Full Idea
Against Kripke's thesis of 'necessity of origin' I will just point out the intuitive force of the claim that Socrates - that very person - could, logically, have had no beginning to his existence at all, or have come into existence ex nihilo.
Gist of Idea
Socrates can't have a necessary origin, because he might have had no 'origin'
Source
comment on Saul A. Kripke (Naming and Necessity lectures [1970], p.110-) by E.J. Lowe - The Possibility of Metaphysics 6.5
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.152
A Reaction
It also strikes me that one base-pair difference in his DNA (by a mutation, or a fractionally different parent) would still leave him as Socrates. People are not good candidates for 'rigid' designation. Counterparts seems a better account here.