the researcher added.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle explained kissing could be employed in reproductive situations to possibly increase reproductive success or assist in selecting between mates, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the behavior of great apes said that as kissing behavior was observed in a broad spectrum of primates it was logical its roots lie deep in 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.

"Things that we think of as characteristics of our species, like kissing, are not exclusive to us if we look closely at different species," he said.

Cultural Aspects

Another professor said that intimate contact had a social component as it was not common to all societies.

"However, as humans we succeed or struggle on the strength of our relationships, and methods of encouraging confidence and closeness will have been important for millions of years," the professor stated. "This could represent an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it should be expected that Neanderthals – and including them and our human ancestors together – engaged intimately."