YOURLIFEASPLANETEARTH

AdvancePraisefor

YourLifeasPlanetEarth

“Whatafunread!Andalotofgreatandimportantconclusions.Ihopeitfindsawideaudience,because theywill love itsuniqueapproachtomakingscience interestingandaccessible,yetcomplete.”

ProfessorErikAsphaug,UniversityofArizona,

authorofImpactCratersintheSolarSystem

“Ihighlyrecommendthisengagingbookforanyonewantingtolearnmoreaboutthe planet we live on. Using the clever metaphor of Earth’s planetary evolution as anindividualpassing through thevarious stagesof life, “YourLifeasPlanetEarth” tellsaremarkabletaleofaplanetthathaswitnessedtumultandturmoil,andexperiencedthefullrangeofextremesfromicehousetoinferno.YetthebookleavesnodoubtthatEarthfacesher greatest challenge of all today, confronted withmassive and unprecedented humaninterference,beitmassextinction,oceanacidification,orperhapsthegreatestthreatofall—human-causedclimatechange.”

MichaelE.Mann,DistinguishedProfessorPennStateUniversity,

authorofTheHockeyStickandtheClimateWars.

YOURLIFEASPLANETEARTH_________

AnewwaytounderstandthestoryoftheEarth,itsclimateandourorigins.

_________

HowardLee

Copyright©2014HowardLee.

Allrightsreserved.Nopartofthisbookmaybereproducedinanymannerwhatsoeverwithoutwrittenpermissionexceptincaseofbriefquotationsembodiedincriticalarticles

andreviews.

1stEdition–e-bookversion.

ISBN:978-0-9766137-1-8

ylape.com

CoverdesignedbyHowardLeeusingimagesbyNASA.

DedicatedtoScoutseverywhere.

Keepleavingtheworldbetterthanyoufindit.

“Thekeystothefutureofourspeciesandourplanet

havealwayslaininthedeeppast”RichardFortey,FRS

Contents

PREFACEPART1:YOURLIFEASPLANETEARTH

Abbreviations

1.TRAUMATICCHILDHOODYournear-deathexperienceYourfirstmemories

2.TROUBLEDTEENAbadcaseofacneYou’reateenparent!

3.MID-LIFECRISESMaturingintoyour30sIt’sagas!

4.RETIREMENTFeelingcoldinyour80s-SlushballEarth#2At86-GettingacomplexAt88–Toprotectandtoprey

5.OLDAGEANDHEARTATTACKSAt90–Heartattack!JawsAt91-TheinventionofwalkingAt92–AdoubleheartattackAge92to93-SwampsandgiantbugsAt93–Theinventionoftheeggshell

6.THEDAYYOUSETYOURSELFONFIREAge94-DawnofthedinosaursBacktotheburnunit95to96–Jurassicgiants

7.ANOVERAGEDHIPPIE!At98–grassandchalk

8.HOTFLASHESAtlanticfizzTrainwreck

9.YOURLASTYEAR-ACHILLERCentralheatingfailureSeasofgrassVolcanoesandinstantdiamondsThedaytheseadriesupOficeandman

10.YOUR100THBIRTHDAY!Yourlonggoodnight

11.BOTTOMLINE

PART2:TECHNIQUESOFTIMETRAVEL1.ORIGIN

CosmicsedimentsMoonformationThelostelements–cluestoEarth’schildhood

2.THETERRAFIRMAILLUSIONDriftingcontinentsTheplumbingofHadesDidtheEarthmoveforyoutoo?Bottomline:

3.PROOFOFLIFEEvolutionbeforeoureyesNotyourgrandfather’sfossilsMissinglinks–found!BreadcrumbsofevolutionBottomLine:

4.THECLOCKSINROCKSSandsoftimeRadiation-freeproofsofanoldEarthRadioactiverock-clocksDatinghumansDatingfossilswithSHRIMPAgesoflavasandgranitesDatingsedimentsCookingtime–datingmetamorphicrocksAntiquityofmeteoritesandimpactsTheupsanddownsofmountainsBottomline:

5. DIPPINGATOE INANCIENTSEAS,BREATHINGTHEAIROFLOSTWORLDS

ThermometersfortimespastIcecoresBarometersforoldatmospheresAbriefhistoryofspaceweatherFossilraindropsandpackraturineRock-solidevidenceofancientclimatesBottomline:

6.THEFUTUREACCORDINGTOTHEPASTThepowerofhumansWearethecauseUncertaintyinclimatechangeSignpostsfromthepastWhycare?-PrognosisWhattodoaboutitBottomline:

ACKNOWLEDGEMENT

SOURCES

Illustrations1.TheInfantEarthandthenewly-ignitedSun

2.Moon-formingimpact

3.Pre-teenEarth

4.Stromatolitesunderpinkmethaneskiesinyourlate30s

5.SupercontinentRodiniabreaksapart.

6.SlushballEarth–aplanetfrozenitothetropicsinyour80s

7. Protect and Prey - life in theCambrian oceans.Anomalocaris about to pounce on aMarrella

8.Earlylandplants:Aglaophytonbristleacrosstheland

9.Theworld450millionyearsago,showingtheHirnantianglaciation

10.Carboniferousswampforest

11.Theworld300millionyearsago,withglaciation

12.Theworld200millionyearsago,atthebirthoftheAtlanticOcean

13.Theworld100millionyearsago

14.Thefirstcook:Homoerectus

15. Embryonic solar systems: Hubble Space Telescope image of the Orion Nebulashowing5youngstarssurroundedbygasanddustdisksthatmaygoontodevelopplanets

16.The“BigSplat”collisionof2moonstocreateourpresentsinglemoonwithremnantsofthesmallermoonformingthemountainousfarside

17.GPSmeasurementsshowtectonicmovement

18.SeismictomographyrevealingamantleplumebeneathYellowstone

19. A Large Igneous Province (LIP) eruption compared to the world’s largest activevolcanotoday

20.“Jason”and“Tuzo”superimposedonthe“Geoid”(Earth’sshape).ColorscaleshowshowtheactualShapeoftheglobediffersfromaslightlyflattenedsphere(inmeters)

21.Humanselectioncausingevolution in farmanimals:HistoricalYieldsperAnimal inBritainandHolland

22.AdiversityofforamsfromasinglebeachinBurma.Thephotoisjust5.2mm(2/10in)across

23.525million-year-oldwormburrowsintheTapeatsSandtone,GrandCanyon

24.HumanEvolutionFamilyTree

25.Annually-layeredrock‘clocks:’coralbands,lakevarvesandastalagmitelayers

26.Atomicracetrack:howamassspectrometerdetectsatomsofdifferentmass.

27.Bubblesofpastatmospheres: the tinyspecks in thephotoareairbubbles trapped inice.Theycanbesampledtotellscientiststhelevelsofgassesinatmospheresupto800,000yearsago

28.400millionyearsofCO2levelsfromproxies

29.Sunspotcyclesobservedbacktothe1600sandcanbeusedtocalculatevariationsintheSun’sbrightness(TotalSolarIrradiance)

30.JurassicdesertdunesinZionCanyon,Utah

31.Buildingenergy:radiativeforcingbygreenhousegassesintheatmosphere,relativeto1750levels

32.HowchangesinseawaterchemistryprocessCO2emissions

33.Somefeedbacksareslowerthanothers:Thetimeframesofinputs(forcings)comparedtoclimatefeedbacks

34. Compounding uncertainties when predicting climate change (left) is a bit likecompoundinguncertaintypredictingahurricanepath(right)

35.Agearleverasasimplifiedanalogyforclimatesensitivity

36.Risingfast:worldenergyconsumption

37.Nolet-upinfossilfueluseforelectricitygeneration

38. Shifting climate: the state of New Hampshire could experience a summer climateformerly associated with South Carolina by end-century, if business-as-usualemissionscontinue

PrefaceIfirstbecameawareofhowmuchweareatthemercyofclimatewhilegrowingup

in East Africa. I was on safari in Tsavo National Park hoping to see herds of bushelephant,butinsteadallIfoundwasbarrenreddirtlitteredwithbleachedbones.Droughthad always stalked East Africa but this time the adults around me were calling it“unprecedented”andweremutteringaboutdeforestationas thecause-nobodyspokeofglobalclimatechangeinthosedays.Wealmostranoutofwaterbeforetherainscame.

Schooled on the rimof theGreatRiftValley, I learned that the valley’s lakes –Magadi,Naivasha,Elementaita,Nakuru,Hannington,BaringoandRudolph(astheywereknownthen)hadoncebeenmuchlarger,andthatourHomohabilisancestors,whomadethestonetoolsscatteredatmyfeet,haddonesoat theshoresoflakesnowreducedanddistant.Dormantvolcanoescamealiveinlegendsofneighborsstealingeachother’sflame,andfierydevilsdancing.ItwasinKenya–Inowrealize–thatIfellinlovewithgeologyandallittellsusofourplanet’spast.

Ihavesplit“YourLifeasPlanetEarth”intotwoparts.

Thefirst isafast-pacedandlightheartedtellingof the twistsandsetbacksofourplanet’sevolution,usingthemetaphorofahumanlifetimetomakethestoryofourworldand its climate more easily relatable. I show how the strands of climate, evolution,tectonics and cosmic events tangled through time to drive changes in life and climate.Similarcausestodaymust,logically,causesimilarresults.

