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From seabirds to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

It is not the first time scientists have suggested Neanderthals and Homo sapiens were intimately acquainted. Among earlier research, scientists have found modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea aligned with research that has found humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Spin

"It certainly puts a different perspective on ancient interactions," Brindle said.

Publishing in the publication a scientific periodical, the researcher and her team detail how, to investigate the historical roots of intimate contact, they first had to come up with a description that was not restricted by how people kiss.

Describing Kissing

"There have been some efforts to describe a intimate act, but it's largely focused on humans, which implies that essentially other animals don't kiss. Now we know that they likely engage, it might just not look from what our intimate contact resembles," explained Brindle.

However, she said some actions that resembled intimate contact were something rather different – such as the processing and food sharing, or "kiss-fighting", observed in fish known as French grunts.

As a result the team developed a description of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but absence of nutrition.

Research Approach

Brindle said they concentrated on accounts of intimate behavior in non-human species from the African continent and Asia, including bonobos, apes and orangutans, and used digital recordings to verify the reports.

Scientists then combined this data with information on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Timeline

Researchers say the findings suggest kissing evolved approximately 21.5m and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this evolutionary lineage means it is probable they, too, indulged in a kiss, the scientists conclude. But the behavior may not have been confined to their own species.

"The fact that modern people engage intimately, the fact that we currently have shown that ancient relatives very likely kissed, suggests that the two [species] are also likely to have engage," Brindle added.

Biological Significance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in reproductive situations to potentially increase reproductive success or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates commented that as intimate contact was seen in a wide range of primates it made sense its roots extend far into our evolutionary past, and an analysis of different forms of kissing among a broader range of species might push its origins back further still.

"Things that we think of as characteristics of our species, like kissing, are not exclusive to us if we look closely at different species," he said.

Social Aspects

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

"However, as humans we succeed or struggle on the strength of our relationships, and ways of promoting trust and intimacy will have been important for eons," the professor stated. "It might be an image that appears a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and even Neanderthals and our own species together – kissed."