A Scientific View of When Life Begins

The question of when human life begins has been answered in a variety of ways by different religious and philosophical traditions throughout the ages, leading many to conclude the question cannot be definitively answered. Yet what does science tell us about when life begins?[1] One of the basic insights of modern biology is that life is continuous, with living cells giving rise to new types of cells and, ultimately, to new individuals. Therefore, in considering the question of when a new human life begins, we must first address the more fundamental question of when a new cell, distinct from sperm and egg, comes into existence. The scientific basis for distinguishing one cell type from another rests on two criteria: differences in what something is made of (its molecular composition) and differences in how the cell behaves. These two criteria are universally agreed upon and employed throughout the scientific enterprise. They are not “religious” beliefs or matters of personal opinion. They are objective, verifiable scientific criteria that determine precisely when a new cell type is formed. Based on these criteria, the joining (or fusion) of sperm and egg clearly produces a new cell type, the zygote or one-cell embryo. Cell fusion is a well studied and very rapid event, occurring in less than a second. Because the zygote arises from the fusion of two different cells, it contains all the components of both sperm and egg, and therefore this new cell has a unique molecular composition that is distinct from either gamete. Thus the zygote that comes into existence at the moment of sperm-egg fusion meets the first scientific criterion...