Many people believe that human beings begin to exist with the emergence of the 1-cell zygote at fertilization. I present a novel argument against this belief, one based on recently discovered facts about human embryo development. I first argue that a human zygote is developmentally plastic: A zygote that naturally develops into a singleton (i.e., develops into exactly one infant/adult without twinning) might have naturally developed into a numerically different singleton. From this, I derive the conclusion that a human infant or adult is numerically distinct from the zygote she came from and so did not begin to exist at fertilization. This implies that a zygote does not have a “future like ours” and strongly suggests that it is not a human being.

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