I hesitate to dip even my littlest toe in the murky water that is the Podhoretz v. Ponnuru vs. George debate on human life, but here goes: the debate is intellectually interesting but actually of little real relevance to issues such as abortion and stem-sex research.
No sane person denies that human life exists at the moment of conception. That is, the species is human and the being, however small and undeveloped, is composed of living cells. The real question is at one point a human person exists for the purposes of legal protection from harm or exploitation. It seems to me that the definition of when a person exists can only meaningfully be determined for a large number of people — that is, within a political community that does not necessarily share a particular religious faith — by inverting the clearer definition of when a human person is dead.
We generally identify death as that point when there is no longer any detectable brain activity. The cells of a corpse may still be dividing, and its bodily functions may be sustained for a time by artificial means. But if there is no functioning brain at all, there is no live person anymore.
If we can agree that brain death is the end of a person, then can we not agree that brain function is the beginning of a person? That would not extend legal protections all the way to conception, but it would protect many unborn babies from being starved, killed, or harvested for tissues. There is still the tricky issue of what to do when the mother has not implicitly agreed to use her body to nurture and birth the baby — cases of rape and underage incest, in other words — but that’s a debate for another day.