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Among Galápagos albatrosses to Arctic mammals, primates to orangutans, certain species appear to kiss. Currently, researchers suggest that ancient hominins did it too – and might even have locked lips with modern humans.

Shared Oral Evidence

This isn't the initial instance experts have proposed Neanderthals and Homo sapiens were intimately acquainted. In earlier research, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the idea aligned with research that has found people of non-African ancestry have bits of Neanderthal DNA in their genome, revealing interbreeding was at play.

Romantic Spin

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

Writing in the publication a scientific periodical, the researcher and her team report how, to investigate the evolutionary origins of kissing, they first had to come up with a description that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been focused on humans, which implies 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," said the evolutionary biologist.

However, she noted some actions that looked like intimate contact were something rather different – such as the processing and food sharing, or "mouth contact", observed in fish known as French grunts.

Consequently the team developed a description of kissing centered around friendly interactions involving directed oral interaction with a individual of the same species, with some motion of the oral area but no transfer of food.

Research Methods

The lead researcher said they concentrated on accounts of kissing in primates from Africa and Asian regions, including primates, chimpanzees and orangutans, and used online videos to confirm the observations.

Scientists then integrated this data with information on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Timeline

Researchers propose the findings indicate kissing evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, indulged in a intimate act, the scientists conclude. But the behavior may not have been limited to their own species.

"Reality that humans engage intimately, the reality that we now have demonstrated that ancient relatives probably engaged, suggests that the two [species] are also likely to have kissed," Brindle noted.

Biological Significance

Although the evolutionary explanation is discussed, the expert said intimate contact could be used in reproductive situations to potentially increase mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of primates said that as intimate contact was observed in a wide range of apes it made sense its roots lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of species might extend its origins back even earlier still.

"Behaviors that we think of as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Aspects

An archaeology expert said that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and methods of encouraging trust and intimacy will have been important for millions of years," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be expected that Neanderthals – and even Neanderthals and our human ancestors together – kissed."