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HOW DOES THE BIOLOGICAL CLOCK TICK?
A.Ourlifespanisrestricted.Everyoneacceptsthisas'biologically'obvious.‘Nothinglives
forever!’However,inthisstatementwethinkofartificiallyproduced,technicalobjects,
productswhicharesubjectedtonaturalwearandtearduringuse.Thisleadstotheresult
thatatsometimeorothertheobjectstopsworkingandisunusable('death'inthebiological
sense).Butarethewearandtearandlossoffunctionoftechnicalobjectsandthedeathof
livingorganismsreallysimilarorcomparable?
B.Our‘dead’productsare‘static’,closedsystems.Itisalwaysthebasicmaterialwhich
constitutestheobjectandwhich,inthenaturalcourseofthings,isworndownandbecomes
'older’.Ageinginthiscasemustoccuraccordingtothelawsofphysicalchemistryandof
thermodynamics.Althoughthesamelawholdsforalivingorganism,theresultofthislawis
notinexorableinthesameway.Atleastaslongasabiologicalsystemhastheabilityto
renewitselfitcouldactuallybecomeolderwithoutageing;anorganismisanopen,dynamic
systemthroughwhichnewmaterialcontinuouslyflows.Destructionofoldmaterialand
formationofnewmaterialarethusinpermanentdynamicequilibrium.Thematerialofwhich
theorganismisformedchangescontinuously.Thusourbodiescontinuouslyexchangeold
substancefornew,justlikeaspringwhichmoreorlessmaintainsitsformandmovement,but
inwhichthewatermoleculesarealwaysdifferent.
C.Thusageinganddeathshouldnotbeseenasinevitable,particularlyastheorganism
possessesmanymechanismsforrepair.Itisnot,inprinciple,necessaryforabiological
systemtoageanddie.Nevertheless,arestrictedlifespan,ageing,andthendeatharebasic
characteristicsoflife.Thereasonforthisiseasytorecognise:innature,theexistent
organismseitheradaptorareregularlyreplacedbynewtypes.Becauseofchangesinthe
geneticmaterial(mutations)thesehavenewcharacteristicsandinthecourseoftheir
individuallivestheyaretestedforoptimalorbetteradaptationtotheenvironmental
conditions.Immortalitywoulddisturbthissystem-itneedsroomfornewandbetterlife.This
isthebasicproblemofevolution
D. Everyorganismhasalifespanwhichishighlycharacteristic.Therearestriking
differencesinlifespanbetweendifferentspecies,butwithinonespeciestheparameteris
relativelyconstant.Forexample,theaveragedurationofhumanlifehashardlychangedin
thousandsofyears.Althoughmoreandmorepeopleattainanadvancedageasaresultof
developmentsinmedicalcareandbetternutrition,thecharacteristicupperlimitformost
remains80years.Afurtherargumentagainstthesimplewearandteartheoryisthe
observationthatthetimewithinwhichorganismsageliesbetweenafewdays(evenafew
hoursforunicellularorganisms)andseveralthousandyears,aswithmammothtrees.
E. Ifalifespanisageneticallydeterminedbiologicalcharacteristic,itislogicallynecessary
toproposetheexistenceofaninternalclock,whichinsomewaymeasuresandcontrolsthe
agingprocessandwhichfinallydeterminesdeathasthelaststepinafixedprogramme.Like
thefifespan,themetabolicratehasfordifferentorganismsafixedmathematicalrelationship
tothebodymass.Incomparisontothelifespanthisrelationshipis‘inverted’:thelargerthe
organismtheloweritsmetabolicrate.Againthisrelationshipisvalidnotonlyforbirds,but
also,similarlyonaveragewithinthesystematicunit,forallotherorganisms(plants,animals,
unicellularorganisms).
F.Animalswhichbehave‘frugally’withenergybecomeparticularlyoldforexample,
crocodilesandtortoises.Parrotsandbirdsofpreyareoftenheldchainedup.Thustheyare
notableto‘experiencelife’andsotheyattainahighlifespanincaptivity.Animalswhich
saveenergybyhibernationorlethargy(e.g.batsorhedgehogs)livemuchlongerthanthose
whicharealwaysactive,Themetabolicrateofmicecanbereducedbyaverylow
consumptionoffood(hungerdiet)Theythenmaylivetwiceaslongastheirwell-fed
comrades.Womenbecomedistinctly(about10percent)olderthanmen.Ifyouexaminethe
metabolicratesofthetwosexesyouestablishthatthehighermalemetabolicrateroughly
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