what? First: there is no soul, or, at the very least, it isn't science and it isn't arguable or testable, so it is irrelevant here. It's an argument from religion, so it has no place in law or any type of debate that depends on testable, fundamental laws of nature.
fertilization, if it somehow happens, starts you at a ~30% chance of maybe an implantation happening, if the zygote even manages to survive the next 3 days. There is no Biologist that would ever defend the preposterous notion that life begins at conception. It's utter nonsense, and completely defeated by the basic work (those of us that work in reproductive biology) do every single day.
As far as a soul goes, it's a loaded question, but it's a point that needs to be addressed regardless.
I don't think it's politically tenable fur democrats to claim people have no souls. You won't get far with that argument.
But yes, it's ambiguous, at least with answering the question when precisely does a developing life transform from a mass of cells into a person with rights?
As far as biology goes, it's also ambiguous. Conception, basic formation of the heart, brain activity, response to stimulus, passing thru the magical walls of the uterus? Some other arbitrary milestone?
Where you draw the line and others draw the line will be different based on different values.
Given this ambiguity, and given the lack of specific constitutional language and specious legal reasoning behind the original ruling (which was the finding of the court, and has been a bipartisan view), it's likely most appropriate that this question is left to states to decide. Federalism has a place here.
CA can codify in their constitution based on their values, and if MS has different traditions and values, they can do the opposite, and they will have to own the messy consequences of such.