It’s funny how thrown off you might feel when some language branches don’t behave like the rest of the family
When looking up etymons and diverse phonological developments amongst Indo-European languages, I get the most upset about the disappearance of initial *p- in Celtic and Armenian for instance
This initial bilabial plosive is a reliable marker of kinship among IE languages (even in languages like Germanic where *p- > *f- which retained the bilabial component) and descendants can be identified/guessed with a certain degree of confidence because of *p-’s stability
Having a look at Wikipedia puts things into perspective quite easily. Most IE language retained *p- but these 2 branches almost systematically got rid of it;
*pH₂tér- "father"
- Celtic: Old Irish athir "father"; Welsh edrydd "paternal domain"
- Armenian: hayr "father"
*pōds, *ped- "foot"
- Celtic: Old Irish īs "below" < PIE loc. pl. *pēd-su; Welsh is(od) "below, under; lower (than)“
- Armenian: otn "foot”, otkʿ "feet"
*peH₃- "to drink"
- Celtic: Old Irish ibid "drinks" < *pibeti; Welsh yfwn "we drink"
- Armenian: əmpem "I drink"
*prek̂-, *pr̥-sk̂- < *pr̥k̂-sk̂- "to ask"
- Celtic: Old Irish imm-chom-arc "mutual questions, greetings" ; Welsh archaf "I ask"
- Armenian: harcʿanem "I ask"
I’m taking PIE *p- as an example because it has been most stable throughout the many phonological stages PIE went through. Celtic and Armenian represent the most drastic departures from archaic *p-, while Germanic seems to be the second most deviant with PIE *p- > Proto-Germanic *f-
The absence of initial *p- in these groups actually appears less “upsetting” when we take into consideration the intermediary steps like PIE *p- > Proto-Celtic *-ɸ > ∅- and PIE *p- > Proto-Armenian *h-
To note that the proto-languages retained fossils of PIE *p- in some specific environments and that words starting with /b/ in Irish or /w/ and /pʰ/ in Armenian retain a fairly transparent kinship with their PIE ancestor but they seem to be rather few. For instance, an absolutely-not-representative research on Wiktionary for words in Armenian that start with փ /pʰ/ show that they mostly tend to have been inherited or strongly influenced by neighbouring Iranic languages
Likewise, for Irish words starting with /b/ seem to demonstrate that the greater provider of the initial bilabial stop are /b/, /bʰ/ or /gw/
celtic
armenian
etymology