Simplify the following text for a Grade 19 native English-speaking person.
Original: While early linguists such as AndrÃ 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 fully fleshed-out theories were simultaneously but independently published in 1973 by Paul Hopper of the United States in the journal Glossa and by Tamaz Gamkrelidze and Vyacheslav Ivanov of the Soviet Union in the journal Phonetica in 1972 .
Simplified: While earlier linguists , such as AndrÃ Martinet and Morris Swadesh , had seen the potential of substituting glottalic sounds for the supposed plain voiced stops of Proto-Indo-European , the request stayed speculative until more evidence for it was published in 1973 by Hopper in the journal Glossa and by Gamkrelidze and Ivanov in the journal Phonetica .