Thatched roofs, flint, pottery and buffalo hides, yep, an advanced civilization, about like global warming, eh?
About as civilized as Europe during their stone age.
It takes a lot of organization and quite a market system to keep 20000 people fed when everything has to be transported in with a basket on your head or in a back pack.
Apparently they disposed of their sewage, or the city would not have lasted two hundred years.
The Maya, Aztec, and Inca built cities to be envied by any nation.
There have been a lot of people speaking over the last couple decades that NA native populations were severely depressed by time America embarked on Manifest Destiny. Much evidence has been shown that bison populations were higher in the early 1800s than any time before, as their sole predator was in such declining numbers.
