The reason religion is so powerful

[John 6IX Breezy]

No one is arguing that a fertilized egg isn't biologically human with the potential to eventually develop into a person, the argument is that there is no evidence that it is a person.

Personhood appears to be a legal term. You don't find evidence of personhood, you have to legally describe it. Perhaps no different from legal adulthood being 18+.

In biology, human is the category—that's our species. And embryo, fetus, infant, childhood, adolescence, adulthood, late adulthood—these are all our developmental stages. That's all there is to it.