Longevity, quality, and the one-hoss shay.

T his special issue of Science is called " Piecing Together Human Aging " and its scientific content, as you will see in the following pages, is devoted mainly to the life and death cycles of the cells and tissues that compose our bodies. The topic ushers in some troubling thoughts about the way we wear out, as well as about the length of life and its quality— two features that are sometimes in conflict with one another. Let's start with the former: longevity. Demographers have always been interested in life expectancy, and in the problem of whether it has a finite, biologically conferred limit. The history of prediction in this area is a trail of busted estimates; proposed limits have been exceeded, one after another, since 1928, and there is no indication that a biological maximum of some kind is being closely approached. Most think such a maximum exists, but evidence from the steady improvement in life expectancy achieved by the best performers shows that it is still at a distance Does that make us all feel better? Well, it depends—and that brings us to the quality-of-life issue, which has a lot to do with how we wear out. Oliver Wendell Holmes provided one metaphor for the perfect lifespan in his poem " The Deacon's Masterpiece Or, the Wonderful One-Hoss Shay: A Logical Story. " The deacon completes this extraordinary project in 1755, the year of the great Lisbon earthquake. Built of carefully selected parts that the builder thought would wear out but not break down, it lasted exactly a hundred years in good condition. Then, on the centenary of the earthquake, the Wonderful One-Hoss Shay collapsed into a mound of dust, going to pieces " …all at once, and nothing first—just as bubbles do when they burst. " Its driver, the parson, was deposited unceremoniously onto the ground, right outside the meeting-house. The shay's life cycle would be an attractive metaphor for us humans if the span were long enough. Alas, those of us at a Certain Age are all too acutely conscious of differential wear-out. As Roth et al. point out (p. 1423) in exploring the similarities between aging in humans and rhesus monkeys, there is a canonical sequence: presbyopia, cataracts, loss of motor activity, decline in memory performance. It would be nice if these things happened all at once instead of sequentially—as long as it wasn't …