Building on a preliminary analysis of the so-called "Optative negative" (Vanhove 2011), which presented a synchronic semantic map of this verb paradigm in the central dialect of Beja, the sole language of the North-Cushitic branch of Afroasiatic, this article discusses a diachronic semantic map of this verb form within the background of van der Auwera & Plunguian (1998) and van der Auwera et al. (2003). It is based on a larger corpus of first hand data in the three main varieties of Beja (North, South and Centre), using a comparative approach with the other verb forms that constitute the verb system of Beja. The first part presents a more refined analysis of the previous syntactic and semantic findings for the central dialect. It provides an account of the usages of the Optative negative, first in independent clauses with optative, hortative and jussive functions, as well as a participant-external impossibility function, second as a dependent verb form in conditional and concessive clauses, then in relative and completive clauses, where this verb form also has participant-external and participant-internal modal values of uncapacity and unnecessity. It also discusses dialectal variation. In a second part, I present the historical development of the verb forms relevant for the understanding of the origin of the Optative negative, a necessary step in order to achieve a correct diachronic semantic map, which will be presented in the concluding part, together with the consequences of the findings for the origin of the negative polarity system.