At www.sciencedaily.com/releases/2015/01/150108084903.htm … we learn that maize was domesticated from wild teosinte grasses in southern Mexico and can be tracked into the American SW, using DNA analysis, to around 4100 years ago – very nearly 2300BC (and within that block of a couple of hundred years encompassing two closely spaced events) the date ascribed by Moe Mandelkehr to an earthwide event involving a heavy meteor bombardment. The study is published in the January 8th issue of Nature Plants.
It found that maize first entered the SW around 4100 years ago via a highland route (overland). However, a lowland coastal spread of maize was also detected – at around 2000 years ago (during the Roman Warm Period). A more exact figure might be more interesting,