The use of the marker a with a direct object in Spanish is an instance of Differential Object Marking (DOM), which is cross linguistically a well-documented phenomenon. In Spanish, there are two main dimensions that determine a-marking of direct objects: (i) the semantic and (discourse) pragmatic properties of the direct object, such as animacy, definiteness, specificity and topicality; and (ii) the lexical semantics of the verb, such as aktionsart properties and selectional restrictions with respect to the position of the direct object. Diachronically, a-marking spreads along the Referentiality Hierarchy from personal pronouns and proper names to definite and finally indefinite noun phrases, a process that has been well examined. In this study, I focus on the influence of the lexical semantics of verb classes on this process. I present original findings from two corpus searches from the 12th to the 20th century. The data show that the diachronic evolution of a-marking crucially depends on the verb semantics of the governing predicate, and that the change proceeds according to the Constant Rate Hypothesis of Kroch. These findings suggest that a-marking in Spanish, and DOM in general, can only be described in a multi-dimensional space consisting of the semantic properties of the direct object and the lexical semantics of the verb.