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From seabirds to Arctic mammals, primates to orangutans, certain species appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Common Oral Clues

It is not the first time experts have suggested ancient relatives 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, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept aligned with research that has found people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Interpretation

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Writing in the journal a scientific periodical, Brindle and her team detail how, to investigate the historical roots of intimate contact, they first had to develop a definition that was not restricted by how humans smooch.

Defining Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's largely focused on humans, 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," explained Brindle.

However, she noted some behaviors that resembled kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species called certain marine animals.

Consequently the team developed a definition of kissing centered around social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of food.

Research Approach

Brindle explained they concentrated on accounts of kissing in primates from Africa and Asia, including bonobos, apes and orangutans, and employed online videos to verify the reports.

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

Evolutionary Timeline

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

Placement of Neanderthals on this family tree suggests it is probable they, too, engaged in a kiss, the researchers conclude. But the behavior may not have been limited to their specific group.

"The fact that modern people kiss, the reality that we currently have shown that Neanderthals very likely engaged, suggests that the both groups are probably did engage," Brindle noted.

Evolutionary Importance

While the evolutionary explanation is discussed, Brindle explained kissing could be used in sexual contexts to possibly increase reproductive success or help choose between mates, while it might help strengthen connections when used in a platonic way.

A separate researcher in the behavior of primates commented that as intimate contact was observed in a wide range of apes it made sense its origins extend far into our evolutionary past, and an examination of different forms of kissing among a wider variety of animals might push its beginnings back further still.

"Things that we think of as characteristics of human life, like kissing, are not exclusive to us if we look closely at different species," the expert noted.

Social Aspects

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and closeness will have been important for eons," she said. "It might be an concept that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but actually it ought to be expected that Neanderthals – and even them and our human ancestors collectively – engaged intimately."