[prot-] or proto- comb form [ME protho-, fr. MF, fr. LL proto-, fr. Gk prot-, proto-, fr. protos; akin to Gk pro before--more at for] 1 a: first in time "protohistory" b: beginning: giving rise to "protoplanet"
2: parent substance of a (specified) substance "protactinium"
3: first formed: primary "protoxylem"
4. cap: relating to or constituting the recorded or assumed language that is ancestral to a language or to a group of related languages or dialects "Proto-Indo-European"