Unskilled workers - GIF

[Toolmaker51]

In the California mining ghost towns, there are examples of old wheelbarrows. I'm sure it depended on what was being moved, but some had no sides, just front and floor. Too late to capitalize staking a plot to mine gold, a fellow [Wheelbarrow Johnny] did next best thing, and he built LOTS of them.
So many in fact.......... https://en.wikipedia.org/wiki/John_Studebaker

Historically, the wheelbarrow seems a likely early material hauler. The problem might have been load size; small enough to lift in, an unstable one wheel cart. That may have turned out result of the American Indians' type travois. Both use same mechanical advantage of leverage.
Thinking details like rollers evolved into wheels, the hangup would be axles, without means to get some degree of round holes first. I'd bet rollers evolved with a reduced diameter forming an axle first. The earliest probable 'bearings' were notches or saddles, then figured out grease. But it would require fairly smooth surfaces to roll [what does a little pebble do to your floor jack?]. Narrowing it, might have been next. That may have progressed into wheels, after learning to keep them in position.
To me, the real brilliant step? Realization that increased wheel diameter was most effective way to traverse rough paths.