"Human life" is defined by DNA. It isn't vague at all.
And I'm saying it isn't your DNA that makes me care about you, Angel. I care about what happens to you and believe that you should have rights. Why? It isn't because your DNA is similar to mine, or that you have a beating heart, or that you have hands and feet, or that you breath oxygen and exhale carbon dioxide. The only thing that makes you
matter to me as a person, morally speaking, is that you have a mind. And a mind is an emergent property of a functioning, sufficiently developed brain.
So it doesn't matter how you define "human life". The phrase "human life" doesn't address what makes a human a
person, morally speaking, not legally speaking (which is settled law).
The following bit of sci-fi is just to make a point, I'm not making any predictions about it: Some day it could be possible to replicate the operations of the human brain via software on a computer or an articial hardware brain. Maybe we could "download" or "copy" our consciousnesses over or created unique artificial beings that posses consciousnesses in their own right. If that ever happens, my position doesn't change. It is the
mind, not the bits and pieces, that make someone, or something, a "person" morally speaking.
Now, I find that many religious people don't buy my above reasoning, and I think I know why. It is because they focus on words like "DNA" and "human life" to keep things secular. But I don't think those are the things they
really care about, just as they aren't the things I really care about. No, just as I believe it is the
mind that makes a person a person, I think a large percentage of pro-life religious people believe it is the
soul that makes a person a person. And if you believe that and you believe that a human life is infused with a "soul" at conception, then of course you are going to think it is immoral to abort a fetus at any stage. But pro-lifers know that in order to have a chance in the courts they have to make their case as secular as possible. They also know that the argument holds no sway on those of us who don't believe in souls or spirits or ghosts and such.
That is how I see things, anyway. There are a minority of atheists who also oppose early term abortions but their reasoning isn't internally consistent to me, while at least the position of pro-lifers who believe in souls
is internally consistent.