They then treated the act of kissing as an evolutionary trait, and used Bayesian statistical modeling to model kissing behavior in the family tree of primate ancestors. That kissing is an ancient practice among large apes, first arising somewhere in the neighborhood of 20 million years ago. That finding is particularly interesting, the researchers point out, because of a previous study that found that humans and Neanderthals share common oral microbes — likely, they say, from swapping spit. “Probably they were kissing,” the study’s lead author and evolutionary biologist at Oxford Matilda Brindle told The Guardian. More on Neanderthals: Neanderthals May Have Been Killed Off By Magnetic Pole Flip