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“
Biology:ASelf-TeachingGuide,ThirdEdition fillsagapingvoid,evenifyouaretakinga coursetaughtbysomeoneelse.Iknow,becauseIhavetaughtbiologycoursesatCornell Universityfornearly50years.SteveGarberwasoneofmystudents.Hereadallthe assignedtexts,butrecognizedthatsomethingreallyimportantwasmissing.Thereisno othertextthatdoeswhatGarber’sbookdoes,ordoesitaswell.Hecoverseverything thatistaughtinbiologycourses,anddoessoclearly,succinctly,andinawaythat isincrediblyeasy-to-read,understand,andremember.Heevenprovidesanexcellent indexandglossary.Irecommendthisbookveryhighly.Fivestars,forsure.”
Dr.KraigAdler,CornellUniversity
“Biology:ASelf-TeachingGuide,ThirdEdition isanindispensabletextandIwouldhighly recommendittoanyone.IwasastudentofDr.Garber’sandhealsomentoredmeon mystudentthesis.Ihighlyvaluedhisincredibleabilityforcommunicatingcomplicated subjectsanddifficultconcepts,whilesimultaneouslykeepingtopicscomprehensiveand enjoyable.Hewasoneoftheuniversity’sbestprofessorsandoneofthemostinspiring teachersIhaveeverhad.Hisclasseswereextremelypopularbecauseofhiscaptivating teachingstyleandencyclopedicknowledgeofhisfield.LearningwithDr.Garberwas likelearningfromatruemasterandIamverygladIwasgrantedtheopportunity.”
MatthewM.Malone
“Igivethisbook5-stars! Biology:ASelf-TeachingGuide,ThirdEdition doeswhatnoother bookdoes.ButitdoeswhatDr.Garberdoes.Itteachesthematerialinanuncanny mannerthatgetsstraighttothepointwhilesucceedingmagnificentlyatachievingwhat here-to-foreseemedimpossibleforothersciencebookwriters.SteveGarberhasturned theenterpriseoflearningbiologyintoacalmanddareIsay,evenenjoyablebusiness. IwasSteve’scolleaguewhenhetaughthighschoolbiology.Hehasalsotaughtcollege biologyatthebestschools.Whenweworkedtogether,lovedtositinonhisclasses. Myfieldsarechemistry,physics,andmath,butIneverreallyunderstoodbiology.Steve madeallhisclassessointeresting,andIimmediatelyunderstoodeverythinghewas teaching.I’vewantedtowritebookslikehis,butinmyfieldsofexpertise,butIcan’t doit.Idon’thaveSteve’sgifts.Ihavemanygifts,butnotthegiftofwritinginsucha perfectlyclearmanner.Hewritesinawaythatletsyougo,oh,thismakessense.Oh, Igetit.Steveisanoutstandingwriter.Iwishsomeonereadingthiswouldconvince himtoworkwithme,soSteveandIcouldco-authorbooksinmyfields,thatdowhat Stevehasaccomplishedforbiologystudents.Ifhewouldagree,wecouldputasmile onthefacesofsomanymorestudents.”
RajinderKhazanchi,Ph.D.
Biology ASelf-TeachingGuide
Biology ASelf-TeachingGuide
ThirdEdition
StevenDanielGarber
Copyright©1989,2002,2020byStevenD.Garber.Allrightsreserved
PublishedbyJossey-Bass
AWileybrand 111RiverSt.
Hoboken,NewJersey07030
Illustrationpermissioncreditslistedbypagenumber. 14B.S.NeylanandB.G.Butterfield, Three-DimensionalStructureofWood, ChapmanandHall Ltd,Publishers/23Dr.W.H.Wilborn(Figure2.5a)/23Dr.D.E.Kelly(Figure2.5b)/25 Dr.D.W.Fawcett/29photomicrographfromT.Naguro,K.Tanaka, BiologicalMedical ResourcesSupplement, 1980,AcademicPress,HarcourtBraceJovanovich,Inc./31Dr.H.J. Arnott/52Dr.H.W.Beams(Figure3.1b)/55R.A.Boolootian,Ph.D.,ScienceSoftware Systems,Inc./70CarolinaBiologicalSupplyCompany/133drawingadaptedfrom BuchsbaumandPearse, AnimalsWithoutBackbones, UniversityofChicagoPress,1987/198 HuxleyM.R.C.,Cambridge,England/227W.B.SaundersCompany/228drawing adaptedfromA.C.Guyton, TextbookofMedicalPhysiology,6thedition, W.B.Saunders Company/236Dr.ThomasC.Hayes/240AbbottLaboratories,Chicago,IL/254Dr.F.A. Eiserling/254Dr.K.Amako/278,279,298drawingsbyAliceBaldwinAddicott/280 drawingadaptedfromGibson, EdibleMushroomsandToadstools, Harper&RowPublishers, Inc.,1899/Drawingsappearingonpages318,322,323,325,325,326byJeromeLo/All linedrawingsnotlistedabovedrawnbyJohnWiley&SonsIllustrationDepartment.
