Nobody knows precisely how many people want to live a lot longer while remaining forever young, but it's probably a safe guess that the number is high. What are the prospects for baby boomers still holding out hope? Handicapping this race between technology and nature is veteran science writer Stephen S. Hall in his latest book, ``Merchants of Immortality: Chasing the Dream of Human Life Extension.''

``From paramecia to primates, from the single-celled denizens of pond scum to poet laureates, natural selection stops caring about us once we have lived long enough to reproduce,'' Hall says. ``Evolution in that sense is a strange ship: it moves ever forward through dark waters, keeping the species alive, even as it throws each and every member of the species overboard.''

Hall is a contributing writer and editor at the New York Times Magazine and author of three other books on biotechnology, including ``Invisible Frontiers: The Race to Synthesize a Human Gene,'' which told of the high-stakes competition to use recombinant DNA and bacterial cloning to mass-produce insulin. He knows the terrain and how to explain it to the rest of us.

``The purpose of this book is not to conflate promising science with the wishful thinking of an entire generation,'' he writes. ``It is enough to note that in the last decade the most skilled, ambitious, and indeed arrogant of our sciences have lined up to tackle the problem of aging (and its faithful sidekick, death) in a way fundamentally different from that of any previous era of medical intervention.''

Contrary to its title, ``Merchants of Immortality'' is not about the elimination of death by natural causes but about postponing it. And Hall's ``merchants,'' while at the business of extending human life expectancy, would also very much like to soften the ravages of aging.

The quest of these biotech Ponce de Leons was launched in 1962 by the experimental results of Leonard Hayflick, a microbiologist who was truly far ahead of his time. Hayflick developed a human cell line he called WI-38 from aborted fetus cells and used it to overturn ``half a century of flawed dogma'' about mammalian cells. With WI-38, he demonstrated that normal human cells have a limited capacity for replication. After about 50 divisions, senescence sets in, the cell stops dividing into new cells, and it eventually dies. This phenomenon, now known as ``the Hayflick limit,'' pointed for the first time to the cell as the source of the aging process.

A Russian biologist, Alexy Olovnikov, later took Hayflick's idea of built-in mortality as a normal function of cells and ran with it. He discovered that human chromosomes are capped at the ends with a protective coating called a telomere that gets shortened with each round of cell division until a threshold is crossed and the cell goes from being young to old. In 1971, Olovnikov published a paper that showed telomere shortening to be the basis for the Hayflick limit. However, thanks in part to Cold War politics, his idea languished for nearly two decades.

``It fell to a collection of brave and resourceful women scientists, working at the margins of molecular biology in the 1980s, to bring telomeres front and center as an area of tremendous scientific importance,'' Hall says.

These scientists discovered that telomeres play a critical role both in the aging process, through cell mortality, and in the process by which cancer develops, through cell immortality. This launched a furious race at the start of the 1990s to identify the gene for telomerase, the protein that makes up a telomere, and marked the coming into prominence of ``a born-again Darwinian'' entrepreneur named Michael West.

Writes Hall: ``Michael West's obsession with death has left an indelible mark on the course of biotechnology, on the priorities of modern-day molecular biology, and to no small degree on some of the agonizing national conversations we have been having in recent years about the ethics of stem cell research, human cloning, and the social and medical consequences of altering, even a little, the normal course of human aging.''

West is the founder of Advanced Cell Technology (ACT), a start-up firm that developed the first successful method for artificially producing human embryonic stem cells -- generic baby cells that can mature into any type of cell in the body. He has written his own book on this subject, ``The Immortal Cell,'' which is due out in October.

ACT's human stem cells were made by injecting a human skin cell into the egg cell of a cow from which the nucleus had been removed. The feat sparked political debates that continue today.

Hall chronicles at length the inability of the Clinton administration to enact a policy, largely, he suggests, because of the Monica Lewinsky scandal. He then examines the Bush administration's fence-riding, which culminated in a televised presidential speech that Hall characterizes as ``a national lesson in sex education.''

President Bush banned federal funding of human embryo stem cell research except for what he claimed were ``more than 60'' existing cell lines. The president's statement was wrong: There may be no more than four usable stem cell lines, and the existing embryonic cells are almost all commercially owned.

As Hall notes, what Bush did was place an incredibly important field of biomedical research, one that could hold the keys to the successful treatment of cancer, heart attack, Alzheimer's, Huntington's and Parkinson's diseases, plus a host of other debilitating afflictions, into the hands of a few ``underfinanced, understaffed, and scientifically overwhelmed boutique biotech companies.''

Hall expresses utter bafflement as to the rationale behind this decision. ``In the president's curious moral universe, destroying leftover embryos to create more stem cell lines is unethical, but allowing fertilization clinics to create and then toss out excess embryos is simply a legitimate cost of doing business.''

In ``Merchants of Immortality,'' Hall covers the science, politics, money and morality behind what he calls ``life-extension technology'' through solid research and numerous interviews with the key players. He's an excellent writer who has produced a mesmerizing tale. ``Merchants of Immortality'' may well be the year's best science book.

MERCHANTS OF IMMORTALITY: Chasing the Dream of Human Life Extension

By Stephen S. Hall

Houghton Mifflin, 439 pp., $25