[ctack]

There are lizards or beetles that tumble down sand dunes.

Maybe cartwheeling humans could lead to some adaptation where the whole body becomes the wheel.

[lupire]

Wheels are not balls. Balls are common in nature. Wheels are not. The difference is that wheels need roads, which are not common nature and a large scale artificial objects.

[ctack]

That makes sense. Even in the Pullman book there were natural roadways for rolling.