In the second part, I explain the scientific evidence behind the story in plainEnglish.Idirectlyaddresssomecommonmythsthatpreventmanypeoplefromacceptinghuman-causedclimatechangeandevolution,andIexplorewhatthesciencetellsusaboutourfutureasfaraswecantellfromwhatweknowofEarth’spast.

Wecan’tmakesenseofourfutureuntilwemakesenseofourpast.

Part1:YourLifeasPlanetEarthTounderstandhowwecametobe,wemuststartatthebeginning.Tounderstand

whereweareheaded,wemust retrace theplanet’sodysseyof tectonicupheavals, life’sadaptations,climatechangesandcosmicchances,totheoriginofitall–Earth,air,water,life.

The Earth made life, and once begotten life began to mold the Earth –fundamentally:chemically,physicallyandclimatically.Earthshapedclimateandclimatewinnowedlife,repeatedlyrewardinginnovation,repeatedlywipingouttheestablishedandthemaladapted.Tounderstandourfuturewemustlearnfromthosetimes.Wearefarfromthefirstinhabitantsoftheplanettohaveexperiencedclimatechange,butwearethefirstcapableofunderstandingit,andthefirsttohavechoicesinhowwerespond.

Iwanttoshowyouhowwegotherebytracingthetangledthreadsoflife,climateandEarththroughdeeptimetotoday.

Butdeep timeisveryhard tograsp.Just like thevastnessofdeepspace, it issomuchbeyondwhatwehumansevolvedtoreckonintuitively.InsteadIwilltellthestoryofus,ourclimateandourEarthonthescaleofahumanlifetime.ForsimplicityIwillmakethatlife100yearslong,andtomakeitrelatableIwillaskyoutoimaginethatYOUareplanetEarth.Afterall,asyouwillsee,yourlifeandthatoftheplanethavebeenuncannilysimilar.

Abbreviationskmkilometers

mimiles

°CdegreesCelsius

°FdegreesFahrenheit

cmcentimeters

ininches

CO2carbondioxide

1.TraumaticChildhood

Agentlesmackbringsyouintoexistence-amerespeckamongspecks,swaddledinsootandbathedinthesoftglowoftheMilkyWay.Anorphanofoldstarswhodiedanexplosivedeath,youaredustwithindustinavastcloudoftheirashes.Yourmidwifeisanearbysupernova,hershockwavethesmackthatawakenedyou.Itnudgedyouintothedustaroundyou,andgentlegravitynowgathersdusttograins,grainstoclumps.

Youarenotalone.Yoursiblingsareallaroundyou,specksandclumpslikeyou,floatingindarkdustyspace.Oneclumpofspecksisgrowingfasterthanyouandallyoursiblings.Beingheavier, its gravitydraws in evermoregas anddust in runawaygrowthwhichbeginstoformtheSun.Theweightofallthatjunkcompressesittighterandtighterinto a ball andheats it to an ember, a crimsonnight light for your inter-stellar nursery.Aroundthisglow, theimmensenebulaofdustandgas–withyouin it–starts tospiralinwardstowardsthenewSun,andthismotionflattensthenebulaintoadiskthatrotatesaroundtheSunatitscenter.Itisthisrotation–yourorbit-thatprotectsyoufromfallingtoaninfantdeathinthehearthatthecenteroftheSolarSystem.Overyourfirst3weeksofexistence(3millionyearsinrealtime)1yourunafever–heatfromthedecayofshort-livedradioactiveleftoversfromthesupernova.ThisheatandthewarmthoftheearlySunmeltsspecksintomineralbeadscall“chondrules”andsticksthemtogetherinsootyrocksasyougrow.Someofyoursiblingsstopgrowingatthisstage,doomedtoremainforeverasasteroids.Butyougoongrowingattheirexpense,consuminganythatstraynearyou!

Yougrowfast.Asyouputonweight,yougatherupevermorerockanddustuntilthepressuredeepinsideyoubuildstomeltingpoint,andheavymetalslikeironandnickelgravitatetoyourcore.Bythetimeyouarecuttingyourfirsttoothyouarealreadyalmostaslargeasyouaretoday.2

A little after your first birthday (50 million years after your birth 3), the battlebetween gravity and nuclear forces in the proto-Sun reaches the point when it crushesatomsofhydrogentogether,ignitingthenuclearfusionreactionthatmakestheSunshine.Aflashofinsipidsunlight4nowilluminatesmyriadembryonicplanetsandasteroidsintheSolarSystemforthefirsttime.

1TheInfantEarthandthenewly-ignitedSun(©WalterMyers)

Bynowthesespacerocksaretravellingatsizzlingspeeds(20-40kmpersecond5or45,000to90,000mph).TheyzingaroundtheSolarSystemincurlyovalorbits,nudgedsubtly in new directions each time they pass a planet or the Sun.With somany flyingaroundtheyinevitablyhitthings.Youarezappedandbatteredcountlesstimesbyasteroidsandcometsofallsizes,addingtheirmaterialtoyou. 6Theybringrockyminerals,metals,carbon, sulfur and water in the form of ice, as well as water chemically combined inasteroidminerals.

Each time theysmash intoyou, theysearyoursurfaceandmeltyou.Thebiggertherock,thelargeranddeepertheareathatmelts,butaseachcratercoolsandsolidifies,steamandothergassesaredrivenoutintotheatmosphere,rejectedbythecrystallizationprocess,untilovertimeathick,humidatmospherebuildsup.7

Yournear-deathexperienceWhen you are aged somewhere between 18 months and 2 years old (70-100

millionyearsaftertheformationoftheSolarSystem8)younearlydie.

In your neighborhood there is an immature planet called “Theia.” Even thoughshe’sonlyaboutatenththesizeofyou, 6she’sdestructiveandheadedrightforyou.Sheslamsintoyouwithenoughforcetoripaholethroughyourdeepmantle6(halfwaytoyourcenter),obliteratinganentirehemisphere, 4 andknockingyouoffyour axis so thatyourNorthPole tiltsawayfromtheSun innorthernwintersand towards theSun innorthernsummerseventoday.

Wreckage and molten rock from the impact are flung out into space, and theintenseheatliquefiestheouterhalfofourworldintoashimmering,yellowballofmagma.Thetemperatureonthesurfacerisestoaround6,000°C(around11,000°F),9hotenoughto

vaporize rock and drive away the gasses of our first atmosphere into space forever. 4

GlowingdropsofmagmaorbitingtheEarthcoalesceinafewdecades(inrealtime)andeitherfallbacktotheEarthorcombineintotwoballsofmoltenrockinorbitaroundyou-yournewsiblings,betterknownasyourmoons. 10Yes,inyourinfancyyouhaveapairofmoons!10

AstheEarthgraduallycoolsoverarounda thousandrealyears, 11 it rains lava!Thesurface resemblescrèmebrûlée,asblack rockcrustsoveranorangeoceanofseethingmagma.4Duringthenext5monthsofyourlife(20millionyears)11therockveneerisfloodedrepeatedlywithfreshlavathatsolidifiestoformathickeningcrustpunctuatedbyvolcanoes.12

Twinmoonsnowriseinvisibleoverhellishskies,thelargerglowingmoltenredinitsshadows,thesmallerathirdasbig10andalreadyafrozen,inertspacerock.

Graduallyyourmagmaoceanbeginstosolidify,withmushypartly-meltedpatcheshereandthere. 12TheironfromTheia’scoretricklestoyourcore,whosechurningmoltenmetals generate a powerfulmagnetic field to protect you from solarwind.TheorbitingmoonsaddtheirownprotectionbystabilizingthetiltoftheEarth.Astheplanetsolidifies,the crystallization of minerals expels water and other volatile elements, driving steam,nitrogen, hydrogen 13 and carbon dioxide (CO2) into the atmosphere, replenishing thegasses thatwere lost in the impact. This is supplemented by a cosmic hailstormof icycomets,addingtheirwatertoyoursteamingatmospherewhentheyvaporize.1415

2Moon-formingimpact.RedrawnfromcomputersimulationsbyCanup(2008)with2-moonideabyJutziandAusphaug2011

At this point there is still no oxygen in the atmosphere to breathe, and no lifecapable of breathing it. Water vapor, hydrogen, nitrogen and methane (from chemicalreactions rather than from life) and the CO2 in this primitive atmosphere form a thickblanket to trap the Sun’s warmth (the “greenhouse effect”) leading to a warm climatedespitesunlightthatis30%weakerthanitistoday.41113

Inanotheryearortwoofyourlife(tensofmillionsto100millionrealyears)thesmallerofyourtwomoonsbecomesunstableinitsorbitandslowlycrumplesintoitsstillpartly-molten 16 bigger sibling, adding 10 km (6 mi) to its circumference and creatingenormous expansion cracks filled with lava. 17 18 The resulting rubbly debris forms themountainousfarsideofthemoontoday.1019

Aroundthistimeyouratmospherecoolstothepointwhenitrains.7Howitrains!Itrainsandpoursanocean’sworth, submergingyourhot crustunderaglobaloceanwith26%morewaterthantoday’soceans. 207Waterandrockbeginanintimaterelationshipasocean water penetrates the fractured crust and reacts with hot minerals to producemethane, hydrogen, carbonmonoxide, and “organic” chemicals as it jets back into theoceanatsubmarinevents. 21Butthis isnotranquilseascape- it isaragingocean,pulledaround the globe in 480 kph (300mph) tsunami tides twice every 12-hour day, by ourmoonwhichismuchcloser,andloomsmuchlargerintheskythaninmoderntimes.222324

It’salsoanoceancapableofincubatinglife.

