It is difficult to find a truly general criterion—a fingerprint—for language, which considers possibilities as different as the song of the mockingbird and the flashing colors of the squid. But no matter how advanced an alien civilization is, it must have evolved from simpler life-forms that communicated in simpler ways. There are universal traits that evolution should favor in the emergence of any language on any planet.

The most important of these seems to be this: Language should be as complex as it needs to be to convey the necessary information, but no more. An infinitely complex language would mean that aliens would need infinitely large brains to process it. Evolution values efficiency, and absurdly complex language is inefficient. This principle applies equally well to humpback whale song and to Michelangelo’s paintings: We can understand the meaning in a painting’s beauty because it is balanced, not because it is complex.

Such a principle should apply to an alien’s language too, and hence to their messages to the people of Earth. Even if an advanced civilization decides to restructure its language to be more regular, as humans have done with Esperanto, it is already too late. Our brains have been shaped by our language as much as the other way round, and Esperanto still carries with it the traits of its earlier, less well-structured predecessors.