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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Shared Oral Clues

It is not the first time experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In earlier research, researchers have discovered modern people and their Neanderthal relatives shared 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 concept aligned with studies that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Interpretation

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

Writing in the publication Evolution and Human Behavior, the researcher and her team detail how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how people smooch.

Describing Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's largely focused on humans, which implies that essentially non-human species do not engage in this. Currently we know that they likely engage, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

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

As a result the research group came up with a definition of kissing based on social behaviors involving intentional oral interaction with a individual of the same species, with some movement of the oral area but absence of food.

Research Methods

The lead researcher explained they concentrated on reports of intimate behavior in non-human species from the African continent and Asian regions, including primates, chimpanzees and orangutans, and employed online videos to verify the reports.

Scientists then integrated this information with details on the evolutionary relationships between extant and extinct types of such primates.

Evolutionary Origins

Researchers propose the results indicate kissing evolved approximately 21.5 million and 16.9 million years ago in the predecessors of the large apes.

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

"Reality that humans engage intimately, the fact that we currently have shown that ancient relatives very likely engaged, suggests that the both groups are also likely to have engage," Brindle added.

Biological Significance

Although the evolutionary explanation is debated, Brindle said intimate contact could be used in sexual contexts to potentially enhance reproductive success or assist in selecting between partners, while it might help strengthen connections when used in a non-sexual manner.

A separate researcher in the activities of primates said that as kissing behavior was observed in a wide range of apes it was logical its origins extend far into our evolutionary past, and an examination of various types of intimate behavior among a broader range of species might push its beginnings back even earlier still.

"Things that we consider as characteristics of human life, like kissing, are not exclusive to us if we examine carefully at different species," he said.

Social Aspects

An archaeology expert explained that kissing had a cultural element as it was not universal to all societies.

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and methods of encouraging trust and intimacy will have been important for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it ought to be expected that Neanderthals – and including them and our human ancestors together – engaged intimately."