HistoryofLifeonEarth

Chapter14

TheAgeofEarth

• Whenearthformed4.6billionyearsagoitwasaafieryballofmoltenrock

• Eventuallythemoltenrockcooledandformedthecrust

• Watervaporintheatmospherecondensedtoformtheoceans

• Mostscientiststhinkthatlifearoseoutoftheseearlyoceans

• Rockshavebeendatedback,sothissupportstheideaofagradualchange

MeasuringEarth’sAge

• Radiometricdating-istheestimationoftheageofanobjectbymeasuringitscontentofcertainradioactiveisotopes

• Anisotopeisaformofanelementwhoseatomicmassdiffersfromthatofotheratomsofthesameelement

• Radioisotopes-areunstableisotopesthatbreakdown&giveoffenergyintheformofchargedparticles(radiation)

MeasuringEarth’sAge• Thebreakdown,calledradioactivedecay,resultsinother

isotopesthataresmallerandmorestable

• Thetimeittakesfor½ofagivenamountofaradioisotope

todecayiscalledtheradioisotope’shalf-life

• Bymeasuringtheproportionsofcertainradioisotopesand

theirproductsofdecay,scientistscancomputehowmany

half-liveshaspassedsincetherockwasformed

FormationoftheBasicChemicalsofLife

• Mostscientistthinkthatlifeonearthdevelopedthru

naturalchemicalandphysicalprocesses

• Peoplethinkthatthepathtodevelopmentoflivingthings

beganwhenmoleculesofnon-livingmatterreacted

chemicallyduringthe1stbillionyearsofEarth’shistory

• Thesechemicalreactionsproducedmanydifferent

simple,organicmolecules

FormationoftheBasicChemicalsofLife

• Energizedbythesunandvolcanicheat,thesesimple,organicmoleculesformedmore-complexmoleculesthateventuallybecamethebuildingblocksofthe1stcells

• Thehypothesisthatorganicmoleculesnecessaryforlifecanbemadefromnon-livingmoleculeshasbeentestedandsupportedbyexperiments

The“PrimordialSoup”Model

• 1920’s,RussianscientistsA.I.OparinandBritishscientists

J.B.SHaldanebothsuggestedthatearlyearth’soceans

containedlargeamountsoforganicmolecules

• Thishypothesisbecameknownastheprimordialsoupmodel

• Theyhypothesizedthatthesemoleculesformed

spontaneouslyinchemicalreactionsactivatedbyenergyfrom

sunlight,lightning,&volcanoes

The“PrimordialSoup”Model

• Oparin,togetherwithAmericanscientistsHaroldUrey,&

othersproposedthat’searth’searlyatmospherelacked

oxygen

• Richinnitrogen,hydrogen,andhydrogen-richgasessuchas

(watervapor;H20,ammoniaNH3;andmethaneCH4)

• Theyreasonedthatelectronsinthesegaseswouldhavebeen

frequentlypushedtohigherenergylevelsbylightparticles

fromthesunorlightning

The“PrimordialSoup”Model

• Today,highenergyelectronsarequicklysoakedupbytheoxygenintheearth’satmosphereb/coxygenatomshaveagreat“thirst”forelectrons

• Sow/outoxygentheelectronswouldhavereactedwithhydrogenmolecules,formingarichvarietyoforganiccompounds

The“PrimordialSoup”Model:Urey/MillerExperiment

• 1953,thishypothesiswastestedbyStanleyMiller,whowasworkingwithUrey

• Millerplacedthegasestheythoughtwasabundantonearlyearthintoadevice

• Tosimulatelightning,heusedsparks

The“PrimordialSoup”Model:Urey/MillerExperiment

• Afterafewdays,Millerfoundacomplexcollectionoforganicmoleculesinhisdevice

• Hefound:aminoacids,fattyacids,andotherhydrocarbons

• Theseresultsupportthehypothesisthatsomebasicchemicalsoflifecouldhaveformedspontaneouslyunderconditionslikethoseintheexperiment

ReevaluatingtheUrey/MillerModel

• WenowknowthatamixtureofgasesusedinMiller’sexperimentcouldn’thaveexistedonearlyearth

• 4bya,earthdidn’thaveaprotectivelayerofozonegas,O3

• TodayozoneprotectsfromharmfulUVRaysfromthesun

• W/outozonetheUVrayswoulddestroyanyammoniaormethaneintheatmosphere

ReevaluatingtheUrey/MillerModel

• W/outthesegases,nokeybiologicalorganicmoleculesweremade

• Ifthechemicalsneededtoformlifewerenotintheatmosphere,wheredidtheycomefrom?

