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Among Galápagos albatrosses to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Now, scientists propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were closely connected. Among previous studies, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the concept chimed with studies that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Romantic Spin

"This offers a different perspective on human-Neanderthal relations," Brindle commented.

Publishing in the journal Evolution and Human Behavior, Brindle and her team detail how, to investigate the historical roots of intimate contact, they first had to develop a definition that was not restricted by how humans kiss.

Describing Intimate Contact

"There have been some previous attempts to define a kiss, but it's largely human-centric, which implies that basically other animals don't kiss. Now we understand that they likely engage, it may appear different from what human kissing resembles," said Brindle.

Nonetheless, she said some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish called certain marine animals.

As a result the research group developed a definition of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but absence of food.

Research Methods

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

Scientists then integrated this information with details on the genetic connections between living and ancient species of such primates.

Evolutionary Origins

The team propose the results indicate kissing developed somewhere between 21.5 million and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree suggests it is likely they, too, indulged in a kiss, the scientists say. But the behavior may not have been limited to their specific group.

"The fact that modern people engage intimately, the reality that we now have shown that Neanderthals probably engaged, suggests that the both groups are also likely to have engage," the researcher added.

Evolutionary Importance

Although the scientific reasoning is debated, the expert explained intimate contact could be employed in sexual contexts to potentially enhance reproductive success or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the behavior of great apes commented that as kissing behavior was seen in a wide range of apes it was logical its roots extend far into our ancient history, and an examination of various types of kissing among a wider variety of species might extend its origins back even earlier still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not unique to us if we look closely at other animals," he said.

Cultural Elements

An archaeology expert said that kissing had a social component as it was not common to all human groups.

"Nonetheless, as people we succeed or struggle on the strength of our relationships, and methods of promoting trust and intimacy will have been important for eons," she said. "This could represent an concept 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 own species collectively – engaged intimately."