Rick I’m a bit confused by your line of reasoning.
science does not prove that the zygote is fully human in the sense of possessing all the intrinsic rights of a human being; that question is a matter of philosophy and religion. And that is the question on which civil law must rest.
Why must that question *only* be a matter for philosophy and religion? Do either of those prove absolutely that a zygote is, or is not, fully human any more than science? Religion cannot prove the existence of God and philosophy cannot conclusively prove its abstractions, so how can they be tasked with the higher level consciousness of provable fact over that of the biological science which states that the zygote is human. Just because it cannot prove sentience, or some other such requirement, does not make it less worthy than the other two; neither of which can prove the very essence of their foundations (i.e. a Deity and abstract thought), something which biology can at least claim for itself.
I am deeply grateful to you for starting this conversation but I find your dismissal of the hard science in favor of the theoretical sciences highly questionable. This particularly in light of your other writings in favor of sound science coupled with sound judgement surrounding, for instance, climate change.