It is the suggestion that not everything is present in the zygote and that external forces subsequently act on the foetus to eventually create human individuality that leads to the conclusion that human life does not begin at conception or kick in at any one time.
I'm not convinced that Edelman's new biological theory adds anything new to the ethical debate. I mean, it's not exactly news that genetic determinism is false. And everyone else recognizes that "not everything is present in the zygote" -- Edelman's "stochastic processes" aside, how we develop as individuals will at least depend upon the cultural environment we're raised in, etc.
Perhaps the thought is that the potential variance here is so great that we're not talking about a single person developing in different ways; rather, the differences are so vast that they would amount to distinct people. That is, it's not just indeterminate how the fetus will develop; it's indeterminate who the fetus will grow to be. And, we might think, if the fetus' future identity is indeterminate, it cannot presently be identical to either person, and so presumably isn't a person at all.
But again, it's not clear how this argument depends on Edelman's theory, given that most of us already believe that a newborn has the potential to develop in very varied ways (within genetic constraints, just as Edelman grants).
In either case, it remains open to the pro-lifer to deny that even vast psychological differences entail that the possible future persons are numerically distinct. (They are simply alternative future states that the one person could grow into.) After all, the most coherent pro-lifers will be animalists about personal identity, in the sense that they identify human persons with human organisms -- and there's no real question that the latter come into existence at conception.
Am I missing something?