As many astute observers have pointed out, controversial new ideas are assimilated in three stages. First they're false and pernicious, then they're true but trivial, and finally they're what everyone claims to have believed all along. I see nothing to disprove this time-tested formula in the case of Francis Fukuyama's thesis about the "end of history." According to Fukuyama, the evolution of social structure has come to a natural terminus in the combination of free markets and liberal democracy. Though once we scoffed, I'm sure that now, as long as everyone else is willing to accept a few modest qualifications -- markets require vigilant and impartial regulation, periodic free elections are a necessary but far from sufficient condition of robust democracy, unequal distribution of economic power can and usually does translate into unequal distribution of political power, present-day levels of solidarity and selfishness are not eternally fixed -- my fellow democratic socialists will join me in graciously acknowledging that Fukuyama's thesis is just what we've always had in mind, or close enough.

Qualifications, however, are for earthbound thinkers; Fukuyama has moved on. In Our Posthuman Future, he has looked past the end of history and descried the end of mankind. Markets and democracy may be the last word for the human nature of 2002, but what if human nature changes?

Until recently, this wouldn't have seemed to most people a compelling, or even an intelligible, question. But the last several decades have seen astonishing progress in the life sciences. Evolutionary psychology has supplemented or superseded Plato, Freud, and Skinner; molecular biology, meanwhile, seems about to usurp the prerogatives of an even more august personage, sometimes known as the Creator. The technology of Aldous Huxley's Brave New World is no longer fantastic or even remote. Of course no one currently wants to end up there, or anywhere similar. But could it happen? And if so, how do we prevent it?

The first part of Our Posthuman Future is an informative survey of contemporary bioscience and its political implications. It's no longer much disputed, Fukuyama writes, that heredity is involved to a nontrivial extent in determining intelligence, criminal behavior, and secondary sexual differences. (Homosexuality remains an open question.) It's equally certain that environmental factors such as diet, education, peers, parents, and social conventions also play an important role. A few diseases, such as cystic fibrosis, can be traced to a single gene, but "higher-level behaviors ... are likely to have far more complex genetic roots, being the product of multiple genes that interact both with each other and with the environment." The findings of the Human Genome Project -- that we have fewer genes than previously thought -- make this even more likely. Still, "it seems almost inevitable that we will know much more about genetic causation even if we never fully understand how behavior is formed."

Neuropharmacology is one possible Huxleyan technology. Though less than 15 years old, serotonin reuptake inhibitors (Prozac, Zoloft, Paxil) have been administered as antidepressants to a sizeable fraction of the population. So has Ritalin, a central nervous system stimulant that increases concentration and curbs hyperactivity in children. These are valuable drugs, but worrisome ones, too. Conservatives worry that opportunities for self-discipline and character development may be lost. Liberals worry that real-world causes of unhappiness and restlessness may be obscured.

Like the relief of emotional pain, the prolongation of life can be too dearly bought. Throughout the developed world, as life expectancy rises and birth rates fall, the median age is increasing sharply. Biomedical technology will probably accentuate this trend. Yet unless ways are found not only to postpone death but to extend the prime of life proportionately -- a much more difficult matter -- disturbing changes are likely. Alzheimer's and other incapacitating diseases could become epidemic. Pressures on younger workers and family members could become intolerable. The rhythms of generational succession could be disrupted.

Genetic engineering is the most tempting and troubling of biomedical prospects. Genetic screening of embryos is on the horizon; indeed it's already in use for single-gene diseases. First attempts at human cloning are just over the horizon. But germ-line modification, the alteration of DNA in the fertilized egg, is the real Pandora's box. Here the line between therapy and enhancement is blurred; the way is open to "designer babies" whose sex, IQ, physique, and other traits are predetermined.

Germ-line engineering may well be impossible; the scientific jury is still out. Like computer-based artificial intelligence, it may turn out to be a will-o'-the-wisp, at least as currently conceived. Gene/environment interaction has scarcely begun to be understood. The emergence of complexity -- in this case, of phenomes from genomes -- may be beyond the grasp of any information-processing technology, present or future. There may be no way to find out whether a particular genetic intervention has unanticipated lethal consequences, except by performing experiments that violate the right of informed consent. Fukuyama is rightly cautious in accepting claims about technical feasibility; in my (admittedly less well-informed) opinion, even stronger skepticism is warranted.

Still, it's only prudent to give some thought to what we should do if genetic engineering turns out to be feasible. The middle of Our Posthuman Future is an effort to lay ethical foundations for policy judgments. Why shouldn't we "seize the power," as one geneticist has put it, to "control what has been left to chance in the past"?

