> So failing to conform to Zipf's law doesn't prove a signal is not meaningful, but [conforming] to it is likely to be very strong evidence that a signal is a form of communication.

Then again, Zipf's law is known to describe many non-communicative, "natural" distributions like the sizes of grains of sand, the sizes of asteroids, and the populations of cities (one of Herbert Zipf's original objects of study). And conversely, many levels of representation of human communication have non-Zipfian distributions, like phoneme distributions. (See my comment history for references.)