One theory is…..wait for….. regurgitation. Yes in many species Mom and Dad will catch some prey or even pick berries, chew them up and then share the pre-chewed food with their young or even their mate. So its a real sign of initmacy in a family group in the wild to share food in this way. Humans are a species that both kiss and have sex for “recreation” as it were. Human children have an incredibly long childhood (maybe 12 to 14 year in Cavman times) and Mom and Dad have to stick together for a long time so the bonds of initmacy are really important to ensure you raise a successful brood and that your genes get passed on: so those humans that shared intimacy stayed together longer, raised better equipped kids and so their genes survived. So I’d say there’s an evolutionary advantage to kissing!
History of kissing is really unknown whether it is a natural behavior, or learned?? In 5th century BC, Herodotus speaks of kissing among the Persians, who greeted men of equal rank with a kiss on the mouth and those of slightly lower rank with a kiss on the cheek. So this means it is not a new thing. People do it as it is a gesture of affection, to develop social bonds.