For the last several decades, a strange liberation movement has grown within the developed world. Its crusaders aim much higher than civil rights campaigners, feminists, or gay-rights advocates. They want nothing less than to liberate the human race from its biological constraints. As “transhumanists” see it, humans must wrest their biological destiny from evolution’s blind process of random variation and adaptation and move to the next stage as a species.

It is tempting to dismiss transhumanists as some sort of odd cult, nothing more than science fiction taken too seriously: Witness their over-the-top Web sites and recent press releases (“Cyborg Thinkers to Address Humanity’s Future,” proclaims one). The plans of some transhumanists to freeze themselves cryogenically in hopes of being revived in a future age seem only to confirm the movement’s place on the intellectual fringe.

But is the fundamental tenet of transhumanism—that we will someday use biotechnology to make ourselves stronger, smarter, less prone to violence, and longer-lived—really so outlandish? Transhumanism of a sort is implicit in much of the research agenda of contemporary biomedicine. The new procedures and technologies emerging from research laboratories and hospitals—whether mood-altering drugs, substances to boost muscle mass or selectively erase memory, prenatal genetic screening, or gene therapy—can as easily be used to “enhance” the species as to ease or ameliorate illness.

Although the rapid advances in biotechnology often leave us vaguely uncomfortable, the intellectual or moral threat they represent is not always easy to identify. The human race, after all, is a pretty sorry mess, with our stubborn diseases, physical limitations, and short lives. Throw in humanity’s jealousies, violence, ...