My case will be made up of a two-pronged argument. One scientific and one philosophical.
First is the scientific prong.
the eight characteristics of life
It is a scientific fact that life begins at conception. Life is defined in biology as anything that fulfills eight requirements. These requirements are as follows
1. displays cellular organization. From the moment of conception, a human child is made of at least one cell. This requirement is fulfilled.
2. Maintains Homeostasis. Homeostasis means that a living thing maintains a constant internal environment that keeps it alive. a human embryo performs operations such as waste removal, transforming energy, and taking in nutrients from outside of the cell through constructs such as the sodium-potassium pump. This requirement is fulfilled.
3. grows and develops. It is an obvious truism that a human embryo grows and develops and begins to do so from the moment of conception. This requirement is fulfilled.
4. Displays metabolism. Metabolism is converting fuel (like food) into energy. From the moment of conception, a human child displays metabolism. This requirement is fulfilled.
5. Displays heredity. From the moment of conception, the first cell (the zygote) has the capacity to divide into more cells and pass a copy of its DNA onto those other cells. This condition is fulfilled.
6. Responds to the environment. The zygote will perform tasks such as pulling nutrients into itself through active transport and maintaining an internal environment responding to any change therein. This condition is fulfilled.
7. Adaptation through evolution. As a member of the human species, the zygote is subject to prior evolution. And, if not killed, will contribute to evolution through reproduction. this requirement is fulfilled.
8. Can reproduce. It is an obvious truism that the zygote can reproduce. dividing into more cells in order to develop into a fully formed baby.
as we can see, from the moment of conception a zygote fulfills every single one of the characteristics of life and is therefore alive.
the humanity of the zygote
The next question that needs to be answered is whether or not the zygote is indeed human. When the two haploid (containing 23 chromosomes) cells (sperm and egg cells) come together during fertilization the diploid (containing 46 chromosomes) is formed. This diploid zygote contains a complete copy of the human genome. Thus making the zygote human. Note that the zygote's copy of the human genome is genetically distinct from both the mother and the father and is the same set of DNA that they will carry for their entire lives.
I have therefore proven that a human child, from the moment of conception, is a living human being.
My body my choice
One of the more infamous arguments from the pro-choice crowd is that a fetus is a part of a woman's body and can, therefore, be killed at will. This argument displays basic scientific illiteracy. Only two types of things can be biological parts of your body. 1. Things made of cells that contain your DNA, or 2. Things made of cells that contain NO DNA such as hair for example. Nothing can be a part of your body that contains DNA distinct from your own. a baby from the moment of conception, as I've already proven, is genetically distinct from both the mother and the father this argument can be shown to be scientifically invalid.
the philosophical prong
the personhood of the child
After being forced to retreat on the scientific front, many pro-choicers will attempt to hide behind the following statement: "Well, the fetus may be a human being, but they're not a human person."
This is a purely philosophical contention that needs to be defined further. This statement raises the question: When is personhood conferred? Birth? Is personhood then based on geographical location? Nothing changed between those few inches of movement through the birth canal. Why then has personhood been conferred? Since most people find it unpalatable to argue in favor of abortion up to point of birth, a more common line is drawn at a heartbeat. The problem with this is that this trait can be removed. Not every adult has a heartbeat of their own. Can I stab Granny because she has a pacemaker? I sure hope not. Is the line drawn at sapience? What does that say about those born with cognitive defects and no sapience to speak of? can I stab them? I sure hope not. Or what about people who fall into a coma and have their sapience removed. We aren't allowed to kill them. Why is a fetus different? I could go on and on. The point is that wherever you draw the line (except for at conception or birth) you create an inconsistency wherein we should be able to kill adults who don't meet this criterion for personhood. If you tie moral value to a heartbeat then I should be able to stab pacemaker granny because she has no intrinsic value.
the rights of man
As Thomas Jefferson famously and eloquently put it "We hold these truths to be self-evident: that all men are created equal. That they are endowed by their Creator with certain inalienable rights, that among these are Life, Liberty, and the Pursuit of Happiness." The Enlightenment gave us the idea that each individual has inherent worth and is therefore entitled to certain rights. The most critical and primal right that all humans are entitled to is that of life. To strip away the right of the human being who is growing in their mother's womb, to violently dismember them and snuff out their life constitutes a gross violation of these intrinsic, inalienable rights. These rights exist only if they are inviolable. If they don't exist for a fetus, then they don't exist. Period.
To wrap up this first argument I would like to say that the question of whether or not abortion should be legal is a very simple one that all too often becomes overcomplicated in political discourse. The dichotomy is self-evident, inherent in the philosophy of our society, and overwhelmingly simple. Either the fetus is human. Or, it isn't. If it's not, then removing it up to the point of birth should be no more morally problematic than extracting a tooth or a tumor. But if it is, then killing that child constitutes a heinous act of evil that infringes upon the most fundamental of the rights of man.
Please vote pro.
Campbell Biology: 11th edition.