This paper attempts to establish the authorship of a milestone in the development of the concept of the Indo-European language family, the dissertation De lingua vetustissima Europae (Wittenberg, 1686). Since the work of G. J. Metcalf in 1966 and 1974, this dissertation has been ascribed to Andreas Jäger (c.1660–1730), the Swedish student who played the part of respondens in the public disputation in which the dissertation was discussed. The paper sets out the arguments for identifying Georg Caspar Kirchmaier, the praeses in that disputation, as at least the collaborative author of the De lingua vetustissima. It examines a crucial mistaken argument of Metcalf’s, shows that it was usual in the late 17th and early 18th centuries for the praeses to write such a dissertation alone or in collaboration with the respondens, discusses the testimony of contemporaries in this particular case, and remarks on the relationship of the dissertation to Kirchmaier’s own scholarly interests.