from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. Simple past tense and past participle of glottalise.
Sorry, no etymologies found.
Thus PIE would have had only two stop series: plain voiceless stops and creaky voiced or pre-glottalised stops.
Apparently, when looking at more languages that have pre-glottalisation, they all tend to lack the pre-glottalised b.
Proto-Indo-European, voiceless, voiced and voiced aspirated (based on Sanskrit), are actually voiceless, glottalised and voiced respectively.
If this is true, the first stage of the shift, voiceless stops to voiceless fricatives, happened as normal, but the glottalised stops were merely weakened to aspirated stops (or not even -- English still glottalises these stops in coda position), and the voiced stops did not change.