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From seabirds to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were closely connected. Among earlier research, scientists have discovered humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with research that has found people of certain genetic backgrounds have bits of ancient genetic material in their genome, demonstrating interbreeding was at play.

Intimate Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher said.

Writing in the journal a scientific periodical, Brindle and colleagues report how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some efforts to define a intimate act, but it's very much been human-centric, which implies that basically 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 the evolutionary biologist.

However, she said some behaviors that resembled kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", seen in fish called French grunts.

Consequently the research group came up with a description of kissing centered around social behaviors involving intentional oral interaction with a member of the same species, with some movement of the mouth but no transfer of food.

Research Approach

Brindle explained they concentrated on reports of kissing in primates from Africa and Asian regions, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the reports.

The researchers then combined this data with details on the evolutionary relationships between living and extinct types of such animals.

Historical Origins

The team say the findings suggest kissing developed approximately 21.5m and 16.9 million years ago in the ancestors of the great primates.

The position of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a intimate act, the scientists conclude. 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 Neanderthals probably engaged, indicates that the both groups are also likely to have engage," the researcher added.

Biological Significance

Although the scientific reasoning is discussed, Brindle said kissing could be used in sexual contexts to possibly enhance mating outcomes or help choose between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes said that as kissing behavior was seen in a wide range of apes it was logical its origins extend far into our ancient history, and an examination of various types of kissing among a wider variety of species might extend its origins back further still.

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

Social Elements

Another professor explained that kissing had a cultural element as it was not common to all human groups.

"However, as humans we succeed or struggle on the strength of our emotional bonds, and methods of promoting trust and closeness will have been important for millions of years," she said. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it should be expected that Neanderthals – and even Neanderthals and our human ancestors collectively – kissed."