I had a longer progression in mind (http://en.wikipedia.org/wiki/History_of_the_tallest_buildings_in_the_world#1300.E2.80.93present), the idea being that steel and industry were a similar discontinuity.
Though it looks like these examples are really just pointing to the idea of agriculture and industry as the two big discontinuities of known history.
I had a longer progression in mind (http://en.wikipedia.org/wiki/History_of_the_tallest_buildings_in_the_world#1300.E2.80.93present), the idea being that steel and industry were a similar discontinuity.
Though it looks like these examples are really just pointing to the idea of agriculture and industry as the two big discontinuities of known history.