Francis Fukuyama, in a class he teaches at Johns Hopkins, once declared history at an end. Now he takes on biotechnology.

That prospect should be worrying a lot more people, in the view of the political theorist Francis Fukuyama, because history's central question — that of what kind of society best suits human needs — has been settled only if human nature remains as it is, warts and all.

Dr. Fukuyama, now at the Washington campus of Johns Hopkins University, is known for his widely discussed book "The End of History and the Last Man," published in 1992, a few years after the fall of the Berlin Wall. In it he argued that with the demise of communism, liberal democracy had emerged without rival as a political system with universal appeal. The challengers of this tempting thesis included Samuel P. Huntington of Harvard, who argued that struggles between the world's major cultural groups would predominate in a post-Communist world.

Even in the wake of the Sept. 11 attacks, Dr. Fukuyama has yielded little ground to his critics. The only objection he acknowledges as serious is the argument that history cannot end without an end to science, or at least to science that alters human nature. In a new book, "Our Posthuman Future," he explores the ways in which biotechnology may change the human essence. Despite his title, Dr. Fukuyama has no taste for a rerun of history and believes a posthuman future is one to avoid.

The danger is the greater because those closest to the action — scientists and bioethicists — cannot in his view be trusted to raise the alarm. Scientists are interested in conquering nature while many bioethicists, Dr. Fukuyama contends, "have become nothing more than sophisticated and sophistic justifiers of whatever it is the scientific community wants to do." His views are not academic; he has an official voice on such matters as a member of the White House's Council on Bioethics.

Genetic engineering of the human germline — making permanent changes to the genes in the egg or sperm — would pose the most direct threat to human nature but other techniques bear watching, in his view. Mood changing drugs could change society if taken widely enough, and Dr. Fukuyama says he wonders whether Caesar or Napoleon would have felt the need to conquer Europe if either had been able to pop a Prozac tablet occasionally.

Major increases in human longevity could also be disruptive, he fears, because "life extension will wreak havoc with most existing age-graded hierarchies," postponing social change in countries with aging dictators and thwarting innovation in others.

But the most serious threat to the stability of human societies is genetic engineering that may alter, by design or inadvertence, the special balance of contrarieties of human nature.

Human nature, Dr. Fukuyama argues, "is fundamental to our notions of justice, morality and the good life."

By messing with the human genome in order to enhance intelligence or physique or other desirable qualities, biotechnology may cause us "to lose our humanity — that is, some essential quality that has always underpinned our sense of who we are and where we are going," he writes. Science has had many critics, but Dr. Fukuyama's warnings come from an unusual direction. h His father, Yoshio, a sociologist of religion, was an American of Japanese descent who escaped internment in World War II, unlike several other members of his family. Francis grew up in New York, not much exposed to Japanese culture, studying classics at Cornell and political science at Harvard.

He spent the first part of his career as a Sovietologist at RAND Corporation, the research group, and in between stints at RAND, he worked at the State Department.

It was in listening to a speech by Soviet President Mikhail Gorbachev that Dr. Fukuyama had the idea for his first book. Hegel, the 19th-century German philosopher, believed history would culminate in a constitutional state or, in modern terms, a liberal democracy, whereas Marx saw a communist state as the likely end point. Hearing Mr. Gorbachev declare surprisingly in a speech one day that the essence of socialism was competition, Dr. Fukuyama recalled in an interview, "I called up a friend and said if Gorbachev was saying that, this is the end of history," meaning that Hegel's prediction had triumphed over Marx's.

Dr. Fukuyama's only expectation of the book he then wrote was that "my political theorist friends would be vaguely amused." But "The End of History" was too powerful and hopeful a guide to the post-Communist world to be ignored.

He argued that history was not a meaningless cycle but had a direction imposed on it by the logic of modern science, a direction that "would seem to dictate a universal evolution in the direction of capitalism." Though the advanced industrialization made possible by science and technology does not necessarily lead to political liberty, Dr. Fukuyama wrote, the human desire for recognition, cited by Hegel as the driving force of history, is best satisfied in a liberal democracy.

As to his critics, he agrees that culture is important but does not consider the fault lines between the seven civilizations identified by Dr. Huntington as likely to be permanent. Dr. Fukuyama believes that liberal democracy, because of its affinity with human wants and desires, holds universal appeal. It evolved in Western Europe but is no more a necessarily European possession than is modern science.

"The Western modernizing package gives access to a standard of living everyone wants," he said in an interview. "Many don't want the whole package. They want a job, not necessarily Hollywood. But there is a certain logic to modernization — to have the TV you have to have certain institutions, including the rule of law."

Though religion and culture can impede modernization, Dr. Fukuyama sees no reason to suppose that the Islamic and other civilizations will not in time adopt their own versions of liberal democracy. "The basic structure of world politics continues to be the juggernaut of modernization as pioneered by the West," he says.

The optimism about science that imbues "The End of History" is strikingly absent from "Our Posthuman Future." Dr. Fukuyama notes scientists' responsible record of self-regulation, but he says he believes too many now have commercial ties to biotechnology companies for the same disinterested behavior to be counted on. And though he credited science and technology for giving history its forward direction for the last 500 years, he says he is much less certain that biotechnology will be handled with the same wisdom as previous innovations.

Dr. Fukuyama says he believes some things should be banned outright, like cloning people, which he regards as immoral in itself and as the opener for worse things, like enhancing human qualities by germline genetic engineering. Despite his membership in the Council on Bioethics, he doubts the ability of national commissions to address the problems raised by biotechnology, calling instead for legislation and `institutions with real enforcement powers."

Dr. Fukuyama plans to devote the next few years to studying how biotechnology should be regulated. He regards as inadequate the present system of oversight by the Food and Drug Administration and the National Institutes of Health but does not yet have a specific remedy: "All I know is that the current American setup is not adequate for the goals I would like to see them achieve."

History may have ended, but it seems that special measures are needed to keep it in a state of finality.