....such complex manipulations of nature could be carried out by primitive yeomen in eight geographical areas over 5,000 years.....
It's no different than the meticulous records for millenia kept to enable predicting eclipses of the sun and moon.
A lot of people have difficulty understanding that "primitive yeomen" were as smart as you or I.
But they were.
Not even you or I are smart enough to breed a domesticated plant from a wild variety.