Interesting recent paper The Origin of Non-Canonical Case Marking of Subjects in Proto-Indo-European: Accusative, Ergative, or Semantic Alignment (2018), by Roland Pooth, Peter Alexander Kerkhof, Leonid Kulikov, and Jóhanna Barðdal, (Ghent University), ERC-funded Project: EVALISA (The Evolution of Case, Alignment and Argument Structure in Indo-European).

Abstract:

For a long time one of the most bewildering conundrums of Indo-European linguistics has been the issue of how to reconstruct the alignment system of this ancient language state, given the lack of distinction between S and O marking in the Proto-Indo-European neuters nouns and the problem of the Hittite ergative. An additional

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