Kissing is ubiquitous now, of course, as a sign of intimate affection; I don't think I'm aware of any cultures currently extant that don't kiss in romantic/sexual/intimate settings.
But was this always true? I found myself wondering as I read a book set in the Roman Empire, and then the curiosity cropped up again when watching a Western film. I also dimly recall that when I read historical texts like Julius Caesar's Commentarii de Bello Civili, Homer's Iliad or the Bible, there's mention of kissing, both in romantic and less romantic settings. It can be hard to tell if, in the case of the book set in the Roman Empire (for example), the modern author made a romantic scene fit our idea of what would appeal (the kissing, touching, sex, etc.), as opposed to how they actually showed affection in that culture and period.
To my mind it seems like a societal, and not an evolutionary behavior -- there's no reproductive reason we do it; it's a social sign of affection, love, lust, etc. Yet, it's everywhere, and seems so natural to do -- anecdotally, even when young, I felt I 'wanted' to kiss the people I was attracted to on a subconscious level, and I didn't have to learn (it seems) how it 'worked' or why I'd want to do so, as was the case with sex or other forms of intimacy to a prepubescent boy.
So, do we have any idea about:
Thanks in advance, AskHistorians, as always!
I don't think it's possible to determine when kissing became a social norm because our closest evolutionary relatives—chimps and bonobos—kiss for largely the same reasons we do. This suggests that the behavior did not originate with H. sapiens and dates back to prehistory, probably to a common ancestor.
Is it a learned behavior? That's not clear. In all of the literature I've read I've not once heard of a culture of chimps that doesn't kiss, and I think you'd be hard pressed to find a culture of humans that doesn't kiss in some respect, either. Because of this I'm of the belief that while kissing isn't quite an innate behavior, it's one that is very easily cultivated in apes for some other evolutionary reason.
Of course, societal norms play a tremendous role in how kissing is perceived and the roles it serves in society. A recent study out of Oxford found that only 46 percent of modern day cultures kiss in the romantic sense. While I question the methodology it does demonstrate that there are a wide variety of meanings and uses for kissing.
So all of this is to say that we don't exactly know but it looks like kissing is a really, really old behavior and it is definitely not unique to humans.
That said I'm sure there's someone on here who specializes in the history of romance that would be able to provide even more context and I'm eager to hear what they have to say.
Sources: Class notes on Frans De Waal Oxford U Press
I came across this question from /r/nostupidquestions.
There was a comment that referenced an article discussing affectionate kissing not being something found in all cultures. It might help you with your last question.