When my son was about 4 years old, he asked me one of those questions for which there seems to be no good answer, and certainly no easy one. As we were pushing the full grocery cart toward the car, he furrowed his little brow and said, "Who decided that people would grow up, have babies and then die, and then their children would grow up and have babies and die?" I may have replied something like "Nobody knows," an answer that has never satisfied his inquisitive mind. Or maybe I said he could try to find out himself one day. But as a former student of biology, I knew that there is, in fact, an answer to part of his question -- why do people age and die? The answer is the "Hayflick limit," the biological law that says cells reproduce themselves only a certain number of times before falling into senescence.

Leonard Hayflick, the brilliant and irascible cell biologist for whom the law is named, discovered that the cells of creatures great and small begin to disintegrate after a set number of divisions, largely because they are no longer able to carry out such housekeeping tasks as exporting wastes and accurately copying the DNA inside their nuclei. Human cells, he found, will divide about 50 times in a Petri dish before running out of gas. This discovery suggested that body parts lose the capacity to repair themselves when the cells reach their allotted span.

Before Hayflick's seminal finding in the 1970s, biologists believed that cells could go on replenishing themselves indefinitely. The Hayflick limit sparked insights not only into the question of why we decline and die, but also into cancer, whose cells somehow escape the limit and continue reproducing ad infinitum. It has also helped spawn the branch of biotechnology devoted to life extension. Few if any serious scientists actually believe that human beings can live forever, but this hasn't stopped biotechnology companies with names like Elixer, Osiris and Geron (short for gerontology) from capitalizing on the possibility that tinkering with the genetic and metabolic machinery controlling cellular senescence will one day allow doctors to slow the inexorable march toward death.

These merchants of immortality, as author Stephen S. Hall calls them, are betting that embryonic stem cells, the primogenitors of all cell types in the body, hold out the promise of treating a host of diseases, from Parkinson's to diabetes to torn rotator cuffs. Of course, stem cells are also the focus of one of the nation's most urgent and rancorous ethical and political debates because they are derived from embryos.

Leading one's readers from the science of the Hayflick limit to the politics of stem cells takes some deft and thoughtful writing. Author Stephen Hall manages to traverse this bumpy and varied terrain with the help of his central character, Michael West, a former antiabortionist and creationist who converted to the church of molecular biology. West has an uncanny ability to see into the future of science. He believes it is his mission to find the means to extend life, and nothing, including ethics or legislation, is going to get in his way. After making a pilgrimage to visit Hayflick in the 1980s, West founded Geron Corporation, which was devoted to life-extension. By the time his own board had booted him from the company, he had already moved on to stem cells and begun raising private funds for academic researchers who were at the earliest stages of finding and understanding them.

West's story makes for compelling reading, but there would be little reason to care about him if not for his Zelig-like knack for being at the center of some of the most troubling controversies surrounding biotechnology and medicine. It was West's second company, Advanced Cell Technologies, that grabbed headlines two years ago with the announcement that it had created a cloned hybrid embryo made from a cow egg and human DNA. He made the nightly news again last year when ACT ballyhooed the creation of a cloned human embryo. West and his company promised they had no interest in creating cloned babies; they simply wanted to be able to harvest embryonic stem cells. The announcement naturally sparked a worldwide furor. Several groups on the fringes of reality responded by vowing to produce a cloned human child. The Vatican condemned the science, and Congress launched another round of hearings to consider banning both human cloning and embryonic stem cell research.

That a public debate as momentous and far-reaching as the one over cloning and stem cells came to be driven by a messianic opportunist like West is a testament to the thoroughness with which abortion politics has cowed more legitimate scientists into silence and prevented a rational and measured discussion of the issues. As Hall puts it, West "was not the most august or the most credible, or the most respected, member of the scientific community to be making the case for stem cells, to say nothing of the way he wanted to push the public debate, pedal to metal, on human cloning. But it was also true that no one else in the scientific community cared to venture so far out on a limb."

One result of scientists' reticence was President George W. Bush's decision to limit federal funding to research on 60 existing stem cells lines, or cells that have been coaxed to grow on their own in the lab. It was an attempt to arrive at a Solomonic solution: Publicly funded research could go forward, but no more embryos would be sacrificed in the name of science and searching for cures. The president's decision has instead had the unintended but entirely predictable consequence of driving researchers into the arms of the private sector. It turns out the administration vastly overestimated the number of existing cell lines. There were perhaps six sets of stem cells, not 60. Researchers will need a lot more varieties of stem cells if the science is going to advance, and the only source is private industry. That means stem cell research will proceed largely hidden from public scrutiny.

Even worse, in Hall's view, the president's edict means that basic researchers will be forced to enter time-consuming and expensive contractual agreements with the private companies that control the majority of viable cell lines. As one researcher from Johns Hopkins University told a Senate committee, "Embryonic stem cell research is crawling like a caterpillar." At least one prominent U.S. stem cell researcher has already uprooted his lab and transplanted it in England, where such research is publicly funded.

Merchants of Immortality is a highly readable and important book. Hall, the author of two previous books about science and medicine, is an expert explicator, able to make the most difficult biology easy to understand. But he is at his best when describing the cast of characters involved in the science and politics and when chronicling recent events. It seems that Hall sides with biologists like West, who would have the rest of us believe that stem cells will be the future of medicine and that science should be allowed to pursue cures unfettered. Whether or not you agree with that assessment, this book will provide new insights into the intersection of science and politics.

It also provided me with additional information for my small son. Altering the human lifespan dramatically would require a delay in development, slowing down the biological machinery that transforms us from tiny, helpless infants into adults. In other words, if you want to live to 150, you can't just tack on 60 extra years as an adult; you also have to extend infancy, childhood and adolescence. That means that children might not reach maturity until they were 30 or 40 years old. Now there's a thought to scare any parent away from life extension.