I have no sources to cite (but none have been in this thread), but seem to have read (Eades, Stephan, Peter ? other Paleo blogs) that corn, rice, soy, grains in general are a very minor part of long lived cultures. Much like the fantasy Mediterranean Diet, there is some debate as to what quantities and what foods certain long lived cultures consume.
For instance, as I understand it (again, no particular source) long lived Okinawan's diet is very much pork based. Also, historic Japanese consumption of Soy is under 2 tblspoons a day and only in fermented form, and always eaten with fatty fish broth.
Do we even know that there are high grain eating long lived cultures?