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From Galápagos albatrosses to polar bears, primates to great apes, various animals appear to kiss. Now, scientists suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Oral Evidence

This isn't the initial instance experts have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, scientists have found modern people and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, adding that the concept chimed with studies that has revealed people of non-African ancestry contain ancient genetic material in their genome, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a more romantic spin on ancient interactions," Brindle commented.

Publishing in the journal a scientific periodical, the researcher and her team detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not restricted by how humans smooch.

Describing Kissing

"Previously there were some previous attempts to define a kiss, but it's largely human-centric, which implies that essentially non-human species don't kiss. Currently we understand that they probably do, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

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

Consequently the research group developed a description of kissing based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but absence of food.

Research Methods

Brindle said they concentrated on accounts of intimate behavior in non-human species from the African continent and Asian regions, including primates, chimpanzees and orangutans, and employed online videos to verify the observations.

The researchers then integrated this data with information on the evolutionary relationships between extant and ancient species of such primates.

Historical Origins

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

Placement of Neanderthals on this evolutionary lineage suggests it is likely they, too, engaged in a intimate act, the researchers say. But the behavior might not have been confined to their specific group.

"The fact that modern people engage intimately, the fact that we currently have demonstrated that Neanderthals very likely engaged, suggests that the two [species] are also likely to have kissed," the researcher noted.

Biological Significance

Although the evolutionary explanation is debated, the expert explained kissing could be used in sexual contexts to potentially increase reproductive success or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of great apes said that as intimate contact was seen in a broad spectrum of apes it made sense its roots extend far into our ancient history, and an examination of various types of intimate behavior among a broader range of species might extend its beginnings back further still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Social Elements

Another professor explained that kissing had a social component as it was not universal to all societies.

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and methods of encouraging confidence and closeness will have been important for eons," the professor stated. "It might be an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."