YourfirstmemoriesHowmanyofuscanrememberanythingfromwhenwewereyoungerthan4years

old?SoitiswithourEarth.

Rocks andminerals from the first traumatic years of your life have since beenerased,orareyettobefound.Yourearliestmemoryisfromwhenyouwere4yearsold(4.4billionyearsago).25Itisintheformoftiny,purplishandverydurablecrystalscalled“zircons”,foundatJackHillsinWesternAustralia.

Thesezirconsareourearliestsurvivingrecordofprimitivecontinents,aglimpseofancientmagma thatcooledandcrystallized in thepresenceofgroundwater. 26 27The tinygrains are only about 1/10th of amillimeter (about 4/1,000th of an inch), and yet theycontain even smaller fragments of other minerals, including microscopic diamonds. 28

Thesepre-schoolmemories aremere flecks today,worn from their original rockswhenyou are much older, washed away and tumbled along with pebbles and sand to formsandstonesandgravelswhenyouareinyour30’s.29Yetthesespecksrecalltheconditionsin the crust of our pre-school Earth, when the planet was still somewhat alien to ourcurrent conditions, full of childish energy and a good deal hotter inside (by 300°C or570°F)thantoday.30

It is hotter because ever since you solidified after your Moon-forming impact,natural radioactiveelements,mainlyuranium-235, 4havebeendecayingandbuildingupheatthroughoutyourinterior.Thesolidplanetcouldn’tchurntolettheheatescape,soitjustgothotterandhotter.Nowit’shotenoughtomeltagain,andsomeofthismeltescapesthrough volcanoes. The Earth is now studded with massive volcanoes supported by athick,deep-rootedcrust28ratherlikeJupiter’smoonIoistoday.3012ThesevolcanoesinjectCO2andwatervaporintotheatmosphere,butatthesametimehotlavareactschemicallywiththehumidatmosphere,capturingsomeCO2.11Asthecrustcools,itisburiedbymorelavauntilitsinksbackintothesoft,hotmantle,3012takingthecapturedCO2withit.

In this way, by gradually reducing CO2, the climate becomes a little morehospitable.11

Asteroids and icy comets still occasionally smash into you, sometimes withenoughforcetoboiltheocean,butthankfullyneveragainonthescaleoftheimpactthatcreated theMoon, andmuch less frequently than in your first two years of life. Theycontinuetoaddwater,organicchemicalsandrockymaterialstoyou.

Thingsarehazyuntilyournextoldestmemories,whicharefromwhenyouare8½(4.2billionyearsago)andthenagainataround13yearsold(4billionyearsago).ThesearenotthetinyflecksoftheJackHillszircons;thesearefully-formedrocks.TheoldestrocksonEarththathavebeenfoundtodate 11areinthe“NuvvuagittuqGreenstoneBelt”byHudsonBayinCanada,andtheslightlymorerecent“AcastaGneiss”innorthwesternCanada.25

It’sawonder theseancient fragmentsexistatall, consideringwhata rough timeyouhadasateenager.

3Pre-teenEarth(bytheauthorbasedonNASAimagesofIoandEarth)

2.TroubledTeen

For humans adolescence can be an awkward, traumatic time. Hormones seethe,driving you towardsmaturity, and profound behavioral and physical changes leave youalmostunrecognizable.TheteenyearsforourEartharethesame,andyettheyleaveyoufertile.

Abadcaseofacne

By the time you are 13 30 the interior of theEarthhascooledsufficientlythatareasofcontinentalcrustcanremainstable,andsomeofthese“cratons”survivewithinourcontinentstodayinnorthernCanada,ScandinaviaandAustralia,forinstance.Butit’sstilltoohot,andtherocksconsequentlyaretooweakandflaccid,toengagein“platetectonics.”31Inotherwords,youarestillimmature.

Manyteenssufferfromacne,andourEarthisnoexception.Whenyouare14(3.9billion years ago) distant changes in the orbits of Jupiter and Saturn knock a host ofasteroidsandcometsoutoftheirpreviouslystableorbits.113215ManyfalltowardstheSun,collidingwithanythingintheirway,includingtheEarthandtheMoon,aswellasMars,VenusandMercury.

Fromage14toaroundage17(3.92toaround3.8billionyearsago)yourfaceishideously pockmarked by large impacts – over 17 thousand of them! 32 5 This event isknown as the “LateHeavyBombardment” and itmay sound like a pelting, but spreadacross that timeframe it averages around 1 impact every 7,000 years. They are morefrequenttostartwith 32andsomewereprobablylargeenoughtoboilanoceanandfillthesky with rock vapor for a while. 32 Through this adolescence you gain some weight.Roughly200,000trilliontonnes(about0.003%ofthepresentmassoftheEarth)isaddedtotheEarthbytheseasteroidsandcomets,includingwater, 15carbon-based,andnitrogen-

based chemicals needed for life, 5 33 34 They also replenish some preciousmetals in thecrust,likegoldandplatinum,whichhadsunktothecoreaftertheimpactthatformedtheMoon. 35 Ifyouarewearingawedding ring, someof itsgoldprobablyarrivedonEarthfromspaceatthistime.

TheMoon isalso litupwithspectacular fireworksand itscratered face today islargelytheresultoftheLateHeavyBombardment.Itremainspermanentlyscarredtothisday,buteven thoughyousufferfromhugerecurring impacts intoyourearly20s, 36yourcomplexionwillrecoverthroughthestrongexfoliationoferosionbyrainandrivers,and(later)bytheactionofplatetectonics.

AndeventhoughtheLateHeavyBombardmentistraumatic,itdoesn’tpreventthedevelopmentoflifeonEarth.5Infactitprobablymadeyoufertile.

You’reateenparent!Congratulations–it’saslime!

The first evidence that you are in fact a parent shows upwhen you are 19 (3.7billionyearsago), 37butyoumusthavebeenharboring life forawhilebefore then.Thebirthof“LUCA”(LastUniversalCommonAncestor) is shrouded inmystery.Somesayshe was carried to Earth, stork-like, on one of those asteroids during the Late HeavyBombardment–but that justbegs thequestionofhowlifestartedwherever theasteroidcamefrom.Itseemsmorelikely thatLUCAwasbornonEartharoundyourunderwaterhotsprings.38

Inthebeginninglifeisrock,androckislife.39

Eversincethefirstoceanappeared,waterseepingthroughfracturesandporesinthehotoceancrusthasbeen reactingwithbasaltminerals to formagreenish-brownish,scaly-looking mineral called “serpentine,” creating methane, hydrogen and “organic”chemicals,asby-products insubmarinevents. 2140And it is in thesevents that lifestarts.Butit’sthebombardmentofasteroidsthatsuppliestheessentialphosphorousrequiredtomakethegeneticchemicalRNA, theenergy-storingchemicalATP,andcellmembranes.4142

The blistered encrustations that form at theventsofsubmarinehotspringsareporous,creatingnatural,tiny,mineralcells.Hotfluidsmeetingtheoxygen-lessoceanwateraretransformedbyiron,sulfurandnickelminerals

intoRNA(usingphosphorousleachedfromasteroids),organicmoleculesessentialinthe“Krebs Cycle” (the basic chemistry for metabolism), and a “chemiosmoticmembrane”(againessential formetabolism)coatingthe insideofeachpore. 39Eventually thesecellsusetheirRNAtocopythemselvesaslifeescapesitsrockywomb,birthingintotheglobalocean,protectedbyasteroid-forgedcellmembranes,sometimeinyourteens.43444142

All lifeonmodernEarth ismadeof cells inherited fromLUCA, and these cellscontainthespecialmoleculesRNAorDNA(DNAisessentiallydoubleRNA)totransfertheirgenestotheiroffspring.LUCAhadthesimplestofthese-“16SrRNA”-whichalllifeonEarthhasinheritedinsomeform,45alongwiththeKrebsCycle,chemiosmosis,andphosphorous-basedmembranes.3942

Fornowlifeisbasicallyslime.

Wheretherearepatchesofslimeontheseafloor,flecksofsiltandsandsticktoit,makingthatpatchthicker.Overyearsthisleadstohummocksandmoundsaboutthesizeofasmallturnip(rutabaga)toalargepumpkin,called“stromatolites.”Manystromatolitefossilshavebeenpreservedtothepresentday,andsomeoftheoldestare3.4billionyearsoldandarefromWesternAustralia.46Tinyfossilmicrobesfromthistimehavealsobeenfound in Australia, along with chemical traces indicating that these bacteria wereextractingenergyfromsulfurchemicalsinthewater.47Otherfossilhintsofmicrobeshavebeenfoundthatare3.5billionyearsoldinSouthAfrica. 48Theseformedinseasaroundtheearlycontinentcalled“Ur”whichincludesbitsofAntarcticaandIndia.49

PurplelifeItdoesn’ttakelongbeforesomebacteriaevolvethatcanusesunlighttogetenergy.

Byyourlatetwenties(3.3billionyearsago)theyareusingsunlighttoextractenergyfromsulfur compounds dissolved in water. 50 This early photosynthesis is based on moreprimitive chemistry than modern plants (it uses purple retinal rather than greenchlorophyll)anditdoesnotgenerateoxygen.Sointhisoxygen-lessworld,purple51slime-lifeisthemostadvancedlifeontheplanetanditreignssupremeforabillionyears(fromyour20stoyour40s)!

