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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists propose that Neanderthals also engaged in this behavior – and might even have locked lips with modern humans.

Shared Oral Evidence

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. Among previous studies, scientists have discovered modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the concept chimed with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle said.

Publishing in the journal a scientific periodical, the researcher and her team detail how, to investigate the historical roots 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 define a intimate act, but it's very much been human-centric, which means that essentially other animals don't kiss. Now we know that they likely engage, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "mouth contact", seen in fish called certain marine animals.

Consequently the research group developed a description of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the oral area but absence of food.

Study Approach

Brindle said they focused on reports of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and great apes, and employed digital recordings to verify the observations.

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

Evolutionary Origins

Researchers say the results indicate intimate contact developed approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a intimate act, the scientists say. But the activity may not have been limited to their own species.

"The fact that modern people kiss, the reality that we currently have shown that ancient relatives very likely engaged, suggests that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Significance

Although the scientific reasoning is debated, the expert said intimate contact could be employed in reproductive situations to potentially enhance mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of primates said that as kissing behavior was seen in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an examination of different forms of kissing among a broader range 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 examine carefully at other animals," the expert noted.

Social Aspects

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

"However, as people we succeed or struggle on the quality of our relationships, and methods of encouraging confidence and intimacy will have been significant for eons," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be expected that ancient hominins – and including them and our human ancestors collectively – engaged intimately."