What I learned:
Laughter is at least 10–16 million years old. It predates humor by an enormous margin. The acoustic structure of tickle-induced vocalizations in orangutans, gorillas, chimpanzees, bonobos, and human infants is similar enough that a phylogenetic tree built from laughter acoustics exactly matches the known genetic relationships of those species. Laughter isn't a metaphor for primate play sounds — it is primate play sounds, evolved.
The original function was play signaling. Tickling triggers laughter across all great apes because tickling is simulated play-fighting — a physical signal that says "this is a game, not a real attack." The vocalization evolved to broadcast that state: I'm safe, we're playing, don't stop. Rats do something structurally similar with 50kHz ultrasonic chirps during rough-and-tumble play, which Panksepp (who studied this obsessively) called "rat laughter." Not metaphoric