[weegee]

Why are scientists so sure of the origins of life when they cannot even "determine" when life begins after a child is conceived?

Certainly there are enough case studies possible that such a determination should be able to be reached.

There are those who say that "when life begins" should be left up to each state. THAT'S bad science.

[Dimensio]

Why are scientists so sure of the origins of life when they cannot even "determine" when life begins after a child is conceived?

Because the issue of when a child exists following the union of sperm and egg is a philosophical question, not a scientific one. There's no mystery of what happens after the union of sperm and egg cells, either immedately after or in the months following, the issue is when the resulting product should be termed a "child".