Why is fertilization a poor marker for the start of a person's life?
Fertilization is the start of a new cell line with a specific pattern of DNA. It's widely known that fertilization occurs when two gamete cells, a sperm and an egg, merge their haploid DNA to become a new diploid cell called a zygote. Less widely known is that no one had ever laid eyes on a human egg until the early 19th century, and no one had ever witnessed the fusion of human egg and sperm until the late 19th century (learn more here).
To be fair and just, a marker for the start of a person's life must also make sense in the context of death. Fertilization as a marker for the start of personhood is strikingly problematic because we are not "unfertilized" when we die.
Some people argue that the result of successful fertilization—a zygote—is alive and has the genetic material of a complete person, therefore it is a person. This argument, that a zygote is a person because it has the DNA of a complete person, is not reasonable. To see why, consider the body you'll leave behind after you die. After your death, cells within the body you left behind can live for days. Some cells in your body can live for more than two weeks after you're gone. Those cells eventually die when they run out of the energy—called ATP—that is required for their cellular metabolism. Their ATP levels diminish because are you are no longer there to control that body, to eat and breathe to provide those cells with the nutrients necessary for them to produce ATP.
This phenomenon of cellular life persisting in a body after death is what makes life-saving organ donation possible. A dead heart is not going to save the life of someone who needs a heart transplant. Someone who needs a heart transplant needs a living, functional heart to replace their broken one. If you die and become an organ donor, those donated cells with your DNA will live for much, much longer than two weeks after your passing.
Signing up to be an organ donor after death is an incredibly precious gift to give. Your post-mortem donation of living organs, tissues, and cells gives other people in need an extended lease on life. Those donations also give doctors the opportunity to ethically develop new life-saving surgical techniques, as well as give scientists the opportunity to ethically develop new life-saving medicines. After your death, other people—namely doctors, nurses, and scientists—keep your donated organs, tissues, and cells living for a long time by simply providing those cells with the nutrients necessary for their cellular metabolism. Additionally, if you are a biological match for someone in need of an organ transplant, then after your donated organs are transplanted, cells with your complete DNA, that used to work to support your life, will now live, grow, work, and reproduce inside a different body to support someone else's life long after you are dead. Because cells with your DNA can live without you, the argument that “the result of successful fertilization—a zygote—is alive and has the genetic material of a complete person, therefore it is a person” must be false.
Before fertilization, an unfertilized egg is a living thing. Mammalian eggs in particular are alive because they are capable of metabolism and other wise meet the seven criteria for being biologically alive. After fertilization, a fertilized egg continues to be a living thing. What changes is that it gains complete set of DNA. However, because a living cell with a complete set of DNA can exist without being part of whole person who has the same DNA, there is no reason to believe fertilization is "the moment" that a new person comes into existence.
Furthermore, people do not wait to declare a person dead until the very last cell with that person's specific pattern of DNA dies (that would take a loooong time, especially if they were an organ donor and had cells living on within someone else's body. That lengthy wait also means dealing with a very smelly, leaky corpse). If we are not waiting until the very last cell with a specific pattern of DNA dies to declare a person dead, then why would we do the equivalent at the start of a person's life—declaring a new person alive when the very first cell with a new specific pattern of DNA is formed?
The American College of Obstetricians and Gynecologists notes that between one-third and one-half of all embryos naturally fail to implant in the uterus and thus never develop into new human people. Of embryos that do successfully implant in a uterine wall, one-third naturally fail to develop to term.
Based on these statistics, if someone were to believe that a person's life begins at fertilization, then they'd also have to believe that two-thirds of all people on earth naturally die before they are born. And they'd have to believe that the average life expectancy of an American is not 78 years but 26 years.
Overall, fertilization is not an appropriate marker for the start of personhood because it creates a double standard where the very first cells with a new complete set of DNA are given a right to life but the very last cells with that same complete set of DNA are not. Moreover, the consequences of that double standard are negatively profound, launching us into a dark alternate reality where the natural fate of two-thirds of all people on earth is to die before ever experiencing a breath of fresh air.
There is one last concern to address regarding the validity of fertilization as a marker for the start of personhood: underlying the argument that a person's life begins at fertilization is a notion that "a unique set of DNA is equivalent to a unique person's life." This notion is untrue, and nature/God's proof that this notion is untrue is the existence of identical twins. Identical twins are two people whose cells have the exact same pattern of DNA. If two different people can have the same set DNA, then the notion that "a unique set of DNA is equivalent to a unique person's life" must be invalid, which ineffect further invalidates the logic supporting fertilization as a marker for personhood on top of it's double standard problem.
Because identical twins are two individual people, even though their bodies have the same DNA, we must conclude that DNA alone is insufficient as a marker for categorizing personhood.
Commentary from an identical twin:
"My identical twin and I are very different people, even though we look alike and have the same DNA. I like weightlifting, and my twin likes yoga. I like the color purple, and my twin likes the color red. I also like protein smoothies, and my twin can't stand them. I don't buy into the idea that "a unique set of DNA is equivalent to a unique person's life." I can't believe that. If I believed that, then I'd have to believe I'm only half a person, that I only have half a soul. I'm a whole person, thank you very much. I have my own soul, separate from my twin's. We are each different people, with minds of our own. DNA to a body is like a blueprint is to a building. Insctructions, but not the building itself. There is much more to a building than its blueprint, and there is much more to a person than DNA."