Pacifier

Personally I dont really believe in a Soul. But the problem of your question is independent from that believe. The problem is, the human mind seems to have difficulties with the concept of "becoming", the human mind always searches for define point at which something happens. Be it "is human" or "has a soul". In both cases I think you cannot tell at some point now has a soul/ is a human and a second before that it hasn't/isn't.
The two cell merging togehter are not a human being, but the baby is of course human, somewhere between this two points in time these two cells became a human being. And I think this is also true for the soul

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"It seems to me that the idea of a personal God is an anthropological concept which I cannot take seriously. I also cannot imagine some will or goal outside the human sphere. Science has been charged with undermining morality, but the charge is unjust. A man's ethical behavior should be based effectually on sympathy, education, and social ties and needs; no religious basis is necessary. Man would indeed be in a poor way if he had to be restrained by fear of punishment and hope of reward after death
— Albert Einstein