Good question HiLo. Hunter Gathers came round the 'top' all the evidence points to that. All the big sites containing large public buildings are found in central and south America they were built by farming communities. It is not possible to farm in the north. Farming did occur in the Mississippi Valley and large earthworks and such like were erected there.

Jared Diamond has written a lot on this in Germs, Guns & Steel:


Here

Best wishes