In this article, a comparison is made of the expression of possibility in West Nyanza Bantu languages in order to reconstruct the origins of Luganda’s two most frequent possibility markers, viz. the near-synonymous auxiliaries -sóból- and -yînz-. Earlier Luganda diachronic corpus-driven analyses showed that -yînz- has been involved in expressing all possibility categories since the 1890s, which is when Luganda was first reduced to writing, while -sóból- acquired deontic possibility as a meaning only in the 1950s. Although this would suggest that -yînz- is the possibility marker with the greatest time depth in Luganda and across West Nyanza, with -sóból- a relative newcomer, the comparative data which is presented in this article indicates the opposite. It is shown that while -yînz- only exists in some West Nyanza languages (namely in the subgroup which includes Luganda, Lusoga and Lugwere), -sóból- is attested in all West Nyanza languages as well as in other Great Lakes Bantu languages outside West Nyanza. The fact that the cognates of -sóból- in all Great Lakes Bantu languages carry a dynamic modal meaning ‘be able’ suggests that its modal usage is older in any of the individual languages considered here than what language-internal Luganda data seems to suggest.