In some ways our young adultEarthisbeginningtoresemblethemodernEarth.Oceanswithtides,riverswithdeltas,anderosionofhighergroundarealloperatingmuchliketoday,depositingsandandgravelintotheoceans.29Thoseoceanshavecooledtotropicaltemperaturesbymodernstandards-the30s°C(90s°F),orlessthan10°Cwarmerthanourmodernwarmestvalues.525320Tidesandoceancurrentshelp regulate the temperatureof theplanet, toningdown theextremesofhotandcold.Rivers,erosionandvolcanoesregulatethechemistryoftheatmosphereandoceans, and the giant magnet in the Earth’s core deflects harmful solar wind particlesaroundyouasinmoderntimes.

But in the same way as a college student isn’t fully mature (despite what theythink),neitherareyouatthisage.TheSunisstillonlyabout75%asbrightasthemodernSun,andtheMoonissome15%closertoEarththantoday,andadayonlylastsabout17hours compared to our modern 24 hour day, and a year is 514 days long. 23 24 Yourprotectivemagneticfieldisonlyabout2/3thestrengththatitistoday,54exposingtheEarthto higher doses of cosmic rays and high energy solar particles, generating spectacularauroraseveninthemid-latitudes.

If a humanwere to landon the surface of the young adultEarth theywould beasphyxiated immediately as there is no oxygen in the air. If amodern fish were to beplopped into the ocean itwould suffer the same fate, as even thewaters are devoid ofoxygen.Insteadtheatmosphereismostlynitrogen,hydrogen,CO2andsmallamountsofmethane.24Inthisoxygen-freeworld,moleculesofhydrogenandnitrogeninteracttokeepyouwarmbytrappingheat,asdoesCO2.13

3.Mid-LifeCrises

Maturingintoyour30sWhenyouturn30(3.2billionyearsago)555657youmatureandsettleintoadultlife

inthewaymanythirty-somethingsdo.

Untilnow,thickblobsofcontinentalcrustcalled“Cratons”havebeensittinglikespoonfuls of clotted cream floating on a warm, treacly mantle. They’re more like bigislands than continents, really, barely poking out from the global ocean. Beneath thatocean,theremainderofthe“crust”hasbeenamalleable,flaccidskincappingthemantle.ButnowtheEarthhascooledsufficientlythat theskinhasstiffenedtoaglobal,peanut-brittle-like slab. 31 12 The churning mantle beneath rents cracks through it and slowly,powerfully,dragsthepiecesintoeachother,drowningoneunderanother(aprocesscalled“subduction”),andforcingmountainsskywardintheprocess.Thedrownedslabsplungeintothehotmantlewheretheyaresmeltedandrecycledintomagmawhichfillsthecracksbetweentheslabs,creatingfreshcrust.Wecallthis“TheWilsonCycle”anditallhappensatthepaceofafingernailgrowing,butovertimeitobliteratesoldcrust,recyclesit,andreplacesitwithfreshnewcrust.

ThisisthebeginningofPlateTectonicsonEarth.

“Plate Tectonics” may sound geeky, but without it our Earth wouldn’t havemountainsortheriversthatdrainfromthem,ortheflowofnutrientsfromlandtosustainlife in the oceans, or the explosive volcanoes that pump CO2 and steam into theatmosphere. It’s the process thatmanufactures the continental crustwe live on. It’s theprocess that exposes fresh rock to react with CO2 in the atmosphere, and plunges thatcarbon into themantle, keeping our greenhouse gasses in balance (until now), and theclimatelivable.PlateTectonicsisfundamentaltoourlandscape,climateandlifetoday.

By your late thirties, crustal recycling has already generated much of yourcontinentalcrust 25byaprocesscalled“fractionation,”whichisabitlikecreamrisingtothesurfaceofwholemilk. It is responsible for separating theasteroid-like rockmixtureEarthstartedwith,intothekindsofrocksthatcontinentsaremadefrom(granitesandtheirlike, which have a high proportion of silica in them), and the low-silica residue thatremainsinthemantle.Asaside-effectitalsodriveswaterandgassesintotheatmosphere.

It’snowfairtosayatthispointyouaremature:Youareaparentoflife,youhavesettledintoyourjobofconstructingcontinents,anddestroyingthemthroughsubductionataboutthesamerate.Whatcouldpossiblygowrong?

It’sagas!

The“big4-0”isamajormilestoneforus,oftentriggeringmid-lifecrisesandre-settingourdirectioninlife.ForEarthitisthesame.

ThefirsticeageIn your late thirties some microbes learn the trick of “methanogenesis” –

combininghydrogenwithCO2tomakemethaneandwater.Theirinventionenablesthemtoextractenergyfromtheirenvironment,buttheybegintouseuphydrogenandCO2fromtheatmosphere,coolingtheclimate,andtriggeringtheplanet’sfirsticeagewhenyouare36(2.9billionyearsago),13leavingtell-taletracesintherocksofSouthernAfrica.Theiceageistemporaryasthisextramethanesoonreplacesthegreenhousewarmththatyouhadbeengettingbeforefromthehydrogen-nitrogenatmosphere,andtheclimatewarmsagainunderpinkskiesofmethanehaze.5813

4Stromatolitesunderpinkmethaneskiesinyourlate30s(©WalterMyers)

But thatextramethaneleaksinto theupperatmosphere,wheresunlightsplitsoffitshydrogen,whichescapesintospace.Losinghydrogenislikelosingwater(H2O),solifeisnow,veryslowly,dryingoutyouroceans.59

Whenyouarearound41(2.7billionyearsago) thefirst tracesofa totallynewkindof lifebegintoshowup. 24Lifeup tonowhascomprised just2broad typesofmicroscopicgerms -bacteriaandarchaea - collectivelycalled“prokaryotes.”Theseare2ofthe3recognized“domains”ofalllifeonEarthtoday(asIwritethis,sciencemayhaverecognizeda4thdomainoflifecontaininggiantviruses6061 but this is still debated).These lowly, single-celledorganismshavedominated life onEarthforthelastbillionyearsdespitetheirsimplicity.

Butnowthere’safundamentalinnovationincells.

In a fateful encounter, an archaeoncell engulfs abacterial cell, 39 forming a newhybrid creature. Over succeeding generations the trapped bacterium simplifies to form“mitochondria,”whichgenerateenergyforthecell.Fornowthesenewlifeformsarestillsingle-celledcreatures 62different from theirprokaryotecousinsmoreon the inside thanthe outside (except they are typically much bigger). Yet their new more-energeticmetabolism,theirinventionofanucleustohousetheirDNA,ashuffledgenome,andtheirlargersizesetsthemupfortheincrediblerevolutionsoflifetocome.

39

Thedescendantsofthis hybrid cell today make up the 3rd domain of life: the “eukarya” or “eukaryotes,”whichincludefungi,plantsandanimals-includingyouandme.

OxygenIt’salsoaroundthistime63thatsomephotosynthesizingbacteriaswitchfromusing

sunlightforprocessingsulfurchemicals,tousingsunlighttomakecarbohydratefoodbycombiningwaterwithCO2.Thisisaneatinnovationforthecyanobacteriathatmanagedit,buttheprocesshasawasteproduct.

Oxygen.

At first these little blue-green cells areminor players on the stage of life. Theycoexistwithsulfurprocessingbacteria inshallowwaterstromatolites,exudingwhiffsofoxygenhereandthere. 63Buteventhesesmallamountsareenoughtobegintochangetheplanet. 64 They are sufficient to allow a kind of air-breathing (well, absorbing really)bacteria to begin to form skins and crusts on land sediments and in freshwater pools,leaching nutrients from rocks. Their action increases the rate of weathering of rocks,sending unprecedented levels of dissolved metals and sulfur-chemicals into rivers and

oceans.6566

Next,cyanobacteriaevolvenewstrainsthatcantoleratesaltierwater,withbiggercells and filaments that enable them to make their own stromatolites. 63 They begin todominateestuariesandcoastalseasallovertheworldwhenyouareabout46(2.5billionyearsago)givingoffoxygenwherevertheygo.63635867

Each tiny cell is insignificant, but the combinedoxygenemissionsofbillionsofindividuals transform the entire planet forever. They are terra-forming our Earth - anepisodecalledthe“GreatOxidationEvent.”

At first, most of the oxygen isconsumedbychemicalreactionsinwater,airandrock.Itbeginstorustthegeosphere(therocksoftheEarth),creatingahostofmineralsandoresthathavenotbeenpossibleuntilnow.24Intheoceansoxygenreactswithdissolvedironcreatingaslowunderwatersnowofrust, lasting millennia, creating vast iron ore deposits known today as “Banded IronFormations.”

Eventually the cyanobacterial production of oxygen overwhelms the oceans’ability to absorb it, andoxygenbecomespervasive in the air.Asoxygen seeps into thestratosphere itbecomesozone (O2becomesO3)which acts as a sunscreen, filteringoutharmfulUVraysthatwoulddamagelifeonland.68

Thisdoesn’tmeanahumancouldbreatheontheEarthatthistime.Oxygenlevelsin the air are only ameasly 5% - 18% of the present level 58 68- not enough for us, oranimalslikeus.

