Matilda Brindle, an evolutionary biologist at the University of Oxford, led research that traced the origins of kissing back 21 million years, before Homo sapiens existed.The study found that Neanderthals, who coexisted with Homo sapiens for thousands of years, also engaged in kissing and may have shared oral microbes through saliva exchange.Published in "Evolution and Human Behavior," the findings suggest kissing evolved among primates and may have developed from grooming or nursing behaviors.