A: Three and a half billion years ago.
In the abortion debate, there's a lot of talk (on one side, anyway) about 'when life begins'. But life is continuous. You're alive, your fetal self was alive, your zygote-y self was alive, your parents' gametes were alive, your parents were alive, and so on, back a few billion years. At no point in this history is everything dead, with life beginning anew.
The same goes for 'human', which is sometimes substituted with pseudoscientific authority. The fetus is human. Well, so are the egg and the sperm. They're not gibbon sperm, or lizard eggs. That only goes back a million years or so, of course, but the point is that humanness does not spring de novo from something non-human every time someone is conceived.
The question is about legal personhood. And this is not some sort of debating shuck and jive. The debate is about legal issues, and the Constitution refers to the rights of the people (aka persons).
And what makes a legal person is not immediately obvious. Since a corporation is a legal person, we can say that personhood begins (I suppose) at the ratification of the articles of incorporation. It is difficult to see how to apply this to people-persons, er... human-persons, er... biological-persons.
Here endeth the blogging.