Science thinks life begins at fertilization too. https://www.princeton.edu/~prolife/articles/embryoquotes2.ht...
It is, for example, perfectly possible to amputate someone's hand an keep it alive by feeding it some type of artificial blood; not sure if it's currently possible with the state of technology, but there is nothing stopping this in principle. I think it would be accurate to say that this hand is "alive", but is it also a human being worthy of the same protections that you or I have? I don't think many people would argue for this.
You don't need to think of exotic experiments either; we shed all sort of living biological material all the time, and no one is concerned with the rights of this.
So when does a "human being" begin? I don't have any clear answer; that question is even more difficult than trying to define "life". But I do think it's important to think of it in these terms.