primogenitor means an initial ancestor. Lexicurio rates it Sui generis — a strength score of 85 out of 100.
Why this word is great
PRIMOGENITOR — [Noun] The initial or earliest ancestor; a forefather from whom a lineage originates. From the Latin primus ("first") and genitor ("begetter, father"). Unlike "progenitor," which typically denotes a direct parent, or "forebear," a general term for any antecedent, "primogenitor" insists upon a singular, inaugural source. It is the faded name on the first page of the family ledger, the immigrant etched on the inaugural ship's manifest, the solitary figure at the muddy headwaters of a river—the profound and isolating truth that every sprawling lineage narrows, at last, to one.