When are the souls handed out?
Natura non facit saltum: "Nature does not make leaps." This adage has been attributed -- along with the invention of calculus -- to both Isaac Newton and Gottfried Wilhelm Leibniz. Like many old saws, this one still has a few sharp teeth, some of which bite very close to home, especially when it comes to abortion and stem-cell research.
Yearn as we might for clear-cut, yes-no boundaries, nature only rarely obliges. There aren't many genuine leaps in the biological world, an observation that contributed greatly to Charles Darwin's insight about the gradual transformation of species. Nature's reluctance to facit saltum also offers little comfort to those opposed to abortion rights or to stem-cell studies.
Intertwined in both these issues is the question of the human soul, which presumably pops into existence at some point in the development of every human being. But when?
The answer is obvious for those who assume that into each life a leap must fall exactly once: the instant of conception, the magical moment of "ensoulment." By this logic, the beneficiary of such a leap is suddenly rendered human. Therefore, using embryos in stem-cell research, or allowing women to get abortions, must be opposed because even the tiniest human embryo, once conceived, is "ensouled."
Here's the problem: There is no moment of conception. In what follows, try to pick out precisely when a person becomes personified.
Aparticular egg and sperm, each destined to contribute one-half the genome of a future human being, is produced via complex processes of oogenesis and spermatogenesis, respectively. (Is that moment now?) The fated sperm cell migrates through a layer of follicle cells before reaching the egg's "extracellular matrix," known as the zona pellucida. The latter consists of three different glycoproteins, one of which acts as a sperm receptor and binds to its complement on the sperm's head. (Maybe now?)
This induces a vesicle at the tip of the sperm, the acrosome, to spill its contents of enzymes, which enable the sperm to penetrate the zona and bump up snugly against the egg's plasma membrane. (Or now?) A protein in the sperm's membrane then binds to and fuses with the egg membrane. (Now?) This in turn triggers depolarization of the latter, which prevents other sperm from entering. (Now?)
