23 Essays on Infinite Lifespans   Michael R. Rose WHAT IS GOING ON? The  evolutionary  theory  of  life-history,  aging,  etc  is  not complete, but it is the best theoretical foundation for under- standing the phenomena of immortality, old and new. Natural selection is an amazing thing, but it is not all-powerful. When natural selection is weak, survival and reproduction should be imperiled; and natural selection is relatively weak later in life. Consider genes that kill everyone who has even one copy of the killer gene. If the gene kills during childhood, it will be eliminated from the population in a single generation. Under these circumstances, natural selection is all-powerful and this is a common intuitive understanding of the action of natural selection. But if killer genes act during old age, they will not be resisted by natural selection. The killer genes have already made it into the next generation. The future survival of the victim of the killer gene does not matter for the transmission of the killer genes. Natural selection is too feeble at advanced ages. [10;11] At early ages, natural selection is strong, and survival rates will be high, but not 100 percent, because we do not need aging to have death. During reproductive adulthood, natural selection becomes progressively weaker. This increases mortal- ity rates, and thus causes aging, even under ideal conditions. These ideas explain both the health of youth and the advance of aging during adulthood. But there is a subtle twist that explains eventual immortal- ity. The force of natural selection steadily falls with adult age. But it cannot fall forever, because it cannot take on negative values. As a result, it finally hits zero, and stops falling, near the end of life. The force of natural selection reaches a plateau from which it does not budge. This explains why immortality arises in late life. As natural selection cannot get any worse, bad evolutionary effects stabilize. Since these bad effects are