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Tomyfamily
RuthSaraHaendler
MicahGarber
JeremyGarber
MiriamGarber
ArielHaendler
StellaHaendler
MichaelHaendler
KristinHaendler
VioletHaendler
HowtoUseThisBook
TheobjectiveofthisSelf-TeachingGuideistomakehighschoolandcollege biologyeasiertolearn.Thegoalistomakeyoumoresuccessful.Thetext coversthemostimportanttopicsinaclearmanner.Iprovidepracticetests andaglossary,andacomprehensiveglossaryisavailableatthebook’swebpage, http://www.wiley.com/go/biologystg3e.Thisbookorganizes,condenses,and clarifiesthemainconceptsandterms,andithighlightsalltheprimarypoints neededtofullygraspthematerialandtodowellinanybiologycourse.
Thechaptersandquestionsprovideexcellentpreparationforquizzes, tests,exams,prelims,andfinals.ThisSelf-TeachingGuidehasastudysection attheendofeachchapterwherekeyterms,questionstothinkabout,and multiple-choicequestionslikethosethatappearonexams(completewith theanswers)arepresented.Plus,thisbookistheperfectstudycompanion whenpreparingforstandardizedexamsinbiologysuchastheScholastic AptitudeTest(SAT),theAmericanCollegeTestingProgramAssessment (ACT),AdmissionsTestingProgramAchievementTest,inBiology,Advanced PlacementProgram:Biology,CollegeLevelExaminationProgram:Subject ExaminationinSciences–Biology,NationalTeacherExaminations(NTE) SpecialtyAreaTest:BiologyandGeneralSciences,andtheGraduateRecord Examinations(GRE):SubjectTest–Biology.
Preface
Thephotographonthecoverofthisbookdepictsacolorectalcancercell(a cancercellthatstartedinthelowerpartofsomeone’sintestines).Biologyisthe studyoflife,soIconsideredcoverdesignswithbeautifulflowers,andevenone withaladybug(aredbeetlewithblackdots).ThereasonIchosetheintriguingly gorgeouspinkcellforthiscoverisbecause,forsomany,biologyisapaininthe butt!Intheend,myhopeisthisbookwillgoalongwaytowardrectifyingthe situation(nopunintended).
IhavetakentoheartalltheconstructivecommentsfromthestudentsIhave taughtinmyclassroom,andthemanycommentspostedonlinefromallover theworld.EachyearIsitdownandincorporatetheserecommendationsinto themanuscriptthatwillbecomethenexteditionofthisbook.Forthisreason, Ithankyousomuchforthecontinualimprovementsthatmakethisbookbetter andbetter.
Thebestbiologyteachers,professors,andtextbooksmakebiologyclearand compelling.Othershaveawayofgivingbiologyabadname.Forme,thisis counterintuitive.Biologyisaboutlife,andforthisreason,itisatopiceveryone isnaturallycuriousabout,andlearningaboutitshouldbepleasurable.Myhope isthisbookwillmakeyourbiologyclassmorefunandeasier,andintheprocess, itwillhelpimproveyourgrades.
Theseclassesandbooksoftenforcestudentstolearnattheequivalentofthe receivingendofafirehose.Weareallforcedtolearnsomuch,sofast.
WhenIwasastudent,IsworethatthefirstchanceIhad,Iwouldwritea biologybookthatprovidedus,thestudents,whatwerepeatedlyasktheprofessortodo.Iwouldwriteabookwiththematerialthathasthegreatestlikelihood ofbeingonthetest.AndIwouldleavetherestout.Iwouldalsobesureto understandwhatIwaswritingabout,foritseems,thisisnotthecasewhenit comestosomanybiologyteachersandtextbookauthors.Itrulybelievethey aren’tclearbecauseoftentheydon’tknowwhattheyaretalkingorwriting about.Theyaremerelyparrotingsomethingtheyread.
