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From Galápagos albatrosses to polar bears, primates to great apes, certain species appear to kiss. Now, researchers suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Evidence

It is not the first time scientists have proposed Neanderthals and Homo sapiens were intimately acquainted. In earlier research, scientists have found modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea chimed with studies that has revealed people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Interpretation

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

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

Describing Intimate Contact

"Previously there were some previous attempts to define a kiss, but it's very much been human-centric, which means that basically non-human species don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

However, she said some actions that looked like kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species known as certain marine animals.

Consequently the research group developed a definition of kissing based on social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but absence of nutrition.

Research Approach

Brindle said they concentrated on reports of intimate behavior in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to confirm the observations.

The researchers then combined this information with details on the evolutionary relationships between living and extinct species of such primates.

Evolutionary Origins

The team propose the findings indicate kissing developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests it is likely they, too, indulged in a intimate act, the researchers conclude. But the behavior might not have been confined to their own species.

"The fact that modern people engage intimately, the reality that we currently have shown that Neanderthals probably engaged, suggests that the both groups are also likely to have engage," the researcher noted.

Evolutionary Importance

While the scientific reasoning is discussed, Brindle explained kissing could be employed in reproductive situations to possibly enhance mating outcomes or help choose between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of great apes said that as kissing behavior was seen in a wide range of primates it made sense its roots lie deep in our evolutionary past, and an examination of different forms of intimate behavior among a broader range of species might push its origins back even earlier still.

"Things that we consider as signatures of our species, like intimate contact, are not unique to us if we look closely at different species," he said.

Cultural Elements

An archaeology expert said that kissing had a cultural element as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and intimacy will have been important for eons," she said. "This could represent an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be no surprise that ancient hominins – and even Neanderthals and our own species collectively – kissed."