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From seabirds to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. Among earlier research, scientists have discovered modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the idea aligned with research that has found humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Intimate Spin

"It certainly puts a different perspective on human-Neanderthal relations," the lead researcher said.

Writing in the journal a scientific periodical, the researcher and colleagues report how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans kiss.

Defining Kissing

"Previously there were some efforts to define a intimate act, but it's largely focused on humans, which means that essentially 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 kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species called certain marine animals.

Consequently the research group developed a definition of kissing based on friendly interactions involving directed oral interaction with a member of the same species, with some movement of the oral area but absence of food.

Research Approach

The lead researcher said they concentrated on reports of kissing in non-human species from Africa and Asian regions, including bonobos, apes and great apes, and employed digital recordings to confirm the reports.

Scientists then combined this data with information on the genetic connections between living and ancient types of such animals.

Evolutionary Origins

The team say the findings suggest intimate contact evolved approximately 21.5m and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the researchers conclude. But the activity may not have been confined to their specific group.

"The fact that modern people engage intimately, the fact that we currently have shown that ancient relatives probably engaged, suggests that the both groups are probably did kissed," the researcher added.

Biological Significance

While the evolutionary explanation is discussed, the expert explained intimate contact could be used in reproductive situations to possibly enhance mating outcomes or assist in selecting between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of primates commented that as kissing behavior was observed in a broad spectrum of primates it was logical its roots extend far into our ancient history, and an examination of different forms of intimate behavior among a broader range of animals might push its beginnings back further still.

"Things that we think of as signatures of our species, like kissing, are not unique to us if we examine carefully at different species," the expert noted.

Social Aspects

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

"However, as people we succeed or struggle on the strength of our emotional bonds, and ways of promoting confidence and intimacy will have been significant for millions of years," she said. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but really it ought to be expected that ancient hominins – and even them and our human ancestors together – engaged intimately."