Based on the results from Google, 'Primogenitor' refers to the first ancestor.
Is there a word that means the last descendant?
This question goes beyond English. You have to specify the group or family of which he or she is the last descendant. For example,
John Adam Doe IV died on Monday, the last descendant of John Adam Doe I, who founded Doe Manufacturing.
Even then, unless the extra-marital issue (if any) of John Adam Doe !V and all his descendants are dead, John Adam Doe IV will not be the last descendant. Moreover, JAD IV may not even be a descendant of JAD I at all, for reasons we are all sophisticated enough to know. This is more common than one would like to believe.
Furthermore, unless someone is literally the last human alive, he is unlikely to be the "last descendant" of anyone who lived more than about a thousand years earlier.
From The Ancestor's Tale, by Richard Dawkins:
....it doesn't take long before everybody is either a common ancestor or has no surviving descendants......Only during .... [a] brief [period] ..... does there exist an intermediate category of people who have some surviving descendants but are not common ancestors of everybody. A surprising deduction....is that....about 80% of individuals in any generation will in theory be ancestors of everybody alive in the distant future.
These results come from computer modeling. As for the timing, Dawkins continues:
If the model applied to Britain, [the time] ...when everybody is either the ancestor of all modern British people or of none, is....about 1000 AD."
(If you want to learn more about the model and its conclusions, see Chang, J.T. (1999) Recent common ancestor of all present-day individuals, Advances in Applied Probability 31:1002-1026.)
You might consider the term remnant, meaning "a surviving trace".