Promotingbiologicalliteracyisanobletask.Thelion’sshareofwhatIwrite inthisbookwasinitiallydiscoveredbysomeoneelse.OnceinawhileIinclude myowninnovations,thoughInevergavemyselfcredit.Forinstance,thesection
onurbanecosystemscomesfromachapterentitled“UrbanEcosystems”in anotherbookIwrote(TheUrbanNaturalist).Theconceptwasnewthen;now it’sinmanybooks.Thereareevenjournalsnowcalled“TheUrbanNaturalist,” “UrbanEcosystems,”and“UrbanEcology.”Soyes,itisstillpossibletodiscover somethingnewthatendsupinbiologybooksallaroundtheworld!
Thisbookwillteachwhatweareexpectedtolearnaboutbiology.This self-teachingbiologybookhasbeenfield-testedbytensofthousandsofstudents,manyofwhomhaveprovidedcommentsthatcontinuallyhelpmeto improvethisbook,sobiologyisinteresting,andcompletinghomeworkiseasier, andpreparingforquizzesandtestsgoesmoresmoothly. Biology:ASelf-Teaching Guide isthenextbestthingtometeachingyouthismaterialone-on-one.
Acknowledgments
Wenaturallylovelife.Sincebiologyisthestudyoflife,Icanmakethecase thatweshouldalsolovebiology.Learningaboutlifecanandshouldalsobe wonderful.I’maluckyduckbecauseinadditiontolovinglife,Igetpaidto learnaboutitandtoteachittoothers.Inadditiontolovingnature,which includesplantsandanimals,andcells,andecology,andhowourbodieswork, andhowecosystemswork,Ialsolovepeople,andmorethanmostpeople,Ilove myfamily:MitchandMimi(parents),Ruthie(wife),Jeremy,Micah,Ariel,and Michael(sons),StellaandKristin(daughters-in-law),andViolet(granddaughter).Thanksforeverythingyoudoasthegluethatkeepsourfamilyfunctional, supportive,understanding,intriguing,andloving.
Masteringbiologyisfulfilling,thoughattimes,alsofrustrating,andoccasionallyevenexasperating.Whenbeginningtolearnthenutsandboltsofany specializedfield,weareforce-fedmorejargonthanseemsnecessary,andsometimeslearningitallinthegivenamountoftimemayseemimpossible.Yet, withoutamasteryoftheterminology,itmaybeimpossibletoearnthegrades wehopefor.Forthisreason,thepersonandbookyouarechannelingtolearn thisinformationaren’talwayseasytoappreciate.Icancommiseratewithyouon this,becauseItooamfrustratedbydifficult-to-understandpeopleandbooks. ThatiswhyIwrotethisbook.
ThisiswhyIowesomuchgratitudetothosewhovisually(sight),olfactorily(smell),gustatorilly(taste),aurally(sound),orthigmotactically(touch)have clarifiedanythingthathasorinthefuturemayleadtosomethingpositive.We alsoowemuchgratitudetothepeopleIlove,andtothoseIdon’tknowaswell, andtothepeopleIdon’tknowatall,whosharedsomethingthatIhavecome acrossinmyyears,thataddedtotheworkingknowledgethatIcouldchannel inthisbook.
WithallmyheartandallmysoulIlovescienceandnature,andIamgrateful forthelawsoftheuniverse.Iwouldalsoliketothankeveryoneandeverything thathaseverelevatedmethroughideas,humor,music,entertainment,conversation,epiphanies(ahamoments),words,andlanguage.Appreciatingsomeone orsomethingcanbeeasy.Learningsomethingwellcanbedifficult,andlearningbiologyisnoexception.Thereasonscientificterminologymatterssomuch whentakingacourselikebiologyisbecausethetermsarethekeytomaking
theconceptscomealive.Admittedly,biologyasksustolearnagaggleofnew words.Eachhighlightedterminthisbookenablesustothinkpurposelyand precisely,whichleadstoclarityandunderstanding.Kudostothewordsmiths whofashionedthesewords,andtothosewhousethesewordsappropriately. AndImustthankthewordsforbringingsomuchmeaningtoourlives.
ThankstoF.JosephSpielerformentoringme,andtotheWileyfamily members,andthoseinthefamily’semploy,whohavecontinuallycraftedclassics fromwriter’sthoughtsandwordsformorethan212years.Ialsothankmy colleaguesatYaleUniversity,CornellUniversity,NewYorkUniversity, RutgersUniversity,theAmericanMuseumofNaturalHistory,theNational ParkService,theNewYorkCityDepartmentofParksandRecreation,and theUSArmywhoexcelinthefieldsoftheirchoice.Iamalsoindebtedto eachplantandcreature,sentientorotherwise,thatsharetheirsignificance inwaysthatenableustovaluetheworldaroundus,andintheprocesshave empoweredmetotransfermyappreciationandunderstandingtoyou.
