Kisikongo (Bantu, H16a) present-future isomorphism : a diachronic conspiracy between semantics and phonology

The North-Angolan Bantu language Kisikongo has a present tense (O-Rang- a; R=root) that is morphologically more marked than the future tense (O-R-a). We reconstruct how this typologically uncommon tense-marking feature came about by drawing on both historical and comparative evidence. Our diachronic corpus covers four centuries that can be subdivided in three periods, viz. (1) mid-17th, (2) late-19th/early-20th, and (3) late-20th/ early-21st centuries. The comparative data stem from several present-day languages of the “Kikongo Language Cluster.” We show that mid-17th century Kisikongo had three distinct constructions: O-R-a (with present progressive, habitual and generic meaning), O-R-ang-a (with present habitual meaning), and ku-R-a (with future meaning). By the end of the 19th century the last construction is no longer attested, and both present and future time reference are expressed by a segmentally identical construction, namely O- R-a. We argue that two seemingly independent but possibly interacting diachronic evolutions conspired towards such present-future isomorphism: (1) the semantic extension of an original present-tense construction from present to future leading to polysemy, and (2) the loss of the future prefix ku-, as part of a broader phenomenon of prefix reduction, inducing homonymy. To resolve the ambiguity, the O-R-ang-a construction evolved into the main present-tense construction