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From seabirds to polar bears, primates to great apes, certain species appear to kiss. Currently, scientists propose that ancient hominins did it too – and possibly locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In earlier research, scientists have discovered humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the concept chimed with studies that has revealed humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing genetic mixing was at play.

Romantic Interpretation

"This offers a different spin on human-Neanderthal relations," Brindle commented.

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 definition that was not restricted by how humans kiss.

Defining Intimate Contact

"There have been some efforts to describe a kiss, but it's very much been focused on humans, which means that basically other animals don't kiss. Currently we understand that they likely engage, it may appear different from what our intimate contact resembles," said Brindle.

Nonetheless, she said some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in fish known as French grunts.

As a result the team developed a definition of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but absence of nutrition.

Study Methods

The lead researcher said they focused on reports of kissing in primates from Africa and Asian regions, including bonobos, apes and great apes, and used online videos to confirm the observations.

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

Historical Timeline

Researchers say the findings suggest kissing developed approximately 21.5 million and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the scientists say. But the activity might not have been limited to their own species.

"The fact that modern people kiss, the reality that we now have demonstrated that ancient relatives very likely kissed, indicates that the both groups are probably did engage," the researcher added.

Evolutionary Importance

Although the evolutionary explanation is discussed, Brindle said kissing could be used in reproductive situations to possibly increase reproductive success or help choose between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was observed in a broad spectrum of primates it was logical its origins extend far into our evolutionary past, and an examination of various types of kissing among a broader range of species might push its beginnings back further still.

"Things that we think of as characteristics of human life, like kissing, are not exclusive to us if we examine carefully at different species," the expert noted.

Cultural Elements

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

"Nonetheless, as people we succeed or struggle on the quality 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 contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and even them and our own species collectively – kissed."