The commonly given PIE etymon of βροτός is *mr̥-to-s
*mr- > br- as in *mr̥g’hu > brakhus
But how do we end up with the initial /o/? One would surely expect *r̥ > ar/ra rather than > ro?
Thank you for your help in this matter!
It’s just an o-grade derivative isn’t it?
This is supposedly an Aeolic word preserved in Homer (either from the Aeolic phase of the epic tradition or by cultural diffusion, take your pick), and thence spread into other dialects. ρο/ορ is the regular reflex of syllabic ρ in Aeolic (and Mycenaean and Arcado-Cypriot).
See Lejeune, Phonétique historique du mycénien et du grec, sec. 201, p. 197; see also Sihler, New Comparative Grammar of Greek and Latin, sec.96a, p.94; Chantraine, Grammaire homérique I sec. 10, p.24 (1st ed.).