A recent study indicates that kissing may have originated as far back as 21 million years ago, long before the emergence of modern humans. The research, published in the Journal of Evolution and Human Behavior, explores the evolutionary history of kissing, suggesting that early ancestors of apes and humans, including Neanderthals, likely engaged in this behavior.

Matilda Brindle, an evolutionary biologist at Oxford University and the study's lead author, explained that kissing is a complex behavior that has not been thoroughly examined from an evolutionary perspective. "Kissing is one of these things that we were just really interested in understanding," she said. The study defines kissing as a non-aggressive, mouth-to-mouth interaction that does not involve food transfer.

The researchers collected data on kissing behaviors observed in modern primates, including chimpanzees, bonobos, and orangutans. They then used phylogenetic analysis to trace the evolutionary lineage of these species, estimating that kissing likely evolved in a common ancestor of large apes between 21.5 million and 16.9 million years ago. This suggests that kissing was also present in extinct relatives like Neanderthals.

Brindle noted that the study supports the idea that early humans and Neanderthals may have kissed, as they interbred and shared saliva. "Given that humans and Neanderthals were interbreeding and sharing saliva, and that both humans and Neanderthals are known to kiss, we suggest that it kind of tips the scale toward that idea that humans and Neanderthals might have been kissing each other," she said.

Despite the findings, the study does not clarify why kissing evolved. Brindle mentioned that kissing could serve various purposes, such as mate assessment, bonding, and social interaction. However, the risks associated with kissing, including disease transmission, raise questions about its evolutionary advantages.

The research also highlights that kissing is not universally practiced among humans. According to a 2015 study, kissing is documented in only 46% of cultures. Brindle emphasized that while kissing has a strong evolutionary signal, it may not be beneficial in all contexts. "Primates are extremely flexible species, very intelligent, and so kissing might be useful in some contexts but not in others," she said.

Experts in the field, including Adriano Reis e Lameira, an evolutionary psychologist, and Justin Garcia, an executive director at The Kinsey Institute, noted that the study opens avenues for further research. They pointed out that kissing behavior varies widely among individuals and cultures, suggesting a complex interplay of biological and cultural factors.

As researchers continue to explore the origins and variations of kissing, Brindle's study serves as a foundational step in understanding this intimate behavior shared across the animal kingdom. "What we've done, which is a really important first step, is showing it's an evolved trait," she said. "It's really ancient. But why? And that's the amazing next step if people want to pick up the mantle."