Then the Axle

With time the sledges started to wear grooves into the rollers and people noticed that the grooved rollers actually worked better, carrying the object further. This was simple physics, if the grooves had a smaller circumference than the unworn parts of the roller, then dragging the sledge in the grooves required less energy to create a turning motion but created a greater distance covered when the larger part of the log roller turned.

The log roller was becoming a wheel, humans cut away the wood between the two inner grooves to create what is called an axle.

(Information for this blog was found at Inventors )