[orwin]

Discovery of the axles and roads.

The 'wheel' itself was discovered everywhere. Round things are easier to move, but you need an axle to make it useful. And roads or flat terrain to make use of that. Incas had pulley systems, which indicates they could probably have built an axle quite easily too, but had no use for it, because, well, no flat roads.

And even then Northern Manchurians knew about the wheel for sure, and knew about roads, but still used sleds until at least the Russian conquest.

Sorry, I'm quite boring about this, but it bothers me when people talk about 'inventing the wheel' like it was something special. The wheel itself is meh. The axles are what makes it usable, and the roads make it useful.

[jeltz]

Roads were also innvented everywhere. There were cultures with flat roads and no wagons. I would say the axle and then the spoked wheel were likely the big deals.