Actually, there's some evidence as to how ancient Americans discovered nixtamalization. Among pre-agricultural societies across the world, a common method of boiling water before the invention of earthenware was to dig a hole with the water, put a rock in a nearby fire, and drop the heated rock, called a pot boiler, into the water. What they discovered is that if you used limestone, which is common across the Americas, as a pot boiler, the resulting reaction between the heated limestone and water made limewater, providing the necessary alkaline environment for nixtamalization.