I had thought that as well but there seems to be some confusion of whether the word is progenitor or primogenitor. As of now according to some people they seem to be leaning toward primogenitor as being the official term but I could of sworn it was progenitor in the anime?
I'd like to point out that these two words are synonyms. A primogenitor is the earliest ancestor of a people, and a progenitor is the most generic term of an ancestor of a set of people, animals, or plants. But the word used most often in the anime is progenitor, at least by translation.