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From seabirds to Arctic mammals, primates to great apes, certain species appear to kiss. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. Among previous studies, researchers have found modern people and their thick-browed cousins possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," she said, adding that the concept aligned with research that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Spin

"It certainly puts a different spin on ancient interactions," Brindle commented.

Writing in the publication a scientific periodical, 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 humans smooch.

Defining Kissing

"There have been some efforts to define a intimate act, but it's largely human-centric, which implies that essentially non-human species don't kiss. Currently we understand that they likely engage, it may appear different from what our intimate contact looks like," said Brindle.

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

As a result the team came up with a definition of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but absence of food.

Research Approach

The lead researcher said they concentrated on accounts of kissing in primates from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and used digital recordings to confirm the observations.

Scientists then integrated this data with information on the evolutionary relationships between extant and extinct species of such animals.

Evolutionary Origins

The team say the findings suggest kissing evolved somewhere between 21.5 million and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this family tree means it is likely they, too, indulged in a kiss, the scientists say. But the activity might not have been limited to their own species.

"Reality that modern people engage intimately, the reality that we currently have shown that ancient relatives very likely kissed, suggests that the both groups are also likely to have engage," the researcher noted.

Biological Significance

Although the scientific reasoning is debated, the expert explained intimate contact could be employed in reproductive situations to possibly increase mating outcomes or help choose between partners, while it might help reinforce bonding when used in a platonic way.

Another expert in the behavior of great apes said that as kissing behavior was observed in a wide range of primates it made sense its origins lie deep in our ancient history, and an analysis of various types of intimate behavior among a wider variety of animals might push its beginnings back further still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not unique to us if we look closely at other animals," he said.

Cultural 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 strength of our relationships, and ways of promoting confidence and intimacy will have been significant for millions of years," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that ancient hominins – and even them and our own species collectively – engaged intimately."