Recent scientific advances in research involving stem cells derived from human embryos have sparked considerable ethical debate concerning the moral status of the human embryo. Scientists believe that research using stem cells might eventually help us cure diseases such as Alzheimer's, Parkinson's, juvenile diabetes, spinal cord injury, and diseases of main organs. Stem cells apparently have the capacity to transform themselves into specific organ tissues. In the future, researchers may be able to use stem cells to develop, for example, liver cells that could cure someone with a malfunctioning liver. Because human embryonic stem cells have the potential to develop into all of the tissues in the human body, stem cells from human embryos are believed to have a greater potential to produce these results than stem cells from adult cells, umbilical cords, or the human placenta.
For research, embryonic stem cells are isolated from human blastocysts--embryos at around day four in the fetal development. In the course of obtaining stem cells from a living human blastocyst, the blastocyst is destroyed. For that reason, the main ethical challenge associated with stem cell research has to do with whether blastocysts have any moral status. There are two main arguments for the view that blastocysts have moral status. One relies on the claim that, if the embryo under favorable circumstances would be identical to the human being as it would exist after birth, then it already has the same moral status as a human being after birth. The other relies on the claim that the embryo is entitled to protection because from the moment of conception it is a potential human being, even if it would not be identical to the future human being under favorable circumstances. The second of these arguments will not concern me here. (1) My focus is on the argument that the embryo has moral status because under fav orable circumstances it would be identical to the human being that would exist after birth. I shall argue that the premise is false.
The most important argument against this claim, I believe, is the twinning argument. (2) The twinning argument, in its simplest form, is familiar: prior to gastrulation (the critical developmental step at which the embryo's three major germ layers, or tissue type divisions, form), the embryo is susceptible to twinning. Accordingly, it has the potential to develop into several human beings. Since the process of becoming a human being has not yet ended, the pre-gastrular embryo is not a human being.3 Unfortunately, the twinning argument in its simplest form is flawed. The premise that if an entity is potentially two, it cannot be one is false. As the American Civil War teaches us, there are cases where identity is inherited even though an entity is susceptible to twinning. The United States in the period immediately prior to the Civil War was actually one but potentially two. (4) But, I will now argue, even though there are clear cases of individuals susceptible to twinning that persist over time, there are emp irical grounds for denying that an embryo susceptible to twinning can be transtemporally identical to a future human being.
To make a case for the proposal that an embryo susceptible to twinning cannot be identical to a future human being, I shall first consider the possible scenarios under which twinning might occur. Second, for each possible scenario I shall determine whether there are empirical grounds for thinking that human embryonic twinning occurs under that scenario. Finally for each possible scenario I shall determine whether it is conceivable that the pre-gastrular embryo under favorable circumstances is identical to the human being as it exists after birth. As we will see, according to the scenario under which embryonic twinning is known to occur, it is impossible that the pre-gastrular embryo is identical to a future human being. …