Research into the mechanisms of aging has suggested the possibility of extending the human life span. But there may be evolutionary biological reasons for senescence and the limits of the cell cycle that explain the infirmities of aging and the eventual demise of all human organisms. Genetic manipulation of the mechanisms of aging could over many generations alter the course of natural selection and shift the majority of deleterious mutations in humans from later to earlier stages of life. This could harm people in the distant future by making them more susceptible to premature disease and mortality. Thus there are biological and moral reasons to carefully consider the implications of exploiting this technology on a broad scale to extend the lives of people in the present and near future.