The theory is presented that the genetic potential for longevity of a given species in a protected environment is determined by the rate of predation in the wild and by the frequency of spontaneous mutations. A common genetically determined lifespan representative of rare, long-lived individuals in the wild results from the slight selective advantage conferred by genotypes which do not cause early death. The selective advantage of reproductive rare survivors is limited by the rate of spontaneous mutations from a common potential longevity to those which limit lifespan to shorter times. The equilibrium frequency of life-limiting genes increases exponentially with increasing age when the rate of mutations is uniform for different degrees of life-shortening.