Dead Last
Authors: Leslie Mitch 
Published: August 2003

Yes, graying baby boomers, you can safely and naturally stop aging! Just live until your mid-90s and your body will quit decaying on its own! Few of us get that chance, but the question of why aging stops in the very old has nettled researchers for the last decade. Now, a study of fruit flies backs an evolutionary explanation and reveals that the age at which deterioration stops is more malleable than anyone expected. Far in the future, researchers might be able to halt human aging before people are too feeble to enjoy the extra time.

Whoever said that life begins at 50 never read an actuarial table. By that milestone, an individual's risk of dying--a measure of physical decline--is about five times higher than a 20-year-old's and is rising fast. But for people who survive into their 90s, the mortality rate levels off. The fortunate oldsters apparently stop aging.

The idea was controversial at first. "Ten years ago, I didn't believe this at all," says Michael Rose, an evolutionary biologist at the University of California, Irvine. But demographers have confirmed the plateaus in organisms as diverse as yeast and wasps. To explain the slowdown, some scientists back the "tough old bastard hypothesis," which ascribes the effect to inborn constitutional differences. Weaker individuals gradually perish, and by advanced ages, only the hardiest remain. Mathematical models of evolution suggest an alternative explanation. Natural selection dictates how fast organisms fall apart. Creatures grow decrepit because natural selection wanes with age, and mortality rates climb as the strength of natural selection falls (see
"Aging Research Grows Up"
). Once an organism has reproduced for the last time, selection's strength reaches zero, and it might seem that aging should shift into overdrive. However, the evolutionary explanation predicts that death rates should level off and physical deterioration should cease.

Rose and colleagues tested this explanation in fruit flies. It posits a close correspondence between the age of last reproduction and the age when mortality peaks--and that's what the researchers saw in four lines of flies. Although flies of different strains stopped laying eggs at different ages, in each strain the "break day" when death rates began to stabilize closely followed the end of breeding. The researchers then tried to shift the mortality pinnacle in late-reproducing flies whose break day came at 69 days of age. Rose and colleagues collected eggs only from mothers under the age of 15 days to form each succeeding generation. After 24 generations, the break day had slipped back to 48 days.

"This is a fascinating result," says Kenneth Wachter, a demographer at the University of California, Berkeley, because the "onset of mortality plateaus evolves so quickly." The age when mortality levels off is changeable, says Rose, but don't expect these fly results to benefit humans soon. Sometime in the future, he predicts, researchers might know enough to halt aging earlier, say at 45 instead of 90. If scientists pull off that feat, saying that life begins at 50 will be more than a comforting fib.

--Mitch Leslie; suggested by Donna Holmes

M. R. Rose, M. D. Drapeau, P. G. Yazdi, K. H. Shah, D. B. Moise, R. R. Thakar, C. L. Rauser, L. D. Mueller, Evolution of late-life mortality in
Drosophila melanogaster
.
Evolution
56
, 1982-1991 (2002).
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