A variation of the first defense for elective abortion builds upon the same false assumption, that the embryo/fetus is not a human. The medical fact that a unique living human being is created at the point of conception is not up for debate. This will focus more on the philosophical aspect of the argument.
Much like skin cells which also contain our dna, the developing embryo is not actually human but merely a mass of cells. We do not consider killing skin cells to be murder, why should removal of a developing embryo be any different?
This argument makes the logical fallacy of composition. It wrongly implies that, that which is true of the parts is true of the whole. It is presumed that because a skin cell shares a common feature (dna) with the whole body (dna) that one is just as valuable — or non-valuable — as the other. Since we can destroy a skin cell with moral benignity, we can therefore destroy an entire human embryo with the same moral benignity. But a skin cell is merely one piece of a greater complete thing that does not depend on a total amount of parts for its value.
A car, for example, is not a car until a sufficient amount of parts are assembled and operational. Its value is dependant on the total sum of its parts. A car with no engine, or no wheels is not a car, and is valuable only in so far as the individual parts have value. A human is not assembled piecemeal like a car. The embryo is a self-contained organism which, left unmolested will develop on its own through all the stages of life: fetus, infant, toddler, teenager, and adult.
Because a human being is not merely a sum of its parts, it maintains its identity through development and maturity. Through each stage of development, a human doesn’t become more human; it maintains the kind of thing it is as it becomes more mature. As such, when someone loses a limb, or has an organ or tooth removed, they remain who they are — because the parts of the human being are not essential to their identity.
What’s more, everyone is technically ‘just a mass of cells’. A newborn, a teenager, even the defender of abortion. What makes the statue of David valuable, is not strictly the bare materials. It is not just a chiseled slab of marble. I’m sure Michelangelo discarded the shards of marble as he fashioned his masterpiece. The shards are of the same material as the finished statue. But does it then follow that because the shards can be discarded without moral concern, that the statue can also be destroyed because it is of the same materials as the shards? The statue of David is more than its physical parts, it is more than a piece of marble. It is an end in itself. In the same way, a human being is more than its components.
This is why the one offering the ‘skin cell’ defense of abortion misses the point. There is a difference between the parts and the whole even though they share a common factor, dna.