I wonder if this is not actually a compound word, combining root of "hear" with root of "learn,know".
hearken

SYLLABICATION: hear·ken
PRONUNCIATION: härkn
VARIANT FORMS: also har·ken
VERB: Inflected forms: hear·kened, hear·ken·ing, hear·kens

INTRANSITIVE VERB: To listen attentively; give heed.
TRANSITIVE VERB: Archaic To listen to; hear.
ETYMOLOGY: Middle English herknen, from Old English hercnian. See keu- in Appendix I.