Mad Scientist: I am going to commit the world’s earliest murder to get into the Evil Genius Book of World Records!
Mad Scientist: I hold here a petri dish containing a single human egg, an eye dropper containing a single capacitated human spermatozoon, and a bottle of ethanol. I’ve been reading this pro life website that says that life begins at the moment of conception, because it is the moment when a new, genetically distinct human being is formed, and there is really no other place during pregnancy to draw the line. I will now combine the egg and the sperm in the petri dish. Then I will zap the cell with this ray gun, destroying the cell and committing the world’s earliest murder!
Biologist: Wait! The sperm has only made contact with the outer coating of the egg. If there was more than one sperm they would now be racing to dissolve the coating. Obviously the moment of conception cannot have occurred yet!
Mad Scientist: Well how long does that take?
Biologist: About 20 minutes.
Mad Scientist: I’m going for a coffee break.
Biologist: Wait! The sperm is physically inside the egg, but its DNA is still separate. Nothing about the DNA has changed. And the female DNA in the egg is still diploid. The half that will combine with the sperm hasn’t even separated yet. Errors in that process could still happen to determine whether the embryo has a chromosomal abnormality like Down’s Syndrome. How can that be the moment of conception?
Mad Scientist: Sigh. What happens now?
Biologist: The female DNA in the egg finishes splitting, and the DNA that will form the embryo becomes the female pronucleus. Then the DNA from the sperm will turn into the male pronucleus.
Mad Scientist: OK, but they are still separate. When will they meet?
Biologist: 12 to 24 hours.
Mad Scientist: Wake me up when this moment is over.
Biologist: The envelopes of the two pronuclei dissolve…
Mad Scientist: Yes…
Mad Scientist: OK…
Mad Scientist: When did the DNA combine?
Mad Scientist: WHAT? Why didn’t the DNA combine before it duplicated?
Biologist: Because it would just have to separate again in order to duplicate. Most mammals, including humans, skip that step, and don’t have diploid DNA until after the first cell division.
Mad Scientist: But obviously the Moment of Conception has to occur before there are two cells! The prolife website says that the process of development is controlled entirely by the embryo’s DNA. How can it be controlling development if it hasn’t even combined yet?
Biologist: It’s not. They are correct that no input from the mother is required for this process, which should be obvious because you are using a petri dish. But the first 2-3 divisions were preprogrammed by the mother’s DNA when the egg was formed. They are a chemical reaction that occurs independent of the DNA. They can be induced chemically without sperm at all.
Mad Scientist: But when was the moment of conception? It seemed like a continuous process!
Biologist: Yes. That’s why embryology textbooks refer to the “process of conception” not the “moment of conception.”