Humans spend a lot more time kissing than they do thinking about why they kiss. But scientists intrigued by this question say the practice, which has no real survival or reproductive benefit, is built into us, likely originating with the common ancestor of humans and other great apes upward of 21 million years ago. For their study, published Wednesday in Evolution and Human Behaviour, they explored what other species engage in non-aggressive, mouth-to-mouth contact involving some mouth movement, but with no food transfer involved. They identified various species, including wolves, prairie dogs, polar bears, and albatrosses, as well as orangutans, bonobos, chimps, and one gorilla species, per the BBC and Washington Post.
By tracing this behavior, researchers were able to build an "evolutionary family tree" that helped pinpoint the likely origin of kissing to between 16.9 million and 21.5 million years ago with the common ancestor of large apes, per the Post. "Humans, chimps, and bonobos all kiss," so "it's likely that their most recent common ancestor kissed," lead researcher Dr. Matilda Brindle, an Oxford evolutionary biologist, tells the BBC. The study concludes that Neanderthals also kissed, and likely swapped spit with modern humans, because they carried an oral bacteria found in our saliva. The question of why kissing arose remains unanswered, though existing theories suggest it started as grooming among ape ancestors or perhaps serves as a way to assess a partner's health.
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