The glottalic theory is that Proto-Indo-European had ejective or otherwise non-pulmonic stops, *pʼ *tʼ *kʼ, instead of the plain voiced ones, *b *d *ɡ as hypothesized by the usual Proto-Indo-European phonological reconstructions.
A forerunner of the theory was proposed by the Danish linguist Holger Pedersen in 1951,[1] but he did not involve glottalized sounds. While early linguists such as André Martinet and Morris Swadesh had seen the potential of substituting glottalic sounds for the supposed plain voiced stops of Proto-Indo-European, the proposal remained speculative until it was fully fleshed out simultaneously but independently in theories in 1973 by Paul Hopper of the United States[2] and by Tamaz V. Gamkrelidze and Vyacheslav Ivanov of the Soviet Union in the journal Phonetica in 1973.
The glottalic theory "enjoyed a not insignificant following for a time, but it has been rejected by most Indo-Europeanists."[3] The most recent publication supporting it is Allan R. Bomhard (2008 and 2011) in a discussion of the controversial Nostratic hypothesis, and its most vocal proponents today are historical linguists at the University of Leiden. An earlier supporter, Theo Vennemann, has abandoned the glottalic theory because of incompatibilities between it and his theory of a Semitic substrate and loanwords in Germanic and Celtic languages (Vennemann 2006). However, Martin Kümmel (2012), although rejecting the ejective hypothesis as implausible, argues for a re-interpretation of these stops as implosive, comparable to the Leiden interpretation as pre-glottalized stops.