Name me one person in the history of humanity whose existence started at birth. In other words, one person who was not conceived and then nurtured in a human womb.
Are you intentionally misunderstanding?
The concept of “person” with legal rights is just not a scientific concept. We can use science to inform us on who we should choose to grant such rights to, but science cannot make the choice for us.
“Person” is a legally and culturally determined concept. Science is no help. Science gives us information about the world. It does not help us choose between good and evil ways of using that knowledge.
FWIW, I agree that personhood begins at conception. I just cannot use science to convince someone else of this fact. Science has nothing to say on the issue.
BTW, under the Nazis “science” was used to show that Jews and Slavs were sub-humans. In our country the Dred Scott decision used “science” to determine that blacks had no rights which white men need respect.
Science is a wonderful tool. Moral and legal issues it cannot solve for us.