When does life begin?
That has got to be one of the most important questions of the day. I posit that this occurs quite early.
The female egg and the male sperm are haploid cells belonging to each person. They have a function and belong to the organism, yet they are not of themselves a unique organism. Their DNA, although haploid, is identical with that of the person. Their function is to unite: the egg works to chemically attract the sperm, and the sperm works to get to the egg. “If fertilization is not accomplished, the oocyte typically ceases to be within twenty-four hours after ovulation; and sperm degenerate within two to five days.”
Once the first sperm enters the egg, the entire scenario changes. Now, you have a new entity with its unique genetic code. How do we know this? Because it is in some way self-aware of the fact that it is now different, and reacts chemically for the protection of the new organism. This is known as the egg cortical reaction.
When the sperm fuses with the egg plasma membrane, it causes a local increase in cytosolic Ca2+, which spreads through the cell in a wave… There is evidence that the Ca2+ wave or oscillations are induced by a protein that is introduced into the egg by the sperm, but the nature of the protein is unknown. The Ca2+ wave or oscillations activate the egg to begin development [Development of what? Of the new organism], and they initiate the cortical reaction, in which the cortical granules release their contents by exocytosis… The contents of the cortical granules include various enzymes that are released by the cortical reaction and change the structure of the zona pellucida. The altered zona becomes “hardened,” so that sperm no longer bind to it, and it therefore provides a block to polyspermy.
“…there now appears to be a distinct organism directing its own processes of growth and development… The [egg cortical reaction] especially seems characteristic of a new organism, whose existence depends upon a structural barrier to outside forces, rather than of a gamete cell, whose existence is fundamentally oriented toward uniting with another gamete…”
 Robert P. George and Christopher Tollefsen, Embryo: A Defense of Human Life (New York: Doubleday, 2008), 36.
 George and Tollefsen, Embryo, 38-39.