As you have no doubt seen with hieroglyphics, early Egyptian wall paintings were outlines that had tinted colourings, and the earliest known wall sculpture was an incised outline.
After these outlines had been used to convey form for centuries, people gradually began to carve out the surface of the wall between these outlines, modelling in low relief.
Chiaroscuro Study of a Young Woman Facing Away, by Antoine Watteau, from an original drawing in the collection of Charles Ricketts and Charles Shannon.
Maybe it was this that suggested to the artist painting their outline to shade between the outlines. This subtle suggestion of form, using an outline that was lightly shaded was the only technique used up until Leonardo, who was the first genius to seriously perceive light and shade.