How did the varied environments in north and south america shape the emergence of vastly different economic, social, and political development among american indian societies?

In North America in the parts north of the present day Mexico, the First Nations had climatically a more hostile environment to deal with than their counterparts in the now Mexico and Mesoamerica and South America. The winters on the Great Plains and in the now Canadian north were harsh and did not favor large populations to develop (with some exceptions like in British Columbia, Canada  which had a mild climate and in which 100's of 1000's of First Nations lived). So the mainly plains Indians had a nomadic existence following the game and fish and so had a more egalitarian less centralized leadership than their counterparts to the south. In Mexico, Mesoamerica and South America, the climate was generally less harsh, and fairly large scale agriculture was practiced and the people were more sedentary and political power was held in the hands of rulers who though they had henchmen, tended to be all-powerful, though the Incas for example had a quite equitable system of compulsory labour for public works and mines, allowing time for the participants to work their own fields to sustain their families.