New Insights Into The Enigma Of Lengthening Longevity

In an article appearing recently in Science (Vaupel et. al., Biodemographic trajectories of Longevity, Science, vol. 280, 8 May 1998) senior author Prof. James W. Vaupel from the Max Planck Institute for Demographic Research in Rostock, Germany, presents the findings of research on longevity conducted over the last several years by members of an international team of demographers, biologists and statisticians. This study is one of the first to come out of the newly emerging field of biodemography.

Since 1950 there have been substantial, largely unexplained reductions in human mortality at older ages. And the rate of reduction has accelerated in most developed countries, especially since 1970. In 1990 there were about four to five times as many centenarians in these countries than there would have been if mortality after age 80 had stayed at 1960 levels. Death rates increase at a slowing rate after age 80. Mathematical analysis of the data indicates that death rates may level off at around 105 and even decline after age 110.

The decline in old-age mortality is not restricted to Homo sapiens. Vaupel et al. estimated age trajectories of death rates for very large cohorts of four species of fruit flies, a parasitoid wasp, a nematode worm, and baker’s yeast. Despite substantial differences in the trajectories, they share a key characteristic: mortality decelerates and, for the largest populations studied, it even declines at older ages. The same seems to be true for old automobiles as well, which suggests that although living organisms are vastly more complex than manufactured products mortality deceleration could be a general property of complex systems.

As far as living organisms are concerned, these findings come as a big surprise in light of the traditional view that posits maximum life-spans. According to this theory, the postreproductive span of life should be short because there is no selection against mutations that are not expressed until reproductive activity has ceased.

While it is not clear how to reconcile these findings with theories about aging, the authors present three biodemographic insights—concerning the correlation of death rates across age, individual differences in survival chances, and induced alterations in age-patterns of fertility and mortality—that offer clues and suggest avenues for future research.

These three concepts can be tied together by a general question: how important are an individual’s "survival attributes" (persistent characteristics, innate or acquired, that affect survival chances) as opposed to current conditions in determining the chance of death? An analysis of data on Danish twins suggests that about 50% of the variation in human life-spans after age 30 can be attributed to survival attributes that are fixed by the time an individual is 30; a third to a half of these attributes are genetic and half to two-thirds nongenetic (related to, for example, socioeconomic status, disease history, etc.). The model used for this analysis indicates that the importance of survival attributes may increase with a person’s life expectancy. Research over the next decade may resolve the question of how many survival attributes account for most of the variation in life-spans.

That genes can alter mortality trajectories is now certain. The emerging field of molecular biodemography seeks to uncover how variation at the microscopic level of genetic polymorphisms alters mortality trajectories at the macroscopic level of entire populations. As for nongenetic determinants of longevity, the importance of diet, stress, and reproduction in inducing alternative mortality schedules has been demonstrated. The authors conclude that progress in understanding these causal relationships and in discovering genes and other survival attributes will lead to even longer lives.

Co-authors are: James R. Carey, Kaare Christensen, Thomas E. Johnson, Anatoli I. Yashin, Niels V. Holm, Ivan A. Iachine, Väinö Kannisto, Aziz A. Khazaeli, Pablo Liedo, Valter D. Longo, Zeng Yi, Kenneth G. Manton, James W. Curtsinger