In Etymonline, the etymology of primogenitor (and primogeniture) is very similar to progenitor. The word's meaning: Ancestor or forefather.
However, nowhere do I find the reason of the split from the word progenitor. Why does such word exist? When and where (near the 1650s) did it first appear? What is the correlation to the primogeniture succession law?
In a related manner, what is the difference to progenitor? When would one use one or the other?
Edit: Is the assertion that progenitor means the ancestor of a person, while primogenitor means the initial common ancestor of a people, correct?