• Somescientistsarguethatchemicalscamefromoceanbubblesordeepseavents

TheBubbleModel

• 1986,geophysicistLouisLermansuggestedthatthekeyprocessesthatformedthechemicalsneededforlifetookplacewithinbubblesontheocean’sfloor.

Precursorsofthe1stCells

• Thereareenormousdifferencesbetweensimpleorganicmolecules&largeorganicmoleculesfoundinlivingcells

• Inlabs,scientistshaven’tbeenabletomakeeitherproteinsorDNAfromspontaneouslyinwater

• However,shortchainsofRNA,thenucleicacidthathelpscarryoutDNA’sinstructions,havebeenmadetoformontheirowninwater

APossibleRoleasCatalysts• Inthe1980s,AmericanscientistsThomasCechandSidneyAltmanfoundthatcertainRNAmoleculescanactlikeenzymes

• RNA’s3-Dstructureprovidesasurfaceonwhichchemicalreactionscanbecatalyzed

• mRNAactsasaninfo-storingmolecule

• ByCech’s&Altman’sworkonotherexperimentsshowingthatRNAmoleculescanformspontaneouslyinwater;ahypothesiswasformed

APossibleRoleasCatalysts

• RNAwasthe1stself-replicatinginformation-storagemoleculeanditcatalyzedtheassemblyofthe1stproteins.Moreimportant,suchamoleculewouldhavebeencapableofchangingfrom1generationtothenext

Microspheres&Coacervates

• Observationsshowthatlipids,whichmakeupcellmembranes,tendtogathertogetherinwater

• Byshakingoilandvinegarinabottleyouseesmallspheresofoilforminthevinegar

• Certainlipids,whencombinedwithothermolecules,canformatinydropletwhosesurfaceresemblesacellmembrane

Microspheres&Coacervates

• Similarly,experimentshaveshownthat,inwater,shortchainsofaminoacidscangatherintotinydropletscalledmicrospheres

• Anothertypeofdropletcalledacoacervateiscomposedofmoleculesofdifferenttypes,includinglinkedaminoacidsandsugars

• Scientiststhinkthatformationofmicrospheresmighthavebeena1ststeptowardscellularorganization

Microspheres&Coacervates

• Accordingtothishypothesis,microspheresformed,persistedforawhile,andthendispersed

• Overmillionsofyears,thosemicrospheresthatcouldpersistlongerbyincorporatingmoleculesandenergywouldhavebecomemorecommonthanshorterlastingmicrosphereswere

Microspheres&Coacervates

• Microspherescouldn’tbeconsideredtruecells,however;unlesstheyhadthetraitsoflivingthings,includingheredity

OriginofHeredity

• ManyagreethatDNAevolvedafterRNAandthat“RNAenzymes”catalyzedtheassemblyoftheearliestproteins

• SomeagreethatmicrosphereswithRNAwereabletotransfertheirtraitstooffspring

• Butwereallydon’tknow

TheEvolutionofProkaryotes

• Whendidlifeform?• Westudyfossils• Fossil-isthepreservedormineralizedremainsorimprintofanorganismthatlivedlongago

• Oldestdateback2.5billionyears

850 million year old fossil “cyanobacteria”

TheEvolutionofProkaryotes

• Recallthatprokaryotesaresingle-celledorganismsthatlackinternalmembrane-boundorganelles

• Amongthe1stweremarinecyanobacteria• Cyanobacteria-arephotosyntheticprokaryotes• B4cyanobacteriaappeared,oxygenwasscarceonearth,butthecyanobacteriagaveoffoxygenasabi-product

• Thisoxygeneventuallyescapedtotheair

2GroupsofProkaryotes

• Earlyinthehistoryoflife,2differentgroupsofprokaryotesevolved-eubacteria(whicharenowknownasbacteria)andarchaebacteria

• Eubacteriacontainachemicalcalledpeptidoglycanintheircellwalls

• Archaebacterialackpeptidoglycanintheircellwallsandhaveuniquelipidsintheircellmembranes

