The wheel was invented a long time ago; how do you think the designer was inspired to come up with the wheel idea?

Probably a log used as a trolley to help shift heavy weights.

Square wheels just wouldn't roll

The inspiration probably was as Slartibartfast suggests, rolling things on logs; but I doubt we'll ever know how they made that jump from complete logs that you have to keep putting back under the front of the load, to Walt's discs at each corners, on axles fixed to the load.

I have wondered further though. It's all very well putting a great big slab of rock on tree-trunks to move it, but what's to stop the logs sinking into the ground? I wonder if they would also lay logs along the path, for the rollers to work on. It would also make the rolling a bit less of an effort.  

It's curious philosophical point that is comparatively easy to experiment to determine how ancient peoples made and used the tools they had - even knapping flints or erecting monoliths - but not to recreate the insight they had to develop something previously totally unknown but which we take for granted, such as the wheel.  Perhaps it is because we are so familiar with it that we cannot imagine firstly life without it, then the flashes of inspiration that first created it.