Stuff has published an examination of the disposable soma theory of aging, complete with a description of the bathtime eureka moment. While widely accepted now, this theory was fairly revolutionary at the time (the late 1970s). From the article: "Ageing is simply a matter of cumulative damage done to the relatively more vulnerable soma cells. In fact, if we can discover just how this damage occurs, we could learn the secret of much longer life." Scientists are working on that now, as it happens. You might also want to read a previous article that expands on this theory to explain why we live for so long after ceasing to reproduce.