going back to Milo's original post, don't the aboriginal people of australia have thousands of languages? geopgraphic isolation was (is?) supposed to be the main contributer to the variety.
American peoples at some early point in populating the continent, killed off many of the large mammals, leaving very few for possible domestication. with no large draft animals, no evolution to domestication and use of the wheel.
and with no large domesticated animals, people who moved off, to a far place, because of war, or banishment, or what ever, would loose contact with the old ways, and the old words.
words that worked in the northern plattues of montana, could not be used to express the rolling hills of iowa. new words needed to be created to express the new animals, geography, weather, trees, and changed life style.
it took thousands and thousands of years for corn (maize) to move from central mexico where it was first domesticated to the rest of the americas. (in contrast, domesticated animals and grains move rapidly in the fertile cresent, and then to all of of the mediterranian) partly it was the climate, (the area in near east share a more common climate) and partly, it was an already established trade route, using domestic animals.
(and inspite of this, there are hundreds of languages in the area from say, israel/syria on the Mediterranian, and say india)
trade and contact might have caused some languages to consolidate, just as distance might have caused american languages to fracture, and change.