The simplest answer is religious: Genetic engineering would set God's will at naught. But Fukuyama respectfully sidesteps religion. Another possible answer is that genetic manipulation unfairly (because irreversibly) "embeds one generation's social preferences" -- about temperament, personality, looks -- "in the next." (Though couldn't such changes be reversed in the generation after next?) Another reason has to do with class and equality: Won't genetic engineering increase inequality if only the rich can afford it? (Perhaps, though eventually, Fukuyama contends, political pressure would force democratic governments to subsidize it for all.) The most compelling reason, I think, is ecological: The human organism is a miniature ecosystem, and "ecosystems are interconnected wholes whose complexity we frequently don't understand; building a dam or introducing a plant monoculture into an area disrupts unseen relationships and destroys the system's balance in totally unanticipated ways." (One hopes Fukuyama's admirers on The Wall Street Journal editorial page heard that.)

None of these reasons fully satisfies Fukuyama, however. He must have philosophical underpinnings. So we get an elaborate argument, over several chapters, to the effect that yes, there is a human nature, extreme social constructionists notwithstanding. It is the only plausible foundation for human rights and political equality and, therefore, we had better not tamper with it. Ultimately, "we want to protect the full range of our complex, evolved natures" -- our souls -- "against attempts at self-modification," attempts that may simplify us, blunting our suffering or elevating our IQ but robbing us of emotional depths or imaginative heights.

After this philosophical flight, the book descends to policy. Though skeptical of most governmental regulation, especially at the international level, Fukuyama recognizes that the biotechnology industry has "too many commercial interests chasing too much money for self-regulation to continue to work." Actually, "continue" is wrong: Self-regulation has not worked. Monsanto, Aventis, Eli Lilly, and many other pharmaceutical and agro-technical giants have acted unwisely or unethically, then bought, bullied, bamboozled, or otherwise successfully neutralized critics and government regulators. But while Fukuyama seems unaware of how frequently industry self-discipline has failed in similar cases, he is at least ready to acknowledge that it will likely fail in the case of human biotechnology.

What kind of regulatory regime, then, should exist and what issues should it address? The FDA, EPA, USDA, and NIH can't do the job, Fukuyama argues persuasively. Human biotechnology is outside their mandate and beyond their competence. A new agency with enforcement powers is needed, perhaps modeled on Britain's Human Fertilization and Embryology Authority. As other states follow suit, varying national policies can be gradually harmonized.

America's national policy, Fukuyama suggests, should roughly be: yes to therapy, no (or at best maybe) to enhancement. Stem-cell research and preimplantation screening would be presumptively legitimate when aimed at inherited diseases but doubtful when aimed at selecting traits or improving performance. Germ-line engineering, with its drastic and open-ended consequences, would be very doubtful. Human-reproductive cloning would be banned outright. These are wise suggestions. As Fukuyama observes: "The original purpose of medicine is, after all, to heal the sick, not to turn healthy people into gods."

Actually, if that observation were applied to politics as well, the question of genetic engineering might appear still more dubious. It might seem that productive as well as medical resources are better directed to meeting basic human needs than to expanding the opportunities of the comparatively healthy and wealthy. The mere possibility of designer babies for some while others in the same society, or other societies, can't afford proper care for their babies might seem ... unnatural.

"Our greatest social philosopher," as his publicists call him, is not, alas, our most acute social critic. One good reason, not mentioned in Our Posthuman Future, for skepticism about biotechnological interventions, is the "economy of means" principle. The right-wing version of this principle: Just say no. The left-wing version: Expensive, complicated, and dangerous solutions are usually more profitable but less effective. Safe, simple, effective, and relatively cheap ways to increase intelligence and longevity in the American population include mounting the equivalent of the antismoking educational campaign against the high levels of fat, sugar, and salt in our national diet; encouraging physical activity, for example by forcing ourselves, through gasoline taxes and urban rezoning, to walk more, if only to get to public transportation; implementing national health insurance, or at least universal free health care for children under five; and subsidizing public radio and television. Reducing economic insecurity and environmental degradation also offers virtually unlimited opportunities for enhancing our beleaguered selves. But these policies won't make investors a lot of money -- on the contrary -- while pharmaceuticals and genomics will.

The best reason of all not to press forward into the posthuman future also goes unmentioned in this book. It's that the enormous resources required could be put to much better use helping the many people who do not now enjoy a human present. According to the United Nations, roughly three billion people do not have access to safe sewers, 1.5 billion lack clean water, 1.25 billion don't have minimally adequate housing, one billion have no health care, and a half-billion don't get the minimum daily caloric requirement. An unknown but very large number are illiterate. Among the poorest fifth of the world's population, between 30,000 and 40,000 children under five die each day from the effects of malnutrition and infection; another 180 million barely hang on. The UN estimates that all these needs could be met, at a basic level, for a yearly expenditure equal to 10 percent of the recently proposed U.S. military budget -- or slightly less than Americans and Europeans spend annually on pet food and ice cream.

We -- the fortunate "we" of Fukuyama's title -- already inhabit a different world than these hapless billions. Their "human dignity" is moot; their "complex, evolved natures" don't have much scope for expression. It's understandable that the biotech industry and the political class take no notice of these nonvoting, nonconsuming, noninvesting multitudes. But shouldn't our greatest social philosopher have thought to put in a word for them?