The social consequences of extending the human life span might be quite bad; perhaps the worst outcome is that power could be concentrated into ever fewer hands, as those who wield it gave way more slowly to death and disease. But the worry that more life would damage individuals' quality of life is not persuasive. Depending on what the science of aging makes possible, and on how people plan their lives, longer life might even facilitate a richer and deeper life. He had passed that great meridian, the age of forty, when for every man the process of spiritual evolution stops, and he goes on thenceforward working out to the end a character that has become fixed and unalterable.--G. Baker, in Tiberius Caesar The American biologist Andrzej Bartke recently showed that a combination of genetic alteration and nutritional restriction can increase the lifespan of a laboratory mouse by around 70 percent. (1) While control mice withered and died, the test animals were still zestfully scurrying about, fleet of foot with glossy fur and unclouded eyes, and apparently as full of joie de vivre as any young rodent. Discoveries of this sort are now far from rare. I recently found that alteration of a gene called daf-2 can increase the maximum life span of male nematode worms from 31 to 199 days--a 6.4-fold increase. (2) If a nematode life were translated into human terms, this would represent a lifespan of around 700 years. Common sense tells us that aging is universal, inevitable, and associated with gradual physical decline. But in this case, common sense is wrong. Some animal species, such as tiny betentacled hydra, do not appear to age at all. (3) There exist, for example, individual colonies of corals that are over 20,000 years old. (4) What is more, within the last decade biologists have found that the rate of aging is remarkably easy to alter in laboratory animals such as nematodes, fruit flies, and mice. It is no longer far-fetched to think that one day it will be possible to retard the aging process in humans and extend the human life span. Do we really want this research to succeed? Some bioethicists have professed horror at the thought of dramatic life extension. Many recoil at the notion of extending the lives of people undergoing irreversible physical decline, like the senile and decrepit Struldbruggs in Gulliver's Travels. Yet recent research shows it may be possible not just to extend life, but to extend youth. What if each of us could live a longer life, in peak physical and mental health, then suddenly shrivel away at the end, like Dracula when he is exposed to the sunlight? Would bioethicists still be so dour? Perhaps so, yet it would no longer be quite so clear why. There may be reasons to worry, but I want to suggest that aging research raises philosophical questions about the shape and purpose of life that bioethics has thus far failed to address. The New Biology of Aging A revolution has occurred in the biology of aging, transforming a sleepy backwater of research into a rapidly advancing discipline. (5) Up until about fifteen years ago, research into the causes of aging was a somewhat disreputable activity occurring at the fringes of biology. Although the researchers working on aging were few, the number of theories they managed to generate were many--by one estimate, over 300. (6) Not many of these theories have proved useful. The Russian immunologist Elie Metchnikoff believed that aging resulted from toxins released by bacteria in the intestinal tract. He suggested that a yoghurt diet would extend human life span to 200 years. Another early theory had it that aging in men resulted from a reduction in the level of secretions from the testicles. This led to a craze in the 1920s for surgically implanting the testicles of goats or monkeys into the scrotum of the recipient. These crank theories have often found an audience among aging souls all too eager to grasp at the hope of cheating death. …