Thebiospheredoesnotliveinavacuum.Weresideinanatmosphere ofconcatenations.Theseurbanandsuburbanecosystemsofourmakingare thenewest,fastestgrowing,mostimportantecosystemsintheworld.Eachhas replacedanothervibrantecosystem,withconsequencestospecies,communities,families,andcultures.Eachwasreplacedbyanotheramalgamofspecies, communities,families,andcultures.Theysaywemakeourownluck.However thisworks,Ihaveproventobeaveryluckyperson.Nooneeversaidliving aconstructiveandproductivelifeiseasy.I’vestayedontrackbecauseI’vehad exceptionalpeoplesupportingmyeffortsateverystep.ThoughIalreadymentionedmostofthemabove,somedeservetobementionedagain.Becausemy great-grandparentsandgrandparentscametothiscountrywhentheydid,my immediatefamilyescapedthe einsatzgruppen (theSchutzstaffelparamilitarydeath squadsofNaziGermany).Thisiswhymygrandparents,DaveandEstherLipmanandSamandEvaGarber,andmyparents,MitchandMimiGarber,andmy wife’sparents,BillandReevaLedewitz,andouroutstandingsons,Micahand JeremyGarber,andMichael,andArielHaendler,andtheirmagnificentwives, KristinHaendlerandStellaYeo,andmysensible,considerate,compassionate, accomplished,andbeautifulwife,RuthieHaendler,andourgranddaughter, VioletHaendler,havethistimetogether,makingthemostofthehereandnow, doingourbesttosecureasafe,healthy,andhappyfuture.Thepersonwho makeseachofmydaysmoreperfectisRuthie.Wemet65yearsagoandthis lifetimewehavetogether,isbetterthananyImighthaveimagined.
Also,becauseMicahandJeremybothtookbiologycoursesveryrecently, theyhavebeenthebestsoundingboardsaboutwhatworksandwhatdoesn’t workinthisbook.Theyhelpedduringeachstepofwritingthethirdedition ofthisbook,andalthoughtheyaren’tproperlycreditedanywhere,perhaps oneorbothwilldomethehonorofcoauthoringthefourtheditionofthis biologybook.
1 OriginofLife
Theevolutionoflifeonearthhasinvolvedthefollowingsequenceofevents. Thefirstlivingthingstoappearwerethesimplestcreatures,one-celledorganisms.Fromthesecamemorecomplex,multicellularorganisms.Becomingmore complexmeantmorethanjustanincreaseincellnumber.Withmorecellscame cellularspecialization,wherecertaincellswithinthemulticellularorganismcarriedoutspecifictasks.Millions,evenbillionsofyearsoforganismalchangesled tothelivingthingswenowcallplantsandanimals.
Sincethisbasicsequenceofeventsisinaccordwiththatagreeduponby mostgeologists,paleontologists,biologists,physicists,andeventheologians,one mightconcludethatMosesandDarwinwereallkeenobserversandsomewere excellentnaturalistswhowereabletologicallyassessthemostprobablecreation story.
Scientistsgenerallyconcurthatthetimefromtheformationofoursolar systemuntilnowhasbeenontheorderofsome4.5billionyears.Those whobelievetheworldasweknowitwascreatedinsixdaysareoftencalled creationists. TheirmethodofinquiryisbasedonthebeliefthattheBible istobeacceptedasacompletelyaccurateaccountingofallaboutwhichit speaks. Scientists relyonanapproachtounderstandingtheworldaround usthatinvolvesthe scientificmethod, whichishowtheytesthypotheses andtheoriestolearnmorewhiledevelopingnewconcepts,ideas,andmodels thatcanalsobetested.Ofcourse,manygoodscientistsarecreationists.Even thoughthetwoareoftencomparedandcontrasted,creationismisnotascience. Idonotmeantosingleoutcreationism.Icouldspeakthesamewayofso manyotherfieldsofendeavor,suchaspoliticalscience,whichisprobablymore likecreationismthananyhardscienceIknow,becausepoliticalbeliefsarelike religiousbeliefs.Whilemanyholdthemclosetotheirhearts,thesebeliefs arebasedonfeelings,andonacamaraderiesimilartowhatsportsfansshare whorootforthesameteam.Supportingapartyorapersonismuchdifferent thanbuildingandtestingnewideas,basedonprovenfacts,butallthatisfor anotherbook.
