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From Galápagos albatrosses to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals did it too – and might even have locked lips with modern humans.

Common Microbial Evidence

It is not the first time experts have proposed Neanderthals and early modern humans were intimately acquainted. Among earlier research, researchers have discovered modern people and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with studies that has revealed humans of non-African ancestry contain ancient genetic material in their genome, demonstrating interbreeding was at play.

Romantic Interpretation

"It certainly puts a different spin on human-Neanderthal relations," Brindle commented.

Publishing in the publication Evolution and Human Behavior, the researcher and her team detail how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how humans kiss.

Defining Kissing

"Previously there were some previous attempts to describe a intimate act, but it's largely human-centric, 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 resembles," said Brindle.

However, she noted some actions that looked like kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species known as French grunts.

Consequently the team developed a definition of kissing based on friendly interactions involving directed oral interaction with a individual of the same species, with some motion of the oral area but no transfer of nutrition.

Research Methods

Brindle said they focused on accounts of intimate behavior in non-human species from Africa and Asia, including primates, chimpanzees and great apes, and employed digital recordings to verify the reports.

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

Evolutionary Timeline

The team say the results suggest kissing evolved approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, engaged in a intimate act, the researchers conclude. But the behavior may not have been confined to their own species.

"Reality that humans kiss, the fact that we now have shown that Neanderthals very likely engaged, indicates that the both groups are also likely to have engage," the researcher added.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle said intimate contact could be employed in sexual contexts to possibly enhance reproductive success or help choose between partners, while it could assist reinforce bonding when used in a non-sexual manner.

Another expert in the activities of primates said that as intimate contact was seen in a broad spectrum of primates it made sense its roots lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of animals might extend its beginnings back even earlier still.

"Things that we consider as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," he said.

Social Elements

An archaeology expert explained that intimate contact had a social component as it was not universal to all societies.

"However, as people we succeed or struggle on the quality of our relationships, and methods of promoting trust and closeness will have been important for eons," she said. "This could represent an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it should be no surprise that ancient hominins – and even Neanderthals and our own species collectively – kissed."