I don't know, but I do the nod thing quite a lot during my evening walks and bike rides. Just a nice acknowledgment that I see them and recognize their presence for that split second we pass each other. Sometimes I also add a smile, which might be tied for being one of the oldest most universal indicators of recognition. I seldom scream when passing people, but yes, a scream is also a very useful method of communication from pre-historic times, one has to think.

Nothing else comes to my mind as being quite as older or universal as a nod. I'm sure the finger came much much later!