SPONTANEOUSGENERATION
Nottooawfullylongago,peoplebelievedthatmanyoftheorganismsthat livearounduscontinuallyarisefromnonlivingmaterialinamannerthey
called spontaneousgeneration. Thisconcepthadmanyadherentsforover athousandyears.Aristotlebelievedinsectsandfrogsweregeneratedfrom moistsoil.Otherelaborationsonthisbasicthemeprevailedforcenturies.It wasn’tuntil1668thatFrancescoRedi,anItalian,challengedtheconceptof spontaneousgenerationwhenhetestedthewidespreadbeliefthatmaggots weregeneratedfromrottingmeat.Heplaceddeadanimalsinaseriesofjars, someofwhichwerecoveredwithafinemuslinthatkeptfliesoutwhile allowingairin.Theflieswereunabletolandonthemeatinthecoveredjars, andnomaggotsappearedthere.Otherjarscontainingdeadanimalswereleft open.Maggotsappearedonlyonthemeatinthejarsthatwereleftopen.In these,flieshadbeenabletolaytheireggs,whichthenhatchedintoflylarvae, ormaggots.Fromthisheconcludedthatmaggotswouldariseonlywhere fliescouldlaytheireggs.ThisexampleshowshowRediusedthescientific methodtotestthehypothesis,whichisanotherwordforanexplanation. Thishypothesisthatwasacceptedatthattimestatedthatfliesarosefrom nonlivingmaterial.Itshouldbenotedthatthishypothesiswasbasedonvery littleevidence.Redi’sexperimentsfailedtosupporthishypothesis.
Onelastvestigeofmysticisminthedebateconcerningspontaneousgenerationhadtobeinvalidatedbeforetheoriesregardingtheoriginoflifecould moveahead;thiswasknownasthe vitalistdoctrine. Adherentsofthevitalist doctrinemaintainedthatlifeprocesseswerenotdeterminedsolelybythelaws ofthephysicaluniverse,butalsopartlybysome vitalforce, or vitalprinciple.
Fortheoriesaboutlifetomoveforward,scientistswouldhavetoagreethat allorganismsarisefromthereproductionofpreexistingorganisms.Forthisto happen,theconceptofspontaneousgenerationwouldhavetobelaidtorest. Duringthenineteenthcentury,manyscientistswerenotyetconvincedthat microorganismsdidnotarisespontaneously,andhopedthescientificmethod wouldbedeployedinwaysthatwouldmovethedebateforward.
ItwasLouisPasteurinFrance,andJohnTyndallinEngland,whousedthe scientificmethodtotestthetheoryofspontaneousgenerationwithmicroorganisms.Throughexperimentation,theydemonstratedthatbacteriaarepresentin theair,andiftheairsurroundingaheat-sterilizednutrientbrothisbacteria-free, thenthebrothremainsbacteria-free.
CONDITIONSFORTHEORIGINOFLIFE
Theleadingtheoryforhowtheuniversebeganisthat13.8billionyearsago, whenspace,time,matter,andenergyaswecurrentlyknowithadyettoform, fromtheexplosionofacondensed,hypotheticsinglepoint.Scientistsalso believethatbillionsofyearslater,afterstarshadformed,onethatexploded
createdacloudofgasanddust,andthenduetogravitationalforces,thesegases anddustparticleseventuallycoalescedintotheplanetarysystemsurrounding astarthatwearepartof.Theseplanetsandoursunformedabout4.5billion yearsago.
Inthissolarsystem,thelargestmasstocoalescebecameoursun,andone ofthesmallermassesbecameourearth.Onearth,theheaviermaterialssankto thecoreoftheplanet,whilethelightersubstancesarenowmoreconcentrated atthesurface.Amongthesearehydrogen,oxygen,andcarbon–important componentsforalllifethateventuallyevolved.
Theprimordialatmosphereonearthwasconsiderablydifferentfromthat whichcurrentlyexists.Thepresentatmosphericgasesarecomposedprimarily ofmolecularnitrogen(N2 ,about78%)andmolecularoxygen(O2 ,almost21%), withasmallamountofwatervapor(H2 Oabout1%atsealevelandabout0.4% onaveragethroughouttheentireatmosphere),aswellasmuchsmalleramounts ofargon(Ar,almost0.1%),carbondioxide(CO2 ,about0.04%)andmanyother gases,suchashelium(He),methane(CH4 ),neon(Ne),andnitrousoxide(N2 O) whichismorecommonlycalledlaughinggas.Theselastgasesoccurinonly traceamounts.Watervaporisagreenhousegas,asarecarbondioxide,methane, andnitrousoxide.Theconcentrationofwatervaporincreasesastheaverage temperatureoftheearth’satmosphereincreases.Theconcentrationofnitrous oxideintheatmosphereincreasesduetoagriculture(fromfarmanimalsand fertilizer),fossilfuelcombustion.Andsewage.
