Tadaksahak, a heavily Berber-influenced Northern Songhay language spoken in northern Mali and Niger and closely related to Korandjé, is a remarkable example of how far language mixture can go. While the core grammar remains Songhay, causatives and passives can only be formed using Berber morphology attached to Berber stems, so every non-Berber verb in the language has a suppletive causative and passive (there are only a couple of hundred of those left, though, so it’s not that impossible to learn.) I recently finally finished a review of Regula Christiansen-Bolli’s Grammar of Tadaksahak (you can read the review here). For various reasons, I ended up taking the opportunity to write an overview of the general problem of how the language came into being. I don’t have a final answer, but I did find that it was even more complicated than it looks.
You see, Tadaksahak speakers are currently mostly bilingual in Tuareg, and well integrated into Tuareg culture. Most of the Berber loanwords in Tadaksahak are from one or another Tuareg variety. But quite a few — including some of those irregular causatives and most numerals up to 20 — are demonstrably not from Tuareg, but from some other Berber language, closely related to Tetserrét (Niger). Today, Tetserrét is nearly extinct, and nobody speaks it as a second language; obviously things must have been different in the past. It looks like most Tadaksahak speakers are visibly of Berber descent, so probably they shifted from Tetserrét to Northern Songhay and then came under Tuareg influence. But why would anyone want to adopt Northern Songhay, currently barely hanging on in one or two remote towns of northern Niger, as a first language? Again, obviously things must have been different, but it’s not easy to see how. My best guess for the moment is that they did so in order to reinforce their identity as religious specialists (ineslemen, “marabouts”), since Songhay was the language of the urban centres where advanced religious studies could be pursued, but there are a lot of question marks over that. To confuse matters further, their neighbours like to claim that Tadaksahak speakers are of Jewish descent – probably just to undermine their religious specialist status, but possibly reflecting some more complex history. [...]
Imagine having to use a suppletive verb, borrowed from another language, every time you wanted to use a passive form! And that messy history, obscurely visible from the present situation, is just the kind of thing I love historical linguistics for.