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?)

Shortly thereafter, granules in the egg’s cortex release enzymes that catalyze additional, long-lasting changes in the zona, achieving a more long-lasting block to other sperm. (Now?) Pseudopod-like extensions of the egg’s interior proceed to transport the sperm into the egg. (Now?)

If you’ve been waiting all this time for the genetic fusing of sperm and egg, note that it doesn’t happen immediately, at least not in mammals such as ourselves.

Rather, the nuclear envelopes around sperm and egg remain fundamentally distinct through the “fertilized” egg’s first mitotic division. Only at this point, with two “daughter” cells already in existence, do the parental chromosomes unite, forming two nuclei. But even at this point, the parental genes remain identifiable and distinct, as either paternally or maternally derived. Paternal and maternal genes thus remain separate for at least 24 hours after sperm successfully breaches those follicle cells, and it takes an additional day or so before their combined influence directs cell function.

There is, to repeat, no cymbal-crashing “moment” of fertilization. Natura non facit saltum.

Although the problem of ensoulment is especially dramatic, comparable difficulties arise if we substitute “mind” for “soul,” because the former unquestionably derives from brain activity and the brain also does not arrive in a sudden flash of incandescence, to be suddenly plugged in with its complex operating system ready to start humming. The two “daughter” cells have no neurons, and certainly no brain. Neither does a subsequent four-cell, eight-cell, or 128-cell descendant. Somewhere along the line, however, between egg and baby, brain cells aggregate and start whispering electrochemically to each other, whereupon a mind gradually coalesces.

Moreover, there is often a reciprocal diminution of self-hood and mental competence at the other end of life, as the Terri Schiavo debacle so painfully revealed.

The moral of all this: Natural boundaries won’t ease our moral quandaries. When we most want them, they aren’t there. As bioethicist Ronald Green has pointed out, we had better give up trying to find such boundaries, and work instead to choose those we can live with.