Thecompositionoftoday’satmospherediffersmarkedlyfromthatfound herewhenlifewasjustbeginningtoevolve.Atthattime,theatmosphere containedfarmorehydrogen,andunlikenow,therewasverylittleoxygen.In suchanatmosphere,thenitrogenprobablycombinedwithhydrogen,forming ammonia(NH3 );theoxygenwasprobablyfoundcombinedwithhydrogenin theformofwatervapor(H2 O),andthecarbonoccurredprimarilyasmethane (CH4 ).Themoderatelyhightemperaturesoftheearth’scrustcontinually evaporatedliquidwaterfromrainintowatervapor.Astheearthcooled, rainwateraccumulatedinlow-lyingareas.Theserainsalsowasheddissolved mineralsintothebodiesofwater,whichdependingonsizeandsalinity,are definedeitheraslakes,seas,oroceans.Inaddition,volcanicactivityerupting intheoceansandonlandbroughtothermineralstotheearth’ssurface,many ofwhicheventuallyaccumulatedintheoceans,suchasthevarioustypes ofsalts.Itshouldalsobementionedthatlongbeforetherewasanylifeon earth,theseascontainedlargeamountsofthesimpleorganiccompound methane.Mostofthecompoundsnecessaryforthedevelopmentoftheinitial stagesoflifearethoughttohaveexistedintheseearlyseas.Otherstudies haveindicatedthatsuitableenvironmentsforthefirststepsleadingtoliving materialcouldhaveexistedelsewhereaswell.Buttheseenvironmentsarestill
poorlyunderstood,andtheirpotentialconnectionwiththeoriginoflifeis unclear.Itwasonlyaftercyanobacteriaevolvedover2billionyearsagoand lateralgaeandmoremodernplantsthattheconcentrationofatmospheric oxygenbegantoincreaseprecipitously.Youmightsayplantspollutedthe earth’searlyatmospherebyreleasingsomuchoxygen.Andyet,ifitwerenot fortheplantsthatcontinuallyproduceoxygenintotheatmosphere,organisms likeusthatneedoxygentosurvivecouldnothaveevolvedandflourished. Nowthathumansareburningcarbonsourcesthatwerestoredinandunder thegroundforthousands,andsometimesformillionsofyears,weareadding thiscarbontotheatmosphereintheformofcarbondioxide.Thisisgoodfor plants,whichthriveinanatmospherewithelevatedlevelsofcarbondioxide. However,manypeopleareconcernedthattheincreasingquantitiesofcarbon dioxidemayaffecttheweather.(Formoreonthistopic,seeChapter16, entitledEcology.)
EXPERIMENTALSEARCHFORLIFE’SBEGINNINGS
Intheearlytwentiethcentury,J.B.S.Haldane,ascientistwhowasbornin BritainanddiedinIndia,andS.I.Oparin,aRussianbiochemist,investigated howlifecouldhaveevolvedfromtheinorganiccompoundsthatoccurredon earthbillionsofyearsago.Theirworkiscreditedwithleadingtoimportantlater advances,mostprominentofwhichwereStanleyMiller’sexperimentsduring the1950s.Millerduplicatedthechemicalconditionsoftheearlyoceansand atmosphereandprovidedanenergysource,intheformofelectricsparks,which generatedchemicalreactions.Whenwarmwaterandgasescontainingthecompoundspresumedtobefoundintheearlyoceansandintheearth’sprimordial atmosphereweresubjectedtosparksforaboutaweek,organiccompounds formed.
Experimentsthatfollowed,suchasthoseperformedbyMelvinCalvinand SydneyFox(bothAmerican),foundimportantso-calledbuildingblocksoflife, theaminoacidsthatmakeupproteins,readilyformundercircumstancessimilar tothosethatwerefirstestablishedexperimentallybyStanleyMiller.
Thethinfilmofwaterfoundonthemicroscopicparticlesthatmakeclayhas beenshowntopossesstheproperconditionsforimportantchemicalreactions. Claysserveasasupportandasacatalystforthediversityoforganicmolecules involvedinwhatwedefineaslivingprocesses.EversinceJ.DesmondBernal presented(duringthelate1940s)hisideasconcerningtheimportanceofclays totheoriginoflife,additionalprebioticscenariosinvolvingclayhavebeen
proposed.Claysstoreenergy,transformit,andreleaseitintheformofchemical energythatcanoperatechemicalreactions.Claysalsohavethecapacitytoact asbuffersandevenastemplates.A.G.Cairns-Smithanalyzedthemicroscopic crystalsofvariousmetalsthatgrewinassociationwithclaysandfoundthatthey hadcontinuallyrepeatinggrowthpatterns.Hesuggestedthatthiscouldhave beenrelatedtotheoriginaltemplatesonwhichcertainmoleculesreproduced themselves.Cairns-SmithandA.Weissbothsuggestclaysmighthavebeenthe firsttemplatesforself-replicatingsystems.
