The wheel was invented in the 4th millennium BC in lower Mesopotamia (modern-day Iraq) where the Sumerian people inserted rotating axes into solid discs of wood. It was only in 2000 BC that discs began to be hollowed out to make a lighter wheel.

This innovation led to major advances in the following to areas:

1. Transport – the wheel used on carts and battle chariots.
2. Agriculture – animation traction and crop irrigation and craft industries – think of windmills.

Source: Wikipedia