Most scientists believe that the earliest domesticated corn was of the pod-pop variety and was grown in the area of central Mexico, Guatemala, or Honduras about eight thousand years ago. But by the time of sixteenth-century European exploration, the Native Americans had developed the other four categories of corn: flint, dent, sweet, and flour. The dissemination of corn into what is geographically the United States was slow. Corn was cultivated in the American Southwest more than 2,500 years ago. But the vast, marginally populated, arid areas of New Mexico, Arizona, and Texas served as natural barriers, slowing the spread of corn northward or eastward. And the Rocky Mountains also provided a barrier for movement northward and westward.