Someresearchersbelievethatthroughthemutationandselectionofsuch simplemolecularsystems,theclayactingastemplatemayeventuallyhavebeen replacedbyothermolecules.Andintime,insteadofmerelyencodinginformationforarotetranscriptionofamolecule,sometemplatesmayhavebeenable toencodestoredinformationthatwouldtranscribespecificmoleculesunder certaincircumstances.
Otherscenarioshavebeensuggestedtoexplainhowthemoleculesthat makemoremoleculescouldhavebecomeenclosedincell-likecontainments. SydneyFoxandcoworkersfirstobservedthatmolecularboundariesbetween protein-nucleicacidsystemscanarisespontaneously.Theyheatedaminoacids underdryconditionsandascertainedthatlongpolypeptidechainswereproduced.Thesepolypeptideswerethenplacedinhot-watersolutions,andupon coolingthem,theresearchersfoundthatthepolypeptidescoalescedintosmall spheres.Withinthesesphericalmembranes,or microspheres, certainsubstancesweretrapped.Also,lipidsfromthesurroundingsolutionbecameincorporatedintothemembranes,creatingaprotein-lipidmembrane.
Oparinsaid“thepathfollowedbynaturefromtheoriginalsystemsofprotobiontstothemostprimitivebacteria ... wasnotintheleastshorteror simplerthanthepathfromtheamoebatoman.”Hispointwasthatalthoughthe explanationsintendedtoshowhoworganicmoleculescouldhavebeenmanufacturedinprimitiveseasoronclaysseemquitesimple,andalthoughonecan seehowsuchmoleculescouldhavebeenenclosedinsidelipid-proteinmembranes,takingtheseexperimentalsituationsandactuallycreatinglivingcellsis atremendousleapthatmayhavetaken,attheveryleast,hundredsofmillions ofyears,perhapsconsiderablylonger.
PROBINGSPACEFORCLUESOFLIFE’SORIGINSONEARTH
Recentinformationconcerningtheoriginoflifehasopenednewavenuesof research.Tothesurpriseofmany,spacecraftthatflewpastHalley’sCometin
1986sentbackinformationshowingthecometwascomposedoffarmore organicmatterthanexpected.Fromthat,andadditionalevidence,somehave concludedthattheuniverseisawashwiththechemicalprecursorsoflife.Lynn Griffiths,chiefofthelifesciencesdivisionoftheNationalAeronauticsand SpaceAdministration(NASA),said,“everywherewelook,wefindbiologically importantprocessesandsubstances.”
Wehaveknownforyears,fromfossilevidence,thatbacteriaappearedon earthabout3.5billionyearsago,alittlemorethan1billionyearsafterthesolar systemformed.Thegreatchallengehasbeentolearnhow,withinthatfirst billionyears,simpleorganicchemicalsevolvedintomorecomplexones,then intoproteins,geneticmaterial,andliving,reproducingcells.
Asthiscurrenttheorystands,itisfeltthatsome4billionyearsago,following theformationofthesolarsystem,vastquantitiesofelementsessentialtolife, includingsuchcomplexorganicmoleculesasaminoacids,wereshoweredonto earthandotherplanetsbycomets,meteorites,andinterstellardust.Nowseenas thealmostinevitableoutcomeof chemicalevolution, theseorganicchemicals evolvedintomorecomplexmolecules,thenintoproteins,geneticmaterial,and living,reproducingcells.
Unfortunately,notracesofearth’schemicalevolutionduringthecriticalfirst billionyearssurvive,havingallbeenobliteratedduringthesubsequentbillion years.Biologistsandchemistsnowfeel,however,thatcluesconcerningthefirst stagesintheoriginoflifeonearthcanbefoundbylookingelsewhereinthesolar system.Planetaryscientistsaretobelaunchingnewprobesthatwilleventually investigatethesequestions,lookingforevidencerevealingthepathsofchemical evolutionthatmayhaveoccurred,ormaystillbeoccurring,onplanets,moons, comets,andasteroids.
PANSPERMIA
Althoughmostmoderntheoristsdonotaccepttheideathatlivingorganismsare generatedspontaneously,atleastnotunderpresentconditions,mostdobelieve thatlifecouldhaveandprobablydidarisespontaneouslyfromnonlivingmatter underconditionsthatprevailedlongago,asdescribedabove.Otherhypotheses havealsobeensuggestedfortheoriginoflifeonearth.
