Letters to the Editor
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@brightstar
Not meaning to make it harder on you, but another complication in your question is this: what would have been the probability, were the tape of history to be replayed with your parents having had sex at that same time, of the same sperm fertilizing that egg? Different sperm, different specific genotype, different starting point...or not. If a different sperm were to have gotten through, it might have borne a lethal mutation at some locus, resulting in a non-viable zygote, or some other variety of spontaneous abortion (as a pretty high proportion of human conceptions do).
Have you by any chance read Stephen Jay Gould's "Wonderful Life"? The last chapter, as I recall, basically plays this game as applied to life on Earth in general, and humans in general. I think you might like it, though it can make for some disturbing thoughts at times.