2GroupsofProkaryotes

• Archaebacteriaarethoughttocloselyresemblethe1stprokaryotestohaveexistedonearth

• Chemicalevidenceindicatesthatthefirsteukaryoticcellsweremorelikelytohaveevolvedfromarchaebacteriathanfromeubacteria

E.ColiEubacteria

Archaebacteria

TheEvolutionofEukaryotes

• ~1.5bya,the1steukaryotesappeared• Almostalleukaryoteshavemitochondriasandchloroplastsarefoundonlyinprotistsandplants

• Mitochondriaandchloroplastarethesizeofprokaryotes,andtheycontaintheirownDNA

TheOriginofMitochondriaandChloroplast

• Mostscientiststhinktheseoriginatedasdescribedbythetheoryofendosymbiosis

• Itwasproposedin1966byLynnMargulis• Itproposesthatmitochondriaarethedescendantsofsymbiotic,aerobiceubacteriaandchloroplastsarethedescendantsofsymbiotic,photosyntheticeubacteria

TheOriginofMitochondriaandChloroplast

• AccordingtoLynnMargulis’stheoryofendosymbiosis,bacteriaenteredlargecellseitherasparasitesorasundigestedprey

• Insteadofbeingdigested,thebacteriastartedtoliveinthehostcell,wheretheyperformedeithercellularrespirationorphotosynthesis

Evidenceforthetheory

• Sizeandstructure:mitoandchloroplastarethesamesizeasmosteubacteria.Botharedoublemembraned.ThesmoothinnermembraneofmitoisthoughttobederivedoffromtheERofalargerhostcell,alsotheinnermembraneisfoldedmuchlikethemembranesofmanyaerobicbacteria

Evidenceforthetheory

• Geneticmaterial-mitoandchloroplasthavecircularDNAsimilartochromosomesfoundinbacteria;mitoandchloroplastcontaingenesthataredifferentfromthosefoundinthenucleusofthehostcell

Evidenceforthetheory

• Ribosomes-mitoandchloroplastribosomeshavesimilarsizeandstructuretothatofbacterialribosomes

• Reproduction-likebacteria,mitoandchloroplastreproducebysimplefission.Thisreplicationtakesplaceindependentlyofthecellcycleofthehostcell

Multicellularity

• Manybiologistgroupalllivingthingsinto6broadcategoriescalledkingdoms

• The3oldestkingdoms,eubacteriaandarchaebacteria,aremadeupofsingle-celledprokaryotes

• The1steukaryotickingdomwasthekingdomProtista• Protistsareagroupthatconsistofmulticellularandunicellularorganisms

Multicellularity

• Theunicellularbodyplanhasbeenverysuccessful,½ofearth’sbiomassisunicellular

• Butmulticellularhashadtheopportunitytoariseb/cmanytimescellsgetspecialized

• Almosteveryorganismthatcanbeseenwiththenakedeyeismulticellular

• Fungi,plants,andanimalsevolvedindependentlyfromprotists

OriginofModernOrganisms

• MostphylathatexisttodayprobablyoriginatedduringtheCambrianperiodfrom540millionto505mya.

• Manyunusualmarineanimalsappearedatthistime,thattherearenocurrentlivingrelatives

• AveryrichcollectionofCambrianfossilswasuncoveredinCanadacalledtheBurgessShalein1909

OriginofModernOrganisms

• TheOrdovicianPeriod,whichfollowedtheCambrianlastedfromabout505to438mya

• Duringthistimemanymarineanimalscontinuedtothriveintheseasuntilabout250mya

MassExtinctions

• ThefossilrecordindicatesthatasuddenchangeoccurredattheendoftheOrdovicianPeriod

• About440mya,alargepercentageofEarth’sorganismsbecameextinct

• Extinction-deathofallthemembersofaspecies• Thiswasthe1stof5majormassextinctions

MassExtinctions

• MassExtinction-isanepisodeduringwhichlargenumbersofspeciesbecomeextinct

• AnotherM.E.ofaboutthesamesizehappenedabout360mya

• The3rdandmostdevastatingofalloccurredattheendofthePermianPeriodabout245mya

• About96%ofalllivingspeciesdied

MassExtinctions

• About65mya,afourth,lessdevastatingM.E.occurred

• Althoughthespecificcausesoftheseextinctionsareunknown,evidenceindicatesthatworldwidegeologicalandweatherchangeswerelikelyfactors

• The5thM.E.occurredabout35mya;andkilled2/3ofalllandbasedanimals