Germany,notesthatonly 3 percent ofhowlongyou’ll
live, compared with theaverage, is explainedby your
parents’longevity;bycontrast,upto 90 percentofhow
tallyou areis explainedbyyour parents’height.Even
geneticallyidenticaltwinsvarywidelyinlifespan:the
typical gap is more than fifteen years.
Ifourgenesexplainlessthanweimagined,theclassical
wear-and-tearmodel mayexplainmore thanweknew.
Leonid Gavrilov, a researcher at the University of
Chicago, argues that human beings fail the way all
complex systems fail: randomly and gradually. As
engineershavelongrecognized,simpledevicestypically
do not age. They function reliably until a critical
componentfails,andthewholethingdiesinaninstant.A
windup toy, forexample, works smoothlyuntil agear
rustsoraspringbreaks,andthenitdoesn’tworkatall.
But complex systems—power plants, say—have to
surviveandfunctiondespitehavingthousandsofcritical,
potentially fragile components. Engineers therefore
design these machines with multiple layers of
redundancy: with backupsystems,and backupsystems
for the backup systems. The backups may not be as
efficientasthefirst-linecomponents,buttheyallowthe
machine to keep going even as damage accumulates.
Gavrilovargues that,within theparametersestablished
byourgenes,that’sexactlyhowhumanbeingsappearto
work.Wehaveanextrakidney,anextralung,anextra
gonad, extrateeth.TheDNA inourcells isfrequently
damagedunder routineconditions,butourcellshavea
number of DNA repair systems. If a key gene is
permanentlydamaged,thereareusuallyextracopiesof