In1821,theFrenchmanSales-GuyondeMontlivaultdescribedhowseeds fromthemoonaccountedfortheearliestlifetooccuronearth.During the1860s,aGerman,H.E.Richter,proposedthepossibilitythatgerms carriedfromonepartoftheuniverseaboardmeteoriteseventuallysettled onearth.However,itwassubsequentlyfoundthatmeteorictransportcould
bediscountedasareasonablepossibilityforthetransportoflivingmatter becauseinterstellarspaceisquitecold(−220 ∘ C)andwouldkillmostformsof microbiallifeknowntoexist.Andevenifsomethinghadsurvivedonameteor, reentrythroughtheearth’satmospherewouldprobablyburnanysurvivorsto acrisp.
Tocounterthesearguments,in1905aSwedishchemist,SvanteArrhenius, proposedacomprehensivetheoryknownas panspermia.Hesuggestedthat theactualspacetravelerswerethesporesofbacteriathatcouldsurvivethelong periodsatcoldtemperatures(somebacterialsporesinCarlsbad,NewMexico, survivedfor250millionyearsandwererecentlyrevived),andinsteadoftraveling onmeteorsthatburnedwhenplummetingthroughtheatmosphere,thesespores movedalone,floatingthroughinterstellarspace,pushedbythephysicalpressure ofstarlight.
Themainproblemwiththistheory,overlookedbyArrhenius,isthatultravioletlightwouldkillbacterialsporeslongbeforetheyeverhadachanceto reachourplanet’satmosphere.Thisexplainsthenextmodificationtothetheory. However,itisconceivablethatlifeexiststhroughouttheuniverseandisspread throughspacebyasteroids,comets,andmeteoroids.Also,itshouldbenoted thataNASAscientistpublishedareportthatfossilizedbacteria-likeorganisms havebeenfoundonthreemeteorites.Hesaidthesefossillifeformsarenot nativetoearth.
Anotherpossiblewaylifehasspreadisduetospacecraft.FrancisCrick, whoalongwithJamesWatsonreceivedtheNobelPrizefordiscoveringthe structureofDNA,coauthoredanarticlewithLeslieOrgel,abiochemist,in 1973.Theirarticle,“DirectedPanspermia,”wasfollowedbythebook LifeItself, inwhichCricksuggeststhatmicroorganisms,duetotheircompactdurability, mayhavebeenpackagedandsentalongonaspaceshipwiththeintentionof infectingotherdistantplanets.TheonlylinkmissingfromCrick’shypothesis wasamotive.However,itispossiblethatmicroorganismsareunintentionally introducedtoplanetsandmoonseachtimeaspacecraftlandsonone.
KEYTERMS
SELF-TEST
Multiple-ChoiceQuestions
1.Peoplewhobelievethebiblicalexplanationthattheworldandallitscreatureswerecreatedinsixdaysareknownas:
a.evolutionarybiologists
b.molecularbiologists
c.systematists
d.cladists
e.creationists
2.Scientistsusewhattheycall__________,whichallowsthemtotesthypothesesandtheoriesandtodevelopconceptsandideas.
a.Occam’srazor
b.religiousdogma
c.religiousfaith
d.scientificmethod
e.creationscience
3.Aristotlebelievedinsectsandfrogsweregeneratedfromnonlivingcomponentsinmoistsoil.Thisearlyhypothesisconcerningtheoriginofliving organismsisknownas__________.
a.evolution
b.spontaneousgeneration
c.materialism
d.creationism
e.Aristoteliangeneration
4.Adherentsofthe__________maintainedthatlifeprocesseswerenotsolely determinedbythelawsofthephysicaluniverse,butrather,theyalsodepend onsomevitalforce,orvitalprinciple.
a.dogmaticprinciple
b.Darwinianapproach
c.vitalistdoctrine
d.Lamarckianprinciple
e.alloftheabove
5.Thecompositionoftoday’satmospherediffersmarkedlyfromthatfound herewhenlifewasjustbeginningtoevolve.Atthattimetheatmosphere containedfarmore__________.
a.hydrogen
b.oxygen
c.potassium
d.iridium
e.alloftheabove
6.Whenthechemicalconditionsoftheearlyoceansandatmosphereareduplicatedinthelabandprovidedwithanenergysourceintheformofelectric sparks,__________(has)havebeenformed.
a.life
b.organicmolecules
c.aminoacids
d.aandb
e.bandc
