A negative is just an inversion of the intensity of visible signal. It even has a manifestation in common experience. Stare at something for a long time, then l

A negative is just an inversion of the intensity of visible signal. It even has a manifestation in common experience. Stare at something for a long time, then look away. A negative will superimpose on whatever you're now looking at it. I can't think of a good reason humans of the past should not have been able to reproduce this kind of effect artistically.