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From Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Oral Clues

This isn't the initial instance experts have proposed ancient relatives and early modern humans were closely connected. In previous studies, scientists have discovered 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.

"Likely they were kissing," she said, adding that the idea aligned with studies that has found humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Romantic Interpretation

"This offers a different perspective on ancient interactions," the lead researcher commented.

Publishing in the publication a scientific periodical, Brindle and her team detail how, to investigate the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how humans kiss.

Defining Intimate Contact

"There have been some previous attempts to describe a kiss, but it's very much been human-centric, which implies that basically other animals don't kiss. Now we know that they likely engage, it might just not look from what human kissing looks like," said Brindle.

However, she noted some behaviors that looked like kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

Consequently the team came up with a description of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some movement of the mouth but no transfer of food.

Study Approach

Brindle explained they focused on accounts of intimate behavior in primates from Africa and Asian regions, including primates, chimpanzees and orangutans, and used digital recordings to verify the observations.

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

Historical Timeline

The team propose the results indicate intimate contact developed somewhere between 21.5m and 16.9 million years ago in the ancestors of the great primates.

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

"Reality that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives very likely kissed, suggests that the both groups are also likely to have engage," the researcher noted.

Biological Significance

While the evolutionary explanation is debated, Brindle explained kissing could be used in sexual contexts to possibly increase reproductive success or assist in selecting between partners, while it could assist reinforce bonding when used in a platonic way.

Another expert in the activities of great apes said that as intimate contact was observed in a wide range of primates it was logical its origins extend far into our evolutionary past, and an examination of different forms of intimate behavior among a wider variety of animals might push its beginnings back further still.

"Behaviors that we think of as characteristics of our species, like intimate contact, are not exclusive to us if we look closely at different species," the expert noted.

Social Aspects

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting confidence and closeness will have been important for eons," the professor stated. "It might be an concept that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be no surprise that ancient hominins – and even Neanderthals and our human ancestors together – engaged intimately."