primogenitor (noun) – an initial ancestor; the progenitor of a people or family
BREAKDOWN: PRIM- (first or foremost) + GEN- (birth) + -TOR (one who does)
— primogeniture is the state of being the firstborn child; also the right of succession or inheritance belonging to a firstborn child or eldest son
This word is built on the root PRIM-, which means first, chief, or foremost.
Common PRIM- words include primary, primarily, primate, prime, and primitive.
More esoteric PRIM- words include primatology, primigenial, and primiparous.