Lifesavesyouroceans

Astheoxygenintheatmospherebuildsupitreactswithmethane69causingtheskytoflip-flopeveryfewmillionyearsbetweenapinkishmethane-dominatedhaze,andclearblue CO2-dominated skies. 67 This chemical reaction saves your oceans! Up to nowsunlighthasbeendestroyingmethaneintheatmosphere,allowinghydrogentoescapeinto

space forever. Since water is made from hydrogen, losing hydrogen has reduced youroceansbyabout1/5sincetheyfirstformed.20Butnow,insteadofescapingintospace,thathydrogenreactswithnewly-mintedoxygentomakewater(H2O),trappingitonEarthandhaltingthelossofouroceans.

But there is a downside to those atmospheric reactions. Methane is a potentgreenhousegas,soreactingitwithoxygenremovesacozyatmosphericblanket.

SotheEarthfreezes.

SlushballEarth#1Wearenottalkingtheiceageofwoolymammothsandanimatedfilmshere–that

won’thappenuntilyouare99yearsold!Wearetalkinghereaboutsomethingmuchmoreextreme–icecoveringmostoftheplanet,evenintothetropics.24Youmayhaveheardof“SnowballEarth”butthattoocomesmuchlater.

This deep freeze involves 3 separate severe ice ages spread through the GreatOxidationEvent; fromyourage46 to52 (spreadover200millionyears 24).This isn’t ahard freeze of the entire planet, so it’s more of a “slushball” than a snowball, and itcoincideswiththeassemblyofanewcontinentcalled“Kenorland”fromseparatebitsofcrust thatwere pushed together by plate tectonics.Kenorland includes parts ofmodernCanadaandWyoming.

Youdon’t stay frozen foreverbecauseyoukeepproducinggas–yourvolcanoescontinuouslyinjectCO2intotheatmosphere,whichslowlybuildsup,trappingmoreoftheSun’senergy through thegreenhouseeffect.Aprecariousbalancingact setsupbetweenremoval ofmethane by oxygen on one hand, and addition ofCO2 byvolcanoeson theother hand. Like a circus high wire act, the climate lurches one way then the other,threateningdisasterateachswing,butitendssafelyastheclimateemergesintowarmerclimesandblueskiesoncemore.

Standbyme–cellsmakefriends.Bythetimeyoureachtheageof52(2.2billionyearsago)lifeundergoesitsnext

fundamentalrevolution.Somecellsgiveupthesolitarylifeoffendingforthemselves,andinsteadjointogetherandcooperateasasingleorganism.Soon,onthecool,rain-drenchedlandofwhatwillonedaybeSouthAfrica,millimeter-sized,urn-shapedlifeformsappearin thesoil,whichmaybe thefirst fungi. 70Theyare followedbymarinecreatures in theformofinch-sizeblobswithcrenellatededges,whichappearintheseasoffshorewhatwilleventuallybecometheAfricannationofGabon.71

Theseearliestmulticellularcreaturesarethedawnofcomplexlife71-ourancestors-showingupjustasthecontinentofKenorlandisbreakingapart.

But you’re still not immune from huge meteor impacts. Just after your 56thbirthday (2.02 billion years ago) you get slammed by an asteroid in a place calledVredefortinSouthAfrica.Thisimpactisfarmoredevastatingthantheonethatwillwipeoutthedinosaursinyour90s!24Asifthatweren’tenough,ithappensagainwhenyouare59 (1.85 billion years ago), this time at Sudbury in Canada. 24 Coincidentally, and forreasonsthataren’tclear,oxygenlevelsreduceagaintoafractionofwhattheywereinthe

GreatOxidationEvent,andstaythatwayuntilyouare78(abillionyearsago).72

Goingcritical–theworld’sfirstnuclearreactor.No I’mnotmaking this up!Whenyou are 57 (1.95 billion years ago) a bizarre

quirkofnaturecomestopassinOklo,Gabon,whereuraniumoreisformingnaturallyinwaterlogged sandstones. Ore formation is all about nature concentrating metals - likeuranium-togetherinoneplace.Well,natureatOkloisveryeffectiveatthis,somuchsothatanuclearfissionreactorsetsupnaturally.Infactitsetsitselfupin16differentplaces,generatingenoughheattoboilawaygroundwater.Thisprocesscontinuesinon-offpulsesforhundredsofthousandsofyears!73

The climate for the next few years of your life flirts with freezing again andanother iceageoccurswhenyouarearound60yearsold(1.8billionyearsago). 24Thiscoincideswiththeformationof“Nuna,”asupercontinentmadebyjoiningseveralseparatesmallercontinents together.Nunaincludesa largechunkofmodernNorthAmerica,andpartsofSiberia,Scandinavia,WestandSouthAfrica,andSouthAmerica.IncidentallytherocksinthedeepestpartoftheGrandCanyoninArizonadatefromthistime.74

After thedrop inoxygen levels, largeparts of theoceanarestillliketheoldtimesbeforeoxygen.Microbestherestillgeneratemethanethatsteadilyseepsintotheatmosphere,warmingtheplanet.69

Life continues to evolve strange evolutionary experiments such as the spiralstrandsknownas“Grypaniaspiralis” 24andsulfate-reducingbacteria in thedeepoceanswhich begin creating new minerals like iron pyrite (“fools gold”). These are probablyresponsiblefortheworldwidedisappearanceofBandedIronformations24andtheyexudehydrogensulfidegas,givingyouthefaintstenchofsewage.75

Theoriginofplants

Itisaroundthistime76thatanambitiouseukaryotemicrobeeatssomethingitcan’tdigest.Themicrobe isswimmingthroughthewater, twitching itspairofhairs topropelitself,whenitbumpsintoacyanobacterium.Theeukaryotetriestoeatthecynobacteriumand engulfs it, but fails to digest it, and a long, fruitful symbiosis ensues. Thecynobacteriumcarriesonphotosynthesizing,feedingsugartoitseukaryotehost,freeingitfromtheneedtogoaroundengulfingprey.Thistinynewchimeraistheancestorofalgae,and (eventually) land plants. The cynobacterium is passed on from generation togenerationasachloroplast–thephotosynthesizingpartofplantcells.77

By the timeyouare74 (1.2billionyearsago)moreweirdbut simple life formshave evolved, including something that resembles a pearl necklace called “Horodyskia”andastalk-likeredalgaecalled“Bangiomorpha.”24Theseliveintheseassurroundinganewsupercontinentcalled“Rodinia,”whichisassemblingatthistime.Inland,Rodinia’slakes and rivers also supportmulticellular life in the form of tiny, enigmatic balls anddisks.

78

As individual continents collide to form Rodinia, they create an impressivemountainchainknownasthe“GrenvilleMountains,”therootsofwhichcanstillbeseentoday in the rocks of New York’s Adirondacks and the bedrock in New York City. 79

Rodinia comprises much of today’s North America, Scandinavia, Siberia, easternAntarctica,India,theCongoandtheKalahari,Madagascar,andpartsofChinaandSouthAmerica.69

4.Retirement

Yesthat’sright,youareinyour70sandyetwestillhaven’tseentheemergenceofseashells,fish,landplantsoranimals,letalonepeople.Themostadvancedlifeformisakindofredalgae.Infactthere’sstilltoolittleoxygenforfishorlandanimalstosurvive,yet.

Atthispointinyourlifewehavereachedanumericalmilestone.Wearenolongertalkingbillionsofyearsago,nowwearetalkingmillionsofyearsago.

Feelingcoldinyour80s-SlushballEarth#2Attheclosingofyour70s,somethingextraordinarystartstohappen.Complexlife

boldlygoeswherenonebutafewbacteriadaredtobefore–outofthewaterandontodryland.66

Greeningtheland

Bythetimeyouarearound78(1,000millionyearsago)lifeintheformofarangeof fungi, lichen,microbes,andalgaebegins tocolonize theequatorial supercontinentofRodinia.69808178Ittakesawhiletotakehold,butwhenyouarearound83(780millionyearsago)Rodiniabeginstoriftapart.Thenorthernpart,whichincludesAustralia,India,Arabia, China, Antarctica and parts of Africa drifts northwards, and the southern part,whichincludesmuchofNorthAmericaandEurope,Brazil,WestAfricaandSiberiadriftsslowlyintheotherdirection. 79Thiswarm,moisttropicallandscapeisidealforearlylandlife,andtheresultisadramaticgreeningoftheland.80

5SupercontinentRodiniabreaksapart.RedrawnfromScoteseGeol.Soc.Spec.Pub.326,2009.

This greening has two hugeeffects on Earth. The first is to increase the area of the planet that is given over tophotosynthesis,whichmakesmoreoxygen. 80

72

Ofcourse,asweexperiencedbefore,thisoxygen reactswith themethane that had built up since theGreatOxidation Event, 69 24makingEarth cooler again, helped by the fact that the Sun is still only around 90%ofpresentbrightness.69

Butthistimesomethingelseturnsdownthethermostattoo.Lifeonland,assimple

asitis,breaksdownmineralstomakesoil. 82Thisboostsweatheringratesofrock,whichsucksCO2outof theatmosphere,making theEarthcolderstill! 69 80Nowas if that isn’tenough, riverswash the soil nutrients into the oceans,where they are a tonic formoreoxygen-producing life in the oceans. 69 80 And to further chill the planet, the southerncontinentofformerRodiniabeginstodriftovertheSouthPole, 79wherehigherelevationscollecticecapsastheEarthcools–furtherexacerbatingthecooling.

