I assume kissing comes from a parent chewing up food and then transferring it to the baby.

This is innate “caring” behavior.

Similarly, mammary glands are for providing milk/nutrition.

These get associated with primal needs and therefore the amygdala, and so is associated with strong emotion.

Or, kissing is just a subset of skin to skin touching, using sensitive body parts. Not so different from holding hands.

Holding hands may also be from when babies latch on and don't let go because of the necessity of holding on to the parent. My friend's 2 year old grabbed my hand recently and it reminded me of their iron grip.

Any touch can be erotic, depending on the context. That's why I'm skeptical about kissing being special. People have sex with their entire bodies, not just using genitals.

Chew your meat for you, pass it back and forth In a passionate kiss from my mouth to yours I like you

One hacker to another says "I'm lucky to've met you."

The song is 34 years old my friend.

It was a terrible day 34 years ago.

I think it’s a lot simpler. It’s touching parts of the body that have a lot of nerve endings. Lips, fingers and hands, genitals, noses etc.