As a cradle Catholic, I took it as an article of faith to mark conception as the beginning of a human life. But the fact that there was public debate made me assume that science was still investigating the issue of when a new human organism comes into existence.
So when I was a university senior studying animal development, I was astonished that the consensus in biology is so clear and concrete. For any sexually reproducing animal, from fruit flies to humans, the moment in which an egg and sperm merge is developmentally and evolutionarily the point at which a new organism is considered to come into existence. There is no scientific debate on the subject.
Embryos and self-interest
Self-interest is a key concept in this discussion. Every cell acts in its own self-interest, and in a multicellular organism such as a human, that means every cell acts in the interest of the body as a whole. (This concept is taken to its extreme conclusion in Richard Dawkins' book The Selfish Gene.)
Immediately upon fusion of the egg and sperm, a human zygote begins acting in its own self-interest with an event called the cortical reaction, which stops additional sperm from penetrating the egg (which would be lethal). Shortly afterward, it begins secreting hormones to cause its mother's body to allow it to implant in the uterus and gestate it. Every event associated with the embryo has as its aim the continued survival of the embryo, not necessarily the well-being of the mother.
Thus the question of when human life begins is not a semantic one — it is a scientific one, and one that has a known answer.
Science and politics
This is why I felt so much disgust when Congresswoman Diana DeGette falsely declared that science supports the position that it is ethical to kill embryos. Now, in an ironic turn-about, another Congressional Democrat, House Speaker Nancy Pelosi, has made remarks that are in plain contradiction to science.
According to a statement reported on several blogs to have been e-mailed to Catholic bishops, Pelosi justifies her position that a just-fertilized zygote is not a human being on an alleged statement by St. Augustine: "[T]here cannot yet be said to be a live soul in a body that lacks sensation…" (On Exodus 21.22).
Other commentators, from bishops to bloggers, have done an outstanding job of refuting Pelosi's words on theological grounds. Her statement is also unsound on scientific grounds.
Irrespective of whether she is quoting St. Augustine in context (I suspect not), and of whether it accurately reflects Catholic moral teaching on the issue (it clearly does not), her statement blatantly disregards the insights we have from developmental biology.
St. Augustine's statement was an expression of the science of his time, and consists of little more than a definition: a thing that senses has a soul and a thing that does not sense does not have a soul. That view is derived from the classical philosophy of his day.
In modern science, there is no such thing as a natural soul (vitalism having been rejected long ago). The soul is a purely theological concept. While supernatural souls are part of Catholic belief, science is not capable of investigating them.
As for lacking sensation, a freshly fertilized zygote may not have a nervous system, but it does respond to its environment.
Pelosi's remarks on the issue of the beginning of life are ill-informed — not only from a Catholic point of view, but from a scientific one.
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Image: Human ovum before fertilization. Illustration from Gray's Anatomy.