Why does diachronic linguistics matter?

Diachronic linguistics is important for the same reason the evolutionary biology is important: you cannot really understand the state of a language at any given time, or why the range of attested languages is what it is, without understanding the mechanisms by which languages come to be.

In computational linguistics, diachrony has often played second fiddle. The focus, during the Age of Rules, was on implementing linguistic formalisms (that captured aspects of language like syntax, semantics, and discourse structure). This reflected the prestige approaches in theoretical linguistics. It also reflected the underlying expectation that, with just a few more rules (or a few more fixes to existing rules), these symbolic systems would be good for something.

Historical linguistics has long lacked the prestige of theoretical syntax and semantics and, at first blush, it is not good for much in the practical realm. Certainly the knowledge of human history and culture that comes from understanding the linguistic past is valuable. However, it is not readily apparent how diachronic linguistics feeds into machine translation, question answering, automatic speech recognition, or any of the other human language technologies that are economically important today.

Certainly, on the margins, historical linguistics can help with a few NLP tasks. For example, historical linguistic relationships (are the resulting similarities in phonology) can be leveraged to facilitate named entity recognition, as we have shown in a few papers like (Bharadwaj et al., 2016) and (Chaudhary et al., 2018). Likewise, phylogentitic information has been used for selecting transfer languages in cross-lingual training. However, it seems unlikely that the next big breakthrough in NLP will be the product of diachronic linguistics (or even linguistics).

What it lacks in terms of engineering potential, though, diachronic linguistics more than makes up for in its scientific implications. Indeed, the persistent question of typology (why languages are so alike and why they differ where they differ) has been treated as a question of human linguistic competence. However, decades of research make it appear more likely that it is the mechanisms of human change the bias languages towards certain structural properties, whether in phonology, morphology, syntax, semantics, etc. The question of why languages vary in the way that they do, then, is ultimately a question of what changes languages are likely to undergo in their histories.

It is not obvious that diachronic linguistics will have a transformative affect upon the world economy or the technological sphere. However, it is almost certain that our understanding of human language as a phenomenon hinges on our understanding of language change and linguistic history. This may, in turn, have practical consequences, but even if it does not investigating linguistic diachrony is worthwhile.