Theory:

Invention of Wheel:
 
Around \(3500 BCE\)., in Mesopotamia, potteries were built to act as potter's wheels, around \(300\) years before wheels were discovered to be used for chariots.
 
wheels for chariot.jpg
 
The ancient Greeks invented the wheelbarrow.
 
invention_wheelbarrow.jpg
 
Some hominins stood on a loose rock or an unstable log in prehistoric times and found that a round object decreases friction with the ground; this is how they discovered wheels. The Egyptians and Mesopotamians used them to build pyramids and transport heavy equipment.
 
wheel for pyramids.jpg
 
On the other hand, rollers are inefficient since they must be replaced when they roll forward, and even when pinned underneath, friction makes movement impossible.
 
wheel.jpg
 
The axle was the answer and a stroke of genius. They could now roll their heavy machinery without having to replace the rollers regularly.
 
caveman_wheel_shadows.gif
 
The axle's ends had to be nearly perfect, smooth and round. There had to be a hole in the middle of the wheel to fix the axle with spinning wheels; otherwise, there would be much friction between these components for the wheels to spin. Furthermore, the axles had to fit snugly within the holes in the spokes, but not too snugly, because they needed to rotate freely.