Some scientists estimate the maximal life span of a human to be approximately 120 years, a full 50 years longer than the Biblical three score and ten (Psalms 90:10). The people who have actually achieved that maximum can be counted on one hand--or one finger. Mme. Jeanne Calment of France was 122 years old at her death in 1997. But although few challengers to her record exist, we are seeing more and more members of our society reach 100. In fact, in the United States today, there are more than 60,000 centenarians, and their ranks are projected to grow to nearly 1 million by 2050. Much of this growth will be due to the convergence of the large aging Boomer demographic and improvements in health and medicine.

Most people who get to 100 do so by avoidance. They shun tobacco and excess alcohol, the sun and pollutants, sloth, anger and isolation. Still, many of us may know at least one smoking, drinking, sunburnt, lazy, cantankerous recluse who has lived to 100--and wondered how he or she did it.

More and more, scientists are finding that part of the explanation lies in our genes. Many lower organisms have been found to carry longevity assurance genes, genes that grant a longer life span. More and more of those genes are found to resemble genes of our own.

On the flip side, we humans carry a number of genes that are deleterious to our health and longevity. These genes increase our risks of heart disease, cancer, gray hair and wrinkles. Though we cannot change our genetic pedigrees, perhaps if we know what unhelpful genes we carry, we can take steps, such as ridding ourselves of bad health habits and adopting good ones, that can overcome the disadvantages our genes confer and live as long as those people with good genes.