Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100

substances for their effects on aging, then we will see if these intriguing studies on animals carry over to humans.
    DO WE HAVE TO DIE?
    William Haseltine, a biotech pioneer, once told me, “The nature of life is not mortality. It’s immortality. DNA is an immortal molecule. That molecule first appeared perhaps 3.5 billion years ago. That selfsame molecule, through duplication, is around today. … It’s true that we run down, but we’ve talked about projecting way into the future the ability to alter that. First to extend our lives two-or threefold. And perhaps, if we understand the brain well enough, to extend both our body and our brain indefinitely. And I don’t think that will be an unnatural process.”
    Evolutionary biologists point out that evolutionary pressure is placed on animals during their reproductive years. After an animal is past its reproductive years, it may in fact become a burden on the group and hence perhaps evolution has programmed it to die of old age. So perhaps we are programmed to die. But maybe we can reprogram ourselves to live longer.
    Actually, if we look at mammals, for example, we find that the larger the mammal, the lower its metabolism rate, and the longer it lives. Mice, for example, burn up an enormous amount of food for their body weight, and live for only about four years. Elephants have a much slower metabolismrate and live to seventy. If metabolism corresponds to the buildup of errors, then this apparently agrees with the concept that you live longer if your metabolism rate is lower. (This may explain the expression “burning the candle at both ends.” I once read a short story about a genie who offered to grant a man any wish he wanted. He promptly asked to live 1,000 years. The genie granted him his wish and turned him into a tree.)
    Evolutionary biologists try to explain life span in terms of how longevity may help a species survive in the wild. To them, a specific life span is determined genetically because it helps the species to survive and flourish. Mice live so briefly, in their view, because they are constantly being hunted by a variety of predators and often freeze to death in winter. The mice that pass on their genes to the next generation are the ones that have the most offspring, not the ones who live longer. (If this theory is correct, then we expect that mice that can somehow fly away from predators would live longer. Indeed, bats, which are the same size as mice, live 3.5 times longer.)
    But one anomaly comes from the reptiles. Apparently, certain reptiles have no known life span. They might even live forever. Alligators and crocodiles simply get larger and larger, but remain as vigorous and energetic as ever. (Textbooks often claim that alligators live to be only seventy years of age. But this is perhaps because the zookeeper died at age seventy. Other textbooks are more honest and simply say that the life span of these creatures is greater than seventy but has never been carefully measured under laboratory conditions.) In reality, these animals are not immortal, because they die of accidents, starvation, disease, etc. But if left in a zoo, they have enormous life spans, almost seeming to live forever.
    BIOLOGICAL CLOCK
    Another intriguing clue comes from the telomeres of a cell, which act like a “biological clock.” Like the plastic tips at the ends of shoelaces, the telomeres are found at the ends of a chromosome. After every reproduction cycle, they get shorter and shorter. Eventually, after sixty or so reproductions (for skin cells), the telomeres unravel. The cell then enters senescence and ceases to perform properly. So the telomeres are like the fuse on a stick of dynamite. If the fuse gets shorter after each reproduction cycle, eventually the fuse disappears and the cell stops reproducing.
    This is called the Hayflick limit, which seems to put an upper limit on the life cycle of certain cells. Cancer cells, for example, have no Hayflick limit and produce an enzyme called telomerase that prevents the telomeres from getting shorter and shorter.
    The enzyme telomerase can be synthesized. When applied to skin cells, they apparently reproduce without limit. They become immortal.
    However, there is a danger here. Cancer cells are also immortal, dividing without limit inside a tumor. In fact, that is why cancer cells are so lethal, because they reproduce without limit, until the body can no longer function. So the enzyme telomerase has to be