It's conception. The penetration of the egg by the sperm that BEGINS the sequence that 9 months (or so) later produces what is legally called an infant. Sort of a mini big bang.
Please read my comment to post 1.
Conception is a continuous process involving millions similar chemical reactions occuring in parallel and in serial. There is no specific time point in the process of conception that meaningfully delineates human life.
Cosmologists speak of the big bang as resulting from a singularity, but strictly speaking, this is unrealistic speculation. No singularity (in the mathematical sense) has ever been observed, nor is it possible to imagine observing one. Furthermore, cosmologists know that the evidence to describe the earliest moments of the big bang are too limited to say anything with much confidence.