Freezingtheland

Asaresult,theEarthfreezesagain,andthemoreitfreezes,themoreicecoverstheland, which reflects the Sun’s warmth back into space, making the climate colder andcolder,untilmuchoftheplanetisgrippedbyice.Sealevelsfall79andglaciersmarchfromthemountainstotheseaeveninthetropics,wheresoilsfreezeandthawwiththeseasons,andaveragetemperaturesarearound6-7°C(42-45°F).82Ice-groundrockturnstoclay,andthen to dust when the ice recedes each summer. Wind whips it away in terrible duststorms, todeposit thickcoatingsof it inplaceslikeAustralia. 83Only landat theequatorstaysice-free,withSiberiaandAntarcticabeing,ironically,twoofthewarmestplacesontheplanetatthetime.79

Covering most of the land with ice and dust largely reverses the greening, sovolcanicCO2buildsupagainanditisthis,probablyinconjunctionwithorbitalchangesinthedistributionandintensityofsunlighttheEarthgets,thatwarmstheplanetagain,onlyto see a repeat of the cycle. The result is a climate that see-saws between ice age andwarmthatleast3times,withmajoriceageswhenyouare84(720-700millionyearsago),againwhen you are 85 (650-630million years ago) and brieflywhen you are 87 (580millionyearsago).84

Thistimehasbeencalled“SnowballEarth”butitisreallymoreofa“slushball,”becauselargeareasofopenoceanandequatoriallandstayice-free.69

85838279

6SlushballEarth–aplanetfrozenitothetropicsinyour80s.Modifiedwithpermissionfrom:Scotese,Geol.Soc.Spec.Pub.326,2009

At86-GettingacomplexThecontinentshavecontinuedwandering throughout theslushballage,andhave

nowamalgamatedintoasupercontinentcalled“Pannotia,” 79whichhasmuchofEurope,NorthandSouthAmericaclusteredaroundtheSouthPole.ThecollisionshavethrownupahugenewmountainchainacrossArabia,Africa,southernIndiaandpartsofAntarctica,calledthe“Pan-AfricanMountainChain.”79

Now,asicerecedesfromthepost-slushballEarth,resurgentlandliferetakeslostground, greening the newly-uncovered rock and generating oxygen. Rain, glaciers andrivers erode the Himalayan-like Pan-African Mountains, flushing huge quantities ofnutrientsintotheoceans,86causingplanktontoflourishoncemore,givingtheatmosphereanotherboostofoxygen.87Therainofdeadplanktonaddstodeepwatermuds,eventuallyformingoneof the oldest petroleumdeposits in theworld inSiberia. 88 The atmosphereslowlystabilizesathigherlevelsofoxygen(somewherearound15%ofpresentlevels 8990)inyourlate80s.

This oxygen boost triggers another revolution in complex life. A strange newmenagerieknownas the “EdiacaranFauna” appears,with scant resemblance tomodernlife.

Animals–theprototypes

Withneitherashellnorabonebetweenthem,andmostof themunabletomove

about on their own, theynonetheless include the ancestors ofmanykindsof life today.ThereisDickinsonia–a10cm(4in)pleateddisk;there’safeatheryfrondthatgrewastallas aman calledCharnia; 49 andKimberella,which resembles an elongate limpetwith atent, which seems to be a kind of mollusk. 91 We have Marywadea (named afterpalaeontologistMaryWade)–apossibleancestorforcrabs,scorpions,insectsandspiders(arthropods), andSpriggina, a many-segmented thing with a crescent-shaped head thatgrows to 3cm (just over an inch) is possibly the ancestor to trilobites. We seeParvancorina,shapedlike2cm(1inch)sliceofpizzawithasliceofmushroomontop,

andArkarua, whichisablobthatmaybetheancestralseaurchin.92Theancestraljellyfishisthere,beforeitevolvesintoseaanemones,coralsandjellyfish,andfloatingintheseaaretinyspikyballsaround½mmbig(1/50thof an inch) calledAcritarchs. There are also sponges, already resembling theirmoderncounterparts. 93 94 Several of these early creatures already have the biochemical tools todetectlight–aprerequisitefordevelopingeyes.95

Ataroundthis time,amedium-sizedasteroidslamsintoaplacecalledAcraman,Australia, blasting a 90 kilometer wide (56 mi) crater and affecting the regionalenvironmentforaround20millionyears.9697

Among the early experiments in animal body plans, the revolutionary idea ofseparating mouth from anus leads to a much more sanitary arrangement that we(thankfully) inherited.Thisseparationofends inevitably leads toanimalshavingafrontwithsensoryorgans,anervecordandabrain.Thesecreaturesalsodevelopmuscles,andit’sthisnew“bilateran”bodyplanthatwillovertaketheEdiacaranFauna.98

At88–ToprotectandtopreyTheendoftheEdiacaransisheraldedbyahugevolcaniceruptionatVolyninthe

Ukraine,whichdisruptstheglobalclimate.99Thisisanageofglobalwarming,despitetheSunstillbeingonly95%ofitscurrentbrightness.100Globalsealevelsrisedramaticallyby10to70meters(33to230feet),100drowningpartsofPannotiaandburyingalreadyancienthillsandvalleysundermarinesands,asmoreandmoreoftheworldbecomescoveredbyshallow, fertileseas. 101Theburied landscape isknownas the“GreatUnconformity”andcanbeseenintheGrandCanyontoday.

Butitislife,andthemultipliedeffectofmillionsofinsignificantindividuals,thattransformstheplanetonceagain.

Wormpower–unexpectedchemistry

TheEdiacarancreatureshavebeenlivingonaluxurywall-to-wallcarpetofslime– a so-called “microbialmat.” 102But 3½months after your 88th birthday (541millionyears ago) some enterprising worms (bilaterans) begin venturing into the sedimentsbeneaththemat,churningthemupanddestroyingthemicrobialshagpile.Thatseeminglyunremarkable vandalism at a stroke turns the 2-dimensional sea bottom into a 3-dimensionalworld, 103 circulating oxygen through a volume of detritus that hadn’t beenexposedtooxygenbefore.100

Perhaps they are motivated by all the nutrients from newly-drowned terrestrialsoils,101ormaybeburrowingisawaytohidefrompredators.102Whatevertheirmotivation,their action triggers a chemical change in seawater, allowing quantities of sulfate andphosphate, 104 105 to be recycled into the oceans at the same time as there is a surge incalciumlevelsturningoceanwateralkaline.106107

Changing the alkalinity oracidityofoceanwater(changingitspH)hasalargeeffectonthebiochemistryofanimalsthat live in it. 108 Suddenly calcium carbonate,which used to be dissolved in their bodyfluids,wantstoprecipitateoutassolids–somethingthatcouldkillthem!Solifeacrossabroad range of animal species now has to find biochemical ways to deal with thisunexpected chemistry. 109 Some solve this problem by depositing the unwanted calciumcarbonateasshells,whichturnouttobequiteuseful.Othercreaturesrespondtothenewoceanchemistrybymakinganewproteincalledcollagen.39Collagenisavitalcomponentof cartilage and bone and therefore a vital component of you and me. Skeletons withmusclesallowmovement,theabilitytogoafterpreyandtheabilitytoescape,eitherintosedimentsorintothewaterabove.

Now we have a predatory arms race, with some creatures hunting and others

hiding in sediments, in protective shells, or swimming away. 110 At first the shells aresimpleandtiny–around1mm(1/16thinch),butoverthenext5monthsofyourlife(20million years) a cascade of new ecological niches – and the expansion of foodwebs -enables a huge increase in the number and size of shelly and soft-bodied, species – aneventknownasthe“CambrianExplosion.”100110

Marine life bursts into new shapes and ways of life, resulting in swimmers,crawlersandburrowers,predatorsandprey,swiftandstationary,largeandtiny,hidinginamarineforestofspongesandsealilies.Thefirstfish,andinthatwayyourancestorandmine,Haikouichthys appears at this time. 111 Sea snails (gastropods) and othermollusksslitheracrossthesilt,recoilingfromtheattentionsofcrawlingtrilobites(marinecreaturesresemblingthemodernwoodlouse).Dashingthroughthewaterarejet-propelledsquid-likecreatures called Nectocaris.With just 2 tentacles they are nonetheless the ancestor ofsquids, ammonites, and nautiloids. 112 The weird and spiky Hallucigena defiedclassification foryearsbut isnowconsidereda relativeofmodernvelvetworms. 49Themonster of the day isAnomalocaris, ameter-long (3 foot) bug-eyed squid-like creaturewithprey-snatchingarms,while theweirdoof theday isOpabinia,a6centimeter (2½inch)creaturewith5eyesonstalks,asegmentedbodyandaspikymouthontheendofanelephantinetrunk!49

7ProtectandPrey-lifeintheCambrianoceans.AnomalocarisabouttopounceonaMarrella.©JohnSibbick

Fireandice

Theexplosiveevolutionofnewspeciestoexploitnewecologicalnicheshappensalongsideequallyexplosivegeologicalchanges,makingthisa turbulentandunforgivingenvironment. As life proliferates it uses up a lot of carbon, 113 causing a drop in CO2levels114andthereisaniceagearound517millionyearsagoknownasthe“MICEevent”115

alongwithsealevelfalls. 115ButthesupercontinentofPannotiaisbeingdismemberedbycontinentaldrift,andasitriftsapart 100violentvolcaniceruptionsletripacrossathirdofAustralia510.0millionyearsago,formingbasaltsyoucanstillfindtodayatKalkarindji.Undergroundmagmatherebakestheoilyremainsofplanktontogiveoffhugequantitiesof natural gas (methane) and CO2, reversing the cooling with strong, greenhouse gasemissions and global warming that turns oceans into oxygen-starved dead zones withglobalsealevelrises.116

The nascent Cambrian Explosion is stopped dead in its tracks by this climatechange, as around 45% of genera (species groups) go extinct in the Mid-CambrianExtinction. 116 You will recognize these global warming symptoms as they recur manytimesasyouage,oftenwithsimilarlydeadlyconsequences.

