Soul, Science and Abortion: Some thoughts

Recently, I had an idiot try to make the 'scientific' argument that a fertilized egg should have the same rights as a human being. I think the person was just trying to wrap his ideological/religious bullshit in a wrapper of prestige and in that way try to convince people to ban abortions in all cases. Anyway, I wrote a elaborate response, and I reproduce that here.

To equate a fertilized egg to a human being is the same as equating a death corps to a human being. They have exactly the same genetic material. So, what is the key difference between a living person and a death person? A theist would say a soul, but we were trying to find a scientific argument to equate a fertilized egg with a human. So, we are limited to substance monism to explain the mind. Three versions exist: behaviorism, functionalism, and mind-brain identity theory. Behaviorism attempts to explain mental states in terms of behavior. Functionalism holds that mental states are defined by their functional role, by the effect that they have on us. Mind-brain identity theory holds that the mind and the brain are one and the same thing, and that the mind arises from the interaction of nerve cells in the brain. Once enough interactions between the nerve cells stop, you are dead.
A kidney taken out of a body for transplant does not have a life on its own, nor is the soul of the donor split and partially merged with the recipient. So, where is the soul/mind located? That location would be the brain. And as I indicated above, once enough nerve cells stop contributing, you are dead. The flip side of this argument is that you cannot have a mind until you have enough interacting nerve cells in the brain to generate a mind. For nerve cells to be able to interact, they have to form synapses. Synaptogenesis happens relative late during development, somewhere after week 20. This implies that scientifically speaking, you do not have a soul/mind before that happens, because the required nerve interactions are absent.