Toward the end of his life, Paul Gauguin painted a compelling scene of human destiny with a title to match: Where Do We Come From? What Are We? Where Are We Going? The great oil painting, which now hangs in Boston's Museum of Fine Art, depicts the stages of life among the women of Tahiti, where Gauguin spent his last days. In the left corner of the canvas, an elderly woman, clasping her head in her hands, contemplates mortality.

In their new book, The Quest for Immortality: Science at the Frontiers of Aging, S. Jay Olshansky and Bruce A. Carnes reflect in similar fashion on the ebb and flow of life. But a shift in the human condition, which would have astonished a person of Gauguin's generation, informs their writing.

In the century since Gauguin painted his masterpiece, a demographic revolution has transformed the lives of a great part of the Earth's human population. An extension of the life spans of tens of millions of people, mainly in North America, western Europe, Japan, and Australia, has taken place. For the first time in human history, we can seriously contemplate the possibility that great numbers of people will live to advanced ages, so that, as the Bible says, "he who fails to reach a hundred will be considered accursed."

At the turn of the 20th century, life expectancy in the United States was 45 years. Gauguin (who was born in 1848), despite much hard living, made it to 55. Today, the average American can expect to live to the age of 78. Life expectancy is similar in other developed nations. And a growing number of researchers in the field of gerontology believe that further gains in human longevity are possible.

Olshansky and Carnes are not among them. The two authors, both scientists based at the University of Chicago's Center on Demographics and Economics of Aging, are pioneers in the field of biodemography. When viewed with the exacting standards of the demographic research they espouse, most claims of potential leaps in life expectancy seem more like wishful thinking than reputable science.

Indeed, in some cases, the "miracle drugs" and "health foods" being marketed today give credulous consumers no more than a "gambler's odds" of living longer. Olshansky and Carnes show that the great upswing of 20th-century life expectancy resulted from a sharp reduction in high infant mortality and the conquest of early childhood diseases. Vaccinations against killers such as diphtheria and polio, the widespread use of antibiotics, and better sanitation and nutrition all played crucial roles in the great health advances of the past 100 years.

The authors make a convincing case that we should not expect to see a comparable improvement at the other end of the age spectrum. The so-called First Longevity Revolution rested on "a redistribution of death from the young to the old," they point out. In all earlier epochs, the highest mortality rates were among the young rather than the elderly, who had built strong immune systems over their long lives.

Now the reverse is true. People who cheated death as children through what the authors call "manufactured survival" must face genetic vulnerabilities that formerly accounted for many fewer deaths. The reason is simple, if disturbing. In the past, relatively few people lived long enough to succumb to diseases of aging like cancer or Alzheimer's disease. These maladies are not susceptible to "magic bullet" cures like antibiotics.

This may seem obvious, but given the inflated claims of proponents that "wonder drugs," hormones, and dietary supplements promote longer life, it is worth underscoring. Despite great advances in genetic research, there is little likelihood of a breakthrough for the elderly comparable to the Salk polio vaccine for children. The authors contend that life expectancy is unlikely to extend beyond the age of 85.

Their enlightening discussion of the phenomenon of free radical molecules in the human body powerfully reinforces their assessment. Free radicals are by-products of metabolic reactions in mitochondria, the organelles that provide energy for cells. These highly reactive chemical species exist for less than a second, but the damage they cause presents a long-term threat, as these defects accumulate (especially in DNA) over a lifetime.

Our bodies do carry some built-in defenses. They produce molecules called "free radical scavengers" that help keep the free radicals in check. Proponents of various vitamins and dietary supplements contend that antioxidants in these products are effective in countering free radicals. One promising cell-protector is a compound called WR-2721 (aka amifostine). It is being investigated as a so-called radioprotectant, to help cancer patients withstand the assault of toxic radiation and chemotherapy, but some believe it may have a broader application to aging. WR-2721 has antioxidant properties, and some evidence suggests it also stretches out the cell cycle, giving the cell's DNA repair mechanisms more chance to do their job. It has been successfully tested in cell cultures and animal studies.

The human body, however, is a very different situation. Olshansky and Carnes warn against regarding WR-2721 as a mystical elixir vitae. The great number of mutations caused by free radicals argues against the effectiveness of a single "wonder drug." Even more importantly, some level of free radicals may be crucial to the survival of the human species, since the mutations they cause "create the genetic diversity needed to survive and thrive in a changing environment."

Olshansky and Carnes are also concerned that the lives of the elderly who are reaching the current limits of longevity will be ones of loneliness and alienation, rather than of creativity and happiness. As the "baby boom" generation approaches retirement age, this will become an ever-increasing challenge, and put great financial demands on society.

Despite the cautionary nature of much of their book, however, Olshanky and Carnes are not latter-day Malthusians. The tone of The Quest for Immortality is one of guarded optimism and liberal values, making it a welcome guide for contemplating the finitude of life.