Origin of Corn Genetics Diversity

About 9000 years ago, give or take 1000 years, people in Southern Mexico were finding ways of using seeds of a weed we now call Teosinte as food.  It was inefficient in that the seed were encased in a hard fruit wall and that these fruit (grain), were easily shattered from the thin rachis, spreading the seed for the next generation. The hard encasement (fruit wall) allowed the next generation to pass through the gut of a bird, causing in spread of the species. These tall grassy, tillered weeds had many flowers per plants and flower structures that encouraged cross-pollination. It was about the time in human history that our species started to switch from being food gathers to farming. Archaeologists now have evidence that about 4500 yrs ago, farmers in southern Mexico had identified and cultivated a variant of Teosinte that had a cob with seed encased in a thin fruit wall (pericarp) that allowed easier preparation for food. We now know that it only took a few major gene mutations to change this plant to one that had a thin pericarp, a rachis to a cob and a drastic increase in number of kernels from the 8-12 on the original weed to 20-50.  From that beginning, the new type was gradually spread throughout North and South America.  As humans moved it to new environments and selected those that best survived and had characteristics best for them as a food source, corn became a mix of local adaptation and maintaining some of its wild Teosinte past. By the time of Columbus arriving in the New World, corn was cultivated from Canada to Argentina, from hot humid tropics to dry areas of western US and Argentina. 
 
Selections made by locals had resistance to local corn diseases and insects, soft kernels for easy flour production, hard kernels for better storage, different kernel colors for local preference, fewer ears per plant for easier hand harvest and many other characters that came along with diverse local needs. There were varieties that rapidly expanded endosperm when heated (popcorn) and those with an enzyme delayed sugars to be converted to starch (sweet corn). After Europeans introduced this wonderful crop to the other continents, selection to each of those environments further allowed selection for adaptation.  Consequently, corn genetics is more diverse than any other crop, always available for the next request that we humans can make from it.