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From seabirds to polar bears, primates to orangutans, various animals appear to kiss. Now, researchers suggest that Neanderthals did it too – and might even have locked lips with modern humans.

Common Oral Evidence

It is not the first time experts have proposed ancient relatives and Homo sapiens were closely connected. Among previous studies, scientists have discovered humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, adding that the idea chimed with research that has revealed humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

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

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to investigate the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how people kiss.

Defining Kissing

"There have been some efforts to define a intimate act, but it's largely human-centric, which implies that essentially non-human species do not engage in this. Now we know that they probably do, it may appear different from what human kissing looks like," said Brindle.

However, she said some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", observed in fish called certain marine animals.

Consequently the team came up with a description of intimate contact based on social behaviors involving intentional oral interaction with a individual of the identical group, with some movement of the mouth but no transfer of nutrition.

Research Approach

Brindle said they focused on accounts of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to confirm the reports.

Scientists then combined this information with information on the evolutionary relationships between extant and extinct species of such animals.

Historical Origins

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

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

"Reality that humans kiss, the fact that we currently have shown that Neanderthals very likely kissed, suggests that the two [species] are also likely to have kissed," the researcher added.

Evolutionary Significance

While the evolutionary explanation is debated, Brindle said kissing could be employed in reproductive situations to possibly increase mating outcomes or assist in selecting between mates, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of primates commented that as intimate contact was observed in a broad spectrum of primates it was logical its roots extend far into our ancient history, and an examination of different forms of intimate behavior among a wider variety of species might extend its origins back even earlier still.

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

Cultural Elements

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and ways of encouraging confidence and intimacy will have been important for millions of years," she said. "It might be an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be no surprise that Neanderthals – and even them and our human ancestors together – engaged intimately."