Challenge: No Scientific Consensus on When Human Life Begins

This challenge comes from the first item in the Pro-Choice Action Network’s article refuting “some common misconceptions about abortion”:

Human life begins at conception.
There is no scientific consensus as to when human life begins. It is a matter of philosophic opinion or religious belief. Human life is a continuum—sperm and eggs are also alive, and represent potential human beings, but virtually all sperm and eggs are wasted. Also, two-thirds of human conceptions are spontaneously aborted by nature.

What mistakes are made here? How would you argue that we can objectively know when a new human being comes into existence? Answer this challenge in the comments below, and then we’ll hear Alan’s answer on Thursday.

[Explore past challenges here and here.]