My three-year-old son wanted a bunch of books about sea creatures from the library. One of these was a pretty children's book about corals and what lives among them. The book contains a thorough description of coral reproduction. After talking of the sperm and egg uniting, the book says: "With fertilization, a new life begins." This, I take it, is a quite uncontroversial claim. That is precisely when a new coral's life begins.
And there does not seem to be very good reason why there should be any more controversy in the case of humans. Corals reproduce sexually, and typically[note 1] so do we. Sperm and egg unite in very much the same way. It seems that if the new coral comes into existence at fertilization, we should say exactly the same thing about a new human, without any controversy.
Of course, I know about the alleged distinctions between being a human organism and being a human person, and all that. But these seem both implausible and not scientifically grounded.
Here is something more like a policy argument, albeit a conditional one. If we do not wish our jurisprudence to rest on controversial "comprehensive" philosophical or religious views (not that there is anything wrong it resting on such a basis, but a lot of people don't like it), we should have a strong preference for formulating our laws in terms of concepts derived from science. Thus, our laws should not make reference to race, if it turns out that race is not a scientifically respectable category. (But note that it could be that although race is not a respectable category of any natural science, being socially classified as a member of race R could still be a respectable category of a social science, and could function as a concept in a law.) As much as we can, then, our laws should understand death in whatever way biologists do, with whatever fuzziness biologists see in death. Similarly, we should not use the non-scientific concept of a "person", since that is a concept tied to controversial metaphysical or ethical views, but in our formulations should divide things up scientifically, presumably using the fairly respectable concept of a "human being" or "human organism" instead. To draw a line between those human organisms who are persons and those who are not would, then, be something we should avoid. Presumably, then, we should legally protect the life of all human organisms (or at least the innocent ones). At least if we don't want the law to rest on controversial "comprehensive" philosophical or religious views.