I had a neighbour who would communicate with his dogs with a series of escalating "no"s.
e.g:
no... No... NO... NOO!... NO!!!! NO! NO! NO!
And all the dogs I've had never needed verbal communication to understand what I wanted. I usually stared at my dogs and they'd go to the bath themselves (unfortunately, they wouldn't bathe themselves).
Koji: I'd like to extrapolate on your discourse. It is marginally correct, but I don't think it paints a good enough picture. I would believe humans have inbuilt, genetic meanings to certain actions. For example, the action of me pointing my finger. Even babies know that when another points a finger at something, it's a sign that you're pointing your finger at SOMETHING and that something is a point of attention.
However, do that on an animal and it's taken as a sign of aggression (you'll also probably end up with a bandaged finger). In this vein, the act of raising a hand (or anything really) is taken as a general greeting... with different levels of meanings behind them which are interpreted differently of course, but the basic idea is that it is still an acknowledgment of someone's presence.
I think you'll find that it is entirely possible to conduct yourself socially (ignoring sign language) without actual words. Even a grunt, an "oh", or a stare can carry complete conversations. Combined with all our other various actions (and we have a lot: arm scratching, tongue clicking, etc.), and we're set. Words merely help this process along.