Anewocean– the“Iapetus”–floods thegapsbetween thedismemberedbitsofPannotia. Widening tears in the ocean crust power an uptick in the activity of “blacksmokers”-underwaterhotspringsthatinjectironandothernutrientsintotheoceans,aslife begins to recover from the Mid-Cambrian Extinction. 100 The new pieces of thecontinental puzzle are now “Gondwana” (a continent comprising the Amazon region,much of Africa, India, Australia and Antarctica), which lies near the South Pole, and“Laurentia” (much ofmodernCanada and northeasternUSA today), “Baltica” (modernScandinavia)andSiberia.10079117MostlandisintheSouthernhemisphere.

Invasionofthelandplants

Asyoureachyour89thbirthday,lifeonlandpreparestheground–literally–forbig changes to come. It’s around this time that freshwater pond-weed algae evolve intoliverworts, 76whichbegin toshowup inGondwanaaround510millionyearsago. 81 118 119Not long after they appear, theEarth gets another sharp boost in oxygen levels, raisingthemfromaround15%toabout25%ofpresentlevels.89Soonaftertheliverwortsappear,there is also a jump in the diversity of plankton – taking advantage of higher oxygenlevels.89

Andit’sthisinvasionoflandbymoreadvancedplants,coupledwiththatboostinoxygen,whichsetsoffanotherstep-changeindiversityoflife,anditsknock-oneffectsonyourglobalclimate.

5.OldAgeandHeartAttacks

At90–Heartattack!

Asyouapproachyour90s,youmaybealittleunsteadybutyouhavenotlostyourappetitefordrama.ThestageissetforyournextactwhenLaurentiaandBalticaconverge.Volcanicislandsemergelikesentinelsahead of their armies, signaling the impending clash of continents by spewingincandescentcloudsofvolcanicashfarandwide.Elsewhereintheworld-nearSiberia–therearesomeespeciallylargevolcanicoutpourings,99anditmaybethosethatinjectsuchanexcessofCO2intotheatmospherethattheclimatewarmsagain,120andglobalsealevelsrise, 121 creating oxygen-less ocean dead zones across the globe, and depositing deadplankton in stinking black mud on the sea bottom. 122 This causes a mass extinction,including the lossmany trilobite species, 121 and a disruption to the global carbon cyclerecognized in rocks around the world as the “SPICE event.” 89 You recover in a fewmillionyears,andlifecontinuestoevolve.

Theinventionofcoralreefs

Yourwarm,moreoxygen-richseasnowsupportaresumptionintheexplosionoflife that started in theCambrian.This secondphase is knownas the “GreatOrdovicianBiodiversificationEvent,”(mercifullyshortenedto“GOBE”)becauselifediversifiesintothe remaining ecological niches that are still occupied today. 123 The open ocean is now

colonized by a brand new foodweb of swimmers feeding on plankton and each other,including swimming trilobites, armored fish that haven’t yet evolved jaws(“ostracoderms”), “caryocaridids” (shrimp-like crustaceans), and the top predators:“nautiloids”(squidsinshells).Sea-scorpionsprowlthedepths,123asfrondsofgraptolites–tinycoloniesoffilter-feederslivingincascadingcups–drift incurrentsorarerootedtothe sea bed. 124 Trilobites evolve awide range of sizes and shapes, including hedonisticgiantsabout1meter(3feet)longthatreproduceinmassorgies!125Noweventhedeepestoceanflooriscolonizedandchurnedupbyanimalsforthefirsttime.123

It’salsothetimethatcoralreefs,hometoaquarterofallmarinespeciestoday, 126andnurseriesforocean-goinglife,firstappear.Theybeginasmodestmoundsmaybe1m(3feet)tall,dominatedbyalgaeandmicrobes,butbythetimeyouturn90(458millionyearsago),muchlargerreefsarebeingbuiltbycorals,spongesandcoral-likebryozoans.They provide a home for a host of sea shells, sea urchins, trilobites, and a complexnetworkofsuspensionfeeders.123

Theinventionofwood

Like a pensioner fending off a gang ofhooligans,youreceiveanuisance-batteringfromastringofsmallasteroidsalittlebeforeyouturn90(470Millionyearsago–duringthe“OrdovicianPeriod”)whichleaveaseriesofimpactsscatteredfromSwedentoEstoniatoIowa. 127128Butthesewoundsaredwarfedby tumultuous tectonic transformations. As Laurentia (North America) and Baltica(Scandinavia) gradually collide, volcanic islands and sediments of the Iapetus Oceanbetween themare squishedupwards, creating the spineofamountainchain thatcanberecognizedtodayintheAppalachianMountains,fromGeorgiathroughWestVirginiaandNewJersey,toNewEnglandthroughNovaScotiaandNewfoundland,andonthroughtheIrish hills of Donegal and the Caledonian Highlands of Scotland, all the way to theNorwegianAlps.

Onthebanksandlakeshoresofgreatriversthatwashfromthesenewmountains,landplantsinventsomethingthatwillchangetheplanetforever,aroundyour90thbirthday(460millionyearsago).81129

Theyinventwood.81

Thewoodytubesof“vascularplants”allowthemtotransportwateruptheirstems,andgivestemsthestrengthtostandupright.Suddenlyplantsaresetfreetocolonizeareasoflandthattheycouldn’tsurviveonbefore.Theycangrowtallertogetmoresunlightandtodispersetheirsporesoverawiderarea.Vascularplantsarebetteradaptedtolivingondrylandthananythingbefore,andsotheyexplodeacrossthelandscape.

Thisnewplanterabegins,again,tomakeasignificantdentinatmosphericCO2. 119

YoumightthinkthatoneplantismuchlikeanotherwhenitcomestoimpactingCO2,sowhy the difference?Well, the difference is becausewoody plants havemore carbon inthemthanotherplants,90sowhentheydieandgetburiedtheyaremoreefficientatlockingaway carbon, reducing CO2 and cooling the planet by reducing the greenhouse effect.Moreimportantly,thesenewvascularplantsbreakdownrockintosolublenutrientsupto10 times faster than plant-free rock. 129 These chemicals fertilize further plant growth,whichsucksupmoreCO2fromtheatmosphere.129

We are still a long way from trees. The earliest woody plants are modestspecimens, reaching an unimpressive height of a few centimeters (a couple of inches).Still, the landbristleswithadesignerstubbleofsimplebranchingplants likeCooksoniaand Aglaophyton in suitably moist soils, because they have not yet evolved roots orleaves.ThetallestthingonlandisactuallyagiantfunguscalledPrototaxitesthatmakesspiresrising8meters(26feet)high.49

8Earlylandplants:Aglaophytonbristleacrosstheland©WalterMyers

But even these early vascular plants are effective enough at locking awayatmosphericCO2 tocauseanother iceagebeginning just afteryou turn90 (458millionyearsago).129

This “Hirnantian IceAge,” is preceded by another pulse ofwarming and oceandeadzonespossiblydue toeruptions inSouthKorea. 99 130Butas rockweathering,plants

andplanktonlockawaycarbonoverthecourseof15millionyears,129CO2levelsdropandthiscools theplanet. 130Ahuge sheetof icebegins in thepolarmountainsofGondwana(Africa, South America, Arabia, India, Australia and Antarctica combined), whichhappenstooccupytheSouthPoleatthistime. 131Astheicecapthickens,itspreadstoanareamuchlargerthanmodernAntarctica, 132lockingupmoreandmorewater.Thiscausesa big drop in global sea levelwhich leaves reefs high anddry, 133 triggering theEarth’sequivalentofaheartattack–ahugemassextinctionthatwipesout86%ofallspecies.134133129Around⅔ofsea-snail,mollusk,seaurchin,coralandbryozoanspeciesdisappearfromourplanetforever,andreefsdisappearforseveralmillionyears.123

9Theworld450millionyearsago,showingtheHirnantianglaciation.RedrawnfromTorsviketal,Earth-ScienceReviews(2012),Moderncoastlinesforreference.HirnantianglaciationfromNutzetal,J.

SedimentaryRes.2013.

JawsThecontinuingcollisionofLaurentia (NorthAmerica)andBaltica (Scandinavia)

createsanewcontinentcalled“Laurussia”andintensevolcaniceruptions. 135These,alongwith eruptions near Siberia andMongolia, 99 ensure that over the course of the next 25millionyears (in the “SilurianPeriod”)youendure at least 8boutsofvolcanic-warmedfeveraccompaniedbysealevelrises,interspersedwithice-agechillswithsealevelfalls. 135Theperiodsofglobalwarmingcauseextinctions inmanygroupsof creatures includingconodonts(eel-likecreatures),graptolites,polychaeteworms,brachiopods(seashells)and

acritarchs(microscopicspikyballs).135

The stresses of climate fluctuations drivethesurvivalofthefittest,solifeinnovatesandelaboratesonlandandinthesea.

