Humans thought to use the round logs and a sledge together.

Humans used several logs or rollers in a row, dragging the sledge over one roller to the next.

With time the sledges started to wear grooves into the rollers and humans 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.Wooden pegs were used to fix the sledge, so that when it rested on the rollers it did not move, but allowed the axle to turn in-between the pegs, the axle and wheels now created all the movement. These were the first carts.