Merriam-Webster Collegiate Dictionary - prot-

 or proto-  combining form  Etymology: Greek prot-, proto-, from protos; akin to Greek 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. capitalized 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