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).
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 complication stems from the existence of argument structure constructions where the subject(-like) argument is case marked in a different case than the nominative, like the accusative or the dative. Our aim with the present article is to fill two needs with one deed and offer a unified account of this century-long bone of contention. In contribution to the ongoing discussion in the field, we claim that a semantic alignment system, in the terms of Donohue & Wichmann (2008), might not only fit better with the morphological data that are currently reconstructed for the ancestral language, but also with the existence of non-canonically case-marked subjects in general (Barðdal et al. 2013; Danesi, Johnson & Barðdal 2017).
Conclusions (emphasis mine):
For the past decades, the general assumption in the field of Indo-European syntax has been that the alignment of Proto-Indo-European must have been ergative-absolutive, rather than nominative-accusative. However, a reconstruction of the case morphology of Proto-Indo-European corroborates neither of the two analyses. Instead, it suggests a ‘Fluid-S’ system where case marking was semantically driven and the case marking of one-participant clauses was motivated by semantic factors such as whether the referent had an agent role or not. We have laid out the morphological details of the reconstructed semantic alignment stage of Proto-Indo-European where an antipassive-like construction played a key role for the development from semantic alignment to the attested accusative system found in the Indo-European daughter languages today. This antipassive-like construction was reanalyzed as a transitive construction and the earlier agentive *-s marker was generalized into a subject marker, irrespective of the semantics of the subject referent, yielding an accusative system. As a part of this general process we have identified the Early PIE protoconstructions that have developed into the attested accusative and dative subject constructions, respectively. The first one involves the older *-m allative-marking of nonneuters which also developed into the accusative object marker. Through the construction with the *-m or the zero-marking on subjects of one-participant clauses with proto-middle and proto-active marking on the verb, respectively, the accusative subject construction emerged. Out of experiencer constructions involving the old locative ending *-i, one subconstruction of the dative subject construction arose.
To conclude, we have presented an attempt to elucidate how non-canonical case marking of subjects are in line within the most recent discussions on PIE alignment. Our aim has been to show how a Fluid-S or semantic alignment model for the Proto-Indo-European ancestor language may aptly explain the presence of archaic instances of non-canonical subject marking in the ancient IE languages. Although some instances of non-canonical subject marking may be relatively young, as is argued for Latin by Matasović (2011, 2013) and for Hittite by Hoffner & Melchert (2009), this does not necessarily entail that noncanonically case-marked subject constructions should be considered an innovation which affected the daughter languages separately. Instead, we have argued that non-canonical case marking of subjects is a relic of the semantically-marked experiencer and undergoer role. Later, the separate daughter languages may have added new predicate-specific oblique subject constructions to these already existing schematic patterns. We therefore believe it worthwhile to reevaluate the evidence of Latin and Hittite with regard to noncanonically case-marked subjects, since fossils of the old PIE intransitive construction may yet be found there. Also, the exact relationship of non-canonically case-marked subjects to PIE labile verbs and the proto-middle voice category (Pooth 2014), should be explored in full. These questions deserve further research and we will return to these in future publications.
Interesting not only for its intrinsic value, but also because an ancestral common trend may have been found to be more likely than multiple independent dialectal innovations, as we propose for the process of laryngeal loss.