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From seabirds to polar bears, primates to orangutans, various animals appear to kiss. Currently, researchers suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Common Oral Evidence

It is not the first time experts have proposed Neanderthals and early modern humans were intimately acquainted. In previous studies, scientists have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the idea aligned with research that has found humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Spin

"This offers a different spin on ancient interactions," Brindle said.

Writing in the journal Evolution and Human Behavior, Brindle and colleagues report 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 kiss.

Describing Kissing

"Previously there were some previous attempts to define a kiss, but it's very much been focused on humans, which means that essentially other animals don't kiss. Now we understand that they likely engage, it may appear different from what our intimate contact looks like," explained Brindle.

However, she noted some behaviors that looked like kissing were distinct activities – such as the processing and food sharing, or "mouth contact", observed in aquatic species known as French grunts.

As a result the team developed a definition of intimate contact centered around social behaviors involving directed oral interaction with a member of the identical group, with some motion of the mouth but absence of food.

Study Methods

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

The researchers then combined this information with details on the evolutionary relationships between extant and ancient species of such animals.

Historical Origins

Researchers propose the results suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree suggests it is likely they, too, engaged in a kiss, the researchers say. But the behavior might not have been confined to their own species.

"The fact that modern people kiss, the fact that we currently have demonstrated that ancient relatives probably engaged, indicates that the both groups are probably did engage," Brindle added.

Biological Importance

While the scientific reasoning is debated, the expert said intimate contact could be used in sexual contexts to potentially increase reproductive success or help choose between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

A separate researcher in the activities of primates commented that as intimate contact was seen in a broad spectrum of primates it made sense its roots lie deep in our ancient history, and an examination of different forms of intimate behavior among a wider variety of animals might push its beginnings back even earlier still.

"Things that we consider as signatures of our species, like intimate contact, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all human groups.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and ways of promoting trust and intimacy will have been significant for millions of years," the professor stated. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be no surprise that ancient hominins – and even them and our human ancestors together – engaged intimately."