[…] we are faced with the existence, in early indo-European languages, of an '–s' nominative limited to the designation of beings or objects conceived as animate. This limitation suggests that in Proto-Indo-European this '–s' marked a function felt to be characteristic of animate creatures, namely that of acting, as opposed to the passivity of things. Such '–s' forms could hardly be subjects, i.e. forms used with the function of compulsory actualizer of a predicate because the zero information of a subject as such would hardly be compatible with a formally existing indicator. All this points to '–s' as the mark of ergative case, the case of the agent, which must have coexisted with a true nominative, the form of the noun used in order to introduce or ‘nominate’ a person or to present a creature or an object.
- Martinet (1962), Pag. 151