The arch a marvel of design and physics (perfected by the romans)

The Elamite tomb (dated 1500 BC) contains a parabolic vault, considered one of Iran's earliest evidence of arches.

How did they get it?

A proper arch comprises wedge-shaped blocks (typically of a durable stone) called voussoirs, with a keystone in the center holding them into place. The weight is transferred from one voussoir down to the next, from the top of the arch to ground level, creating a sturdy building tool.

The arch comprises vousssoir (a wedge-shaped stone) kept in place by a wooden frame beneath it until they are locked together at the top by the keystone. By itself, the stone can support very little. If you add weight to the top, the sides will push outward and crumble. So, the Romans placed stones around the arch to counteract the outer thrust. Not the weight at the top presses the individual stones together,  making the entire arch rigid and capable or supporting very heavy loads. 

Physically, the weight goes down from the voussoir into the springer's bottom face: the impost.