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Among seabirds to polar bears, primates to orangutans, various animals appear to kiss. Currently, researchers propose that ancient hominins did it too – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have discovered humans and their thick-browed cousins 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 concept chimed with studies that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a different perspective on human-Neanderthal relations," Brindle said.

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people kiss.

Describing Intimate Contact

"There have been some previous attempts to define a kiss, but it's largely human-centric, which means that essentially other animals do not engage in this. Now we understand that they likely engage, it may appear different from what our intimate contact looks like," explained Brindle.

Nonetheless, she said some actions that looked like kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", seen in fish known as French grunts.

Consequently the research group developed a description of intimate contact centered around social behaviors involving directed oral interaction with a individual of the same species, with some motion of the mouth but no transfer of nutrition.

Research Approach

The lead researcher explained they focused on accounts of intimate behavior in primates from Africa and Asian regions, including bonobos, chimpanzees and great apes, and employed digital recordings to verify the reports.

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

Historical Origins

The team propose the findings suggest intimate contact evolved approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

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

"Reality that humans kiss, the reality that we currently have demonstrated that ancient relatives probably engaged, indicates that the two [species] are probably did engage," Brindle noted.

Biological Significance

Although the evolutionary explanation is debated, the expert said kissing could be employed in sexual contexts to possibly increase reproductive success or assist in selecting between partners, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates said that as kissing behavior was seen in a wide range 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 extend its beginnings back even earlier still.

"Things that we think of as characteristics of human life, like kissing, are not unique to us if we examine carefully at other animals," he said.

Social Elements

Another professor said that kissing had a cultural element as it was not universal to all human groups.

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and ways of promoting trust and closeness will have been significant for eons," she said. "This could represent an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it ought to be expected that Neanderthals – and even them and our own species together – kissed."