Da Vinci's Ghost: Genius, Obsession, and How Leonardo Created the World in His Own Image actually goes into detail on this specific question. In a short and dirty summary: Vitruvius was a Roman architect who believed building should imitate the perfect human’s dimensions - which he also gives in detail, basing every part of the human body off of the top of the smallest finger. Vitruvius also talks about using squares to represent the mundane and circles to represent the divine.

In the Middle Ages, the Vetruvian Man (i.e. this idea of a perfect human being both mundane and divine, represented in both a square and a circle) becomes a somewhat useful way to depict Jesus. Lester gives examples and pictures of paintings that have Jesus inside the circle and square, but they are often forced- often Jesus is standing straight or sideways and so the circle is smaller or the square is more rectangular in shape.

Da Vinci does two things with his Vitruvian Man. Firstly, he places the body in motion - one pose making the square (feet straight and arms out) and one pose making the circle (the jumping jack pose). Secondly, and far more importantly, his Vitruvian Man isn’t Jesus. Da Vinci instead draws a self portrait, by extension claiming that he, a mortal, contains both the divine and mundane. Lester continues by saying that since all things in the universe are made up of divine and mundane things, if we humans possess both qualities as the Da Vinci’s rendition claims, we humans possess the ability to understand all things in the universe.