Are all humans susceptible to the checker shadow illusion? Or just those that have been trained to interpret flat arrangements of color as accurate representations of 3D scenes and objects? If you showed the checker illusion to someone who had never seen a photograph or representative painting, would they see different colors or the same color?

I don't know the answer. I do find it fascinating that something as simple as perspective (which we take for granted in a graphic like the checkers illusion) is a fairly recent technique, invented by Renaissance artists.