The giggling rat sounds definitely got my cat's attention.

Why did it never occur to me, in my entire life, to tickle a rat? I guess that correlates to frequency of unprovoked toad-licking.

Okay WildA, I get how laughter can predate speech, especially since humor doesn't need to be provoked by words; fart-lighting, for example, is not usually accompanied by soliloquies.

But now I realize laughter has a variety of different sources:
1 = Physical stimulation, e.g. being tickled.
2 = Dissonance - a new perspective, e.g. discovering that a rat laughs when tickled.
3 = The evil laugh - your link below suggested this angle, where excessive laughter can be caused by "some sudden conception of some eminency in ourselves" e.g. gloating. Batman doesn't like people who do this.
4 = What is up with Santa Claus?

I mean, bottom line, why do I respond to Jack Benny with the same physiological reaction as a rat being tickled? Did nature recycle some part of our brains in an unexpected way?

Similarly, I know a woman who sneezes every time the gets sexually aroused. And she's a limousine driver. I keep expecting to find her name in the obituaries some morning after prom night.

stay away from those trees in Berkeley