Excerpted from "Our Posthuman Future," by Francis Fukuyama, by permission from Farrar, Straus and Giroux.

In his novel "Brave New World," Aldous Huxley suggests that one source for a definition of what it means to be a human being is religion. "In Brave New World," religion has been abolished and Christianity is a distant memory. The Christian tradition maintains that man is created in God's image, which is the source of human dignity.

To use biotechnology to engage in what another Christian writer, C. S. Lewis, called the "abolition of man" is thus a violation of God's will. But I don't think that a careful reading of Huxley or Lewis leads to the conclusion that either writer believed religion to be the only grounds on which one could understand the meaning of being human. Both writers suggest that nature itself, and in particular human nature, has a special role in defining for us what is right and wrong, just and unjust, important and unimportant. So our final judgment on "what's wrong" with Huxley's brave new world stands or falls with our view of how important human nature is as a source of values.

In my book "Our Postmodern Future," I argue that Huxley was right, that the most significant threat posed by contemporary biotechnology is the possibility that it will alter human nature and thereby move us into a "posthuman" stage of history. This is important because human nature exists, is a meaningful concept, and has provided a stable continuity to our experience as a species. It is, conjointly with religion, what defines our most basic values. Human nature shapes and constrains the possible kinds of political regimes, so a technology powerful enough to reshape what we are will have possibly malign consequences for liberal democracy and the nature of politics itself.

It may be that, as in George Orwell's "1984," we will eventually find biotechnology's consequences are completely and surprisingly benign, and that we were wrong to lose sleep over it. It may be that the technology will in the end prove much less powerful than it seems today, or that people will be moderate and careful in their application of it. But one of the reasons I am not quite so sanguine is that biotechnology, in contrast to many other scientific advances, mixes obvious benefits with subtle harms in one seamless package.

Nuclear weapons and nuclear energy were perceived as dangerous from the start, and therefore were subject to strict regulation from the moment the Manhattan Project created the first atomic bomb in 1945. Observers like Bill Joy have worried about nanotechnology--that is, molecular self-replicating machines capable of reproducing out of control and destroying their creators.

But such threats are actually the easiest to deal with because they are so obvious. If you are likely to be killed by a machine you've created, you take measures to protect yourself. And so far we've had a reasonable record in keeping our machines under control.

There may be products of biotechnology that will be similarly obvious in the dangers they pose to mankind--for example, superbugs, new viruses, or genetically modified foods that produce toxic reactions. Like nuclear weapons or nanotechnology, these are in a way the easiest to deal with because once we have identified them as dangerous, we can treat them as a straightforward threat.

The more typical threats raised by biotechnology, on the other hand, are those captured so well by Huxley, and are summed up in the title of an article by novelist Tom Wolfe, "Sorry, but Your Soul Just Died." Medical technology offers us in many cases a devil's bargain: longer life, but with reduced mental capacity; freedom from depression, together with freedom from creativity or spirit; therapies that blur the line between what we can achieve on our own and what we achieve because of the various levels of chemicals in our brains.