>> it's difficult to imagine someone coming up with the lever overnight - and it doesn't really seem to occour in nature.

Nature is full of levers, but that's different matter. It's often fun to try and think of the circumstances in which things were discovered. The lever does make one wonder, but many events could have made people realise the power of levers.

Imagine Ig & Ug have just shot a large deer and they want to get it back to the cave. One of them could carry it. They could drag it. They could tie it to a pole and carry it between them - that works (and would have led to spit roasting too). Then when they put it down Ig decides to move it, so he picks up one end of the pole it's tied to and realises it ain't so heavy after all! Voila ... the second class lever is born !