When does life begin? is a question that seems simple and straightforward, but really isn't.
Defining life to begin with is hardly straightforward, but for the purposes of this question we mean "human life," and more specifically we mean the life of an individual human being, as "life" in general only began once, and has existed continuously since the dawn of the first cellular organisms billions of years ago. Hence, this is more a question of "when does personhood begin?" for a particular person, rather than any definition regarding abiogenesis. This question is crucial to a number of complex ethical debates regarding abortion, premature births and, at the other end of the spectrum, brain-dead patients. It is almost as complicated as defining life itself. Ultimately, it is not a question of science, but of morality, politics, and ethics.