According to [Ramsey's] theory, what is essential to being human, what each human being must retain if he is to continue to exist, is the possession of a functioning (actually or potentially) heart, lung, or brain. It is only when a human being possesses none of these that he dies and goes out of existence; and the fetus comes into humanity, so to speak, when he acquires one of these.I appreciate Brody's opposition to abortion past the three-month stage. And I think he masterfully takes down arguments from bodily rights in his essay. Brody's essay is interesting to say the least, but has a number of fatal flaws.
First, Brody confuses "essential for humanity" with "essential for life." In Brody's essay, he doesn't really seem too concerned about the distinction between biological human life and personhood, so it's difficult to really understand what his argument is. I'm going to assume he's referring to personhood, or at least the personal identity of the individual, since he likely understands that biological life begins at fertilization (which is a fact beyond dispute).
The problem is that while a functioning brain, heart, and lungs are all necessary for an individual's continued life, it does not follow that they are essential to the essence of who they are as a human person. After all, people who obtain heart or lung transplants do not cease to be themselves during the operation. There is a more fundamental essence to a human person, which grounds their capacities and their properties. The reason that human beings develop a rational brain is because they have a rational nature. It is this rational nature that makes them who they are, not a functioning brain, heart, and lungs. They simply need those organs to continue living.
Another flaw in Brody's reasoning is that he mentions that the organ must be there, even if it doesn't function, because it's the potentiality for functioning that grounds the humanness of the unborn human entity. But the problem here is that even before the lungs develop, the unborn human being has the active potentiality to develop functioning lungs. Why is it the lungs must be there, even if only to function potentially, but the potentiality for functioning lungs does not ground the entity's humanity even if they haven't developed yet?
So Brody's and Ramsey's account of human essentialism is flawed. The human organism is a person from fertilization because of the fundamental nature that grounds their development and their capacities. Arguing that certain properties must be present for the human to be valuable is still a form of functionalism, albeit one that includes more human beings in the moral community than that offered by Singer or Tooley. However, a proper pro-life account of human personhood establishes that personhood from fertilization because you are a person based on the kind of thing you are, not the kinds of things you can do.
 From The Problem of Abortion, 3rd Ed., ed. Susan Dwyer and Joel Feinberg, Wadsworth Publishing Company, Belmont, CA, 1997, pp. 88-97. The essay was taken from his book Abortion and the Sanctity of Human Life, and another essay, "Fetal Humanity and Essentialism" in Philosophy and Sex, ed. Robert Baker and Frederick Elliston.