Agricultural societies cannot exist without staple crops  Several food plants, such as kola and okra, are known to have been domesticated in western Africa, but they are all supplemental, not staple, foods  All the recorded staple crops grown in western Africa were introduced from elsewhere, beginning, at some unknown date, with rice and yams
Ttherefore, discovering when rice and yams were introduced into western Africa would establish the earliest date at which agricultural societies could have arisen there
People in western Africa did not develop staple crops that they stopped cultivating once rice and yams were introduced.