Reefseventuallyrecover,andlandplants take-off inabigway,developingmorecomplexbranchingstems81119andraisingoxygenlevelsevenhigher.Asthehigherlevelofoxygendissolvesintheoceans, it inspirestheevolutionofbigfishasifanevolutionaryhand-brake is released. 90Thesenewkingsof the foodwebhaveanewweapon– jaws.Theyalsohaveevolvedsomethingelsethatweinheritedfromthem–livebirth.136

TheSilurianoceanisnowadistinctlyscaryplace:placoderms–largearmoredfishwith faces like vampire turtles – stalk great expanses of coral reef, hunting jawlessostracodermfish,nautiloids,andprettymuchanythingelse.Bytheendofyour91styearthe first sharks have appeared 137 but to this day they still have not evolved bones. Aseparate group of truly bony fishes – again, your ancestors andmine - appears on thescenearoundthesametimeinthelateSilurian(about420millionyearsago).138

At91-TheinventionofwalkingPlantshavenowfoundwaystonurtureandprotecttheirspores(seedshaven’tbeen

inventedyet), so theycannowcolonizedryerareas. 76Tocomplement this theydeveloproots toextractwater fromdryer soils, and this allowsplants tomold theplanet, again.Rootedplantsnowstabilizeriverbanks,leadingtothefirstmeanderingriverswithmuddyfloodplains. 139Rootsgetdowndeepandphysicalwithrocks,helpingtobreakthemdownintodeeper,nutrient-richsoil,causinganotherstep-changeintherateofrockweatheringwhich draws down atmospheric CO2. 76 Plants suck water up through those roots andtranspire water vapor into the air through newly-invented leaves, promoting morecontinentalcloudformationandrain,divertingwaterthatwouldpreviouslyhaverunofftotheocean.140

The diminutive Cooksonia plant has now been joined by these more advancedplantswithleaves,suchasBaragwanathiawhichcangrow25cm(10in)tall.4910inchesmaynotseemlikemuch,butit’s5timesbiggerthanCooksonia,convertingthelandfroma2-dimensionalphotosynthesizingsurfacetoa3-dimensionalphotosynthesizingvolume.Together the land plants have now elevated oxygen levels in the atmosphere to near

present day levels by the time you are 91 (about 400 million years ago - during the“Devonian Period”). 90 You could say that the terra-forming project started bycyanobacteria37yearsagowhenyouwere54(1.7billionyearsbeforethistime)isnowcomplete.

Firstfires

Nowthatoxygen levelsareatnearpresent-day levelsand the land iscovered incombustibleplantmaterial, it’s inevitable thatyouexperienceyour firstwildfiresat thistime(around418millionyearsago).141Butthisnewoxygen-richworldalsoallowslifetoget evenbigger andmore energetic, and in your 91st year evolution takes several stepsforward–literally.

Fishevolvefinsonstubbystalks,liketheCoelacanth,thefossilfishKenichthys, 142and lungfish. 143 They continue to evolve increasingly land-adapted characteristics andreduce their fishy features in steps through Gogonasus, 144 Eusthenopteron, Tinirau, 142Panderichthys,Tiktaalik,145146andotherspecies.142Finsbecomefeet,tailslengthenandlosetheir fins, gill covers are lost, ears develop and snouts get longer, until animals likeTiktaalik vaguely resemble a 3-9 feet long (1-3 meters) crocodile, with both gills andlungs.Butitsstubbytoe-lesslegsstilllimitTiktaaliktospendingmostofitslifeinshallowrivers and ponds, avoiding giant 3 to 4meter (9 to 12 foot) predatory fish that lurk indeeperriverchannels.145

Sooncreaturesliketheseevolveinto“tetrapods”(4-footedcreatures)suchasthesalamander-likeAcanthostegaorthecrocodile-like,7-toed49Ichthyostega 146even if they can’t reallywalk somuch as shuffle.By themiddle of theDevonianPeriod,however,tetrapodsimprovetheirlegsandpelvis,enablingthemactuallyto walk on land, leaving their tracks in Polish mud halfway through your 92nd year(around390millionyearsago).147

Whatadvantagecould therebe in thismove todry land?Well,bynowthere’sawholemenuoflandcritterstoeat.Insects,148spiders,millipedes,centipedesandscorpionsarecrawlinginamonsoonaljungleof10meterhigh(33foot)fern-frondedArchaeopteristrees rooted in rich soilwith an understory of shrubs and bushes.But the climate isn’talwayssohospitableandbecomessemi-arideveryhundredthousandyearsorso,duetovariations in Earth’s orbit, 149 so tetrapods also thrive on tidal flats 147 where a feast isguaranteedtwicedailyfromcreaturesstrandedbythetides,andwheretetrapodsfearnopredators.147Itisafreeluncheveryday!

At92–AdoubleheartattackEconomiststalkaboutdisruptiveinnovationbeinggoodfortheeconomy,butwhen

wearecaughtup in thosechanges itdoesn’t feel thatway. Itworks inasimilarway inEarth’sevolution.Theplant innovationsandvolcaniceruptionswhenyouare91(in theDevonianperiod)createtheirowndisruption.

Thearrivalofthefirstforests 141allowspeatdepositstoform,creatingreservesofcarbon (coal) locked away in continents. The spreadof treesmeans the spreadof rootsystemswhichliberatephosphorousthateventuallyfindsitswaytothesea,fertilizingtheoceans, where the resulting fecundity of shelly creatures locks yet more CO2 away inlimestone.ThecombinedeffectreducesCO2levelstosomethinglikepresentdaylevels. 15076AsCO2 reduces,plant leavesevolvemorepores (stomata)on their leaves, 151allowingthemtocoolmoreeffectivelyandthereforeallowingthemtogetmuchbigger–lockingawayyetmorecarbon.

At the same time, an archipelago that constitutes the advanced guard of thecontinentofGondwana(AfricaandSouthAmericacombined)beginsitsslowsmearintoLaurussia(NorthAmerica,Britain,ScandinaviaandRussiacombined)toformwhatwillbecomesouthernEurope.117EnormousvolcaniceruptionsoccurinRussia,easternSiberiaandelsewhere15215399knockingtheclimateout-of-whack,triggeringbigfluctuationsinsealevel 154 and causing widespread, stinking dead-zones in the oceans with acid, shell-dissolvingwater.155Thefluctuatingclimatestressesyoutothepointthatyousufferamajordoubleheart-attack,a1-2punchofmassextinctionat372andthen359millionyearsagoastheDevonianperiodends.156

Intheoceans,thefronded-graptolitesthathadbeensoprevalentallbutdieout. 157Placoderms,thosebone-platedvampire-facedfish,vanishas50%ofvertebratediversityislost.156OnlandArchaeopteristreesdisappear158asdosomanyotherspeciesinthis,oneofthe 5 biggest mass-extinctions in all your life. Oxygen levels plummet to about 15%,whichisequivalenttolivingatanaltitudeof3,000m(10,000feet)today,alevelnotseensincebeforetheCambrianExplosion.159

But it’snotbadnews for the survivorswho inherit theEarth. In theoceans ray-finnedfishandsharksdoverywellafterthemassextinction,156andammonites(squid-likeanimals in spiral shells) evolve fromnautiloids 160 to navigate the seas until you are 98.New seed-plant forests rapidly diversify into the plains and uplands vacated byArchaeopteris158asabewilderingvarietyoftreesandshrubsclaimeveryavailablenicheofland.Theextinctionalsosetsupthedeckofvertebratesthatisinplaytothisday,andthesurvivingtetrapodsstandardizeon5toes,156pavingthewayforreptiles.

Age92to93-SwampsandgiantbugsTheCarboniferousperiod(359-299millionyearsago) issocalledbecauseofall

the carbon fuel (coal) that forms at this time acrossEurope,America andChina.Greatrivers meander from the Appalachian – Caledonian Mountains into sluggish, densely-forestedswampsanddeltas.Thebuzzingofflyinginsects148emanatesfromthecanopiesofscaly-barked lycopsid trees (relatives ofmodern club-mosses) as tall as 35meters (115feet) high, that tower over bottle-brush-like Calamites trees (relatives of modern

horsetails), in vast expanses of Everglades-like wetlands. 161 162 Amphibians and reptileshunt insects,whilemillipedes and snails crawlover rotting tree stumps. 163As each treedies,itsplashesintothestinkingswampwhereitswoodslowlyblackenstocoal,lockingatmosphericCO2 away in thegrounduntil humansdig it upandburn it inhundredsofmillionsofyears’time.

10Carboniferousswampforest©WalterMyers

Inland,earlyconiferforestsaredominatedby25meter(80foot)tallWalchiatreesthat resemblemodernpine trees, andbygiant 45meter (150 foot)Cordaites treeswithlongstrap-like leaves.Anundergrowthoffernsandpalm-likecycadsspreadsacross thedryerhinterland.49164

Withmoreandmoreoftheworld’slanddedicatedto forests, oxygen production bumps up several notches, rising to about 35% of theatmosphereinyour93rdyear165159(it’sabout21%today).Freakishlylargeinsectsevolvetotakeadvantage.ScorpionsthesizeofaLabradorretriever(Pulmonoscorpius)cowerfromgiant 2.6meter (9 foot) longmillipedes (Arthropleura)while dragonflies the size of an

albatross(Meganeuramonyi)buzzthