Simplicity is the ultimate sophistication.
With very few exceptions, the art that preceded Leonardo was for the most part two dimensional. Leonardo mastered the art of sfumato and chiaroscuro.
Sfumato, (from Italian sfumare, “to tone down” or “to evaporate like smoke”), in painting or drawing, the fine shading that produces soft, imperceptible transitions between colours and tones.
Chiaroscuro, (from Italian chiaro, “light,” and scuro, “dark”), technique employed in the visual arts to represent light and shadow as they define three-dimensional objects.
Perhaps his obsession with making things look three-dimensional stemmed from the fact that he is the father of stereoscopy, the science of 3D. He was the first person to figure out that you had to boil a human eyeball in order to dissect it. (Doing these experiments on deceased human beings at the risk of the death penalty for being caught)
From there he figured out that the eye was transposing an image upside down in the brain and only then could the brain understand what it was receiving. He also calculated that the parallax between one eye and the second eye is what makes three dimensions perceptible.
In simplifying everything to variations of light and shadow, he created the ultimate new sophistication in art.