The verbal morphosyntax of Mande languages is characterized by an important degree of isomorphism between verbs and nouns and between the forms of verbs used in transitive and intransitive constructions. In the former case, the isomorphism is often described in terms of conversion, usually from verbs to nouns. In the latter case, the isomorphism is often described in terms of lability, typically P-lability of the decausative and passive types. The latter type, passive P-lability, is said to be typologically highly unusual (cf. Letuchiy 2006, Creissels 2014). Interestingly, the two types of isomorphism are both most prominent within one particular subgroup of Western Mande (WM) languages, viz. Central-Southwestern Mande (CSWM). Thus, in CSWM languages such as Bambara (cf. Dumestre 2003), roughly speaking, any verb can be used as a noun, viz. action nominal (while the reverse is not true), and any verb that is used transitively can also be used intransitively with a passive-like meaning without any changes in its form (while the reverse is not true). In the remaining subgroups of WM and in Southeastern Mande (SEM), the other major branch of Mande, the two types of isomorphism are much more constrained. I argue that both the convergence of the two types of isomorphism itself and the fact that they are most prominent in the CSWM subgroup are related phenomena. They both result from one small change in the verbal morphology in the history of Mande. In particular, they are both endpoints of an evolution of the same light verb construction comparable to do-periphrasis in English. Semantically, the evolution of this construction went from a kind of verb focus through (with transitive verbs) antipassive to a more general detransitivization device and simultaneously to a marker of action nominalization (possibly through the use of this construction as dependent predication). This scenario is composed of changes of which every stage is attested or transparently reconstructible in various Mande languages (cf. Creissels 2012 on some WM examples). Formally, the light verb became a bound suffix and later fused with the verb root often resulting in irregular stem pairs. This complicated allomorphy was subsequently resolved by generalizing one of the two stem forms. The joint result of these changes was the rise of the two isomorphism types in question. In WM groups other than CSWM, the relevant changes have not been completed and/or were superseded by later changes. In SEM, very few possible traces of the relevant construction can be found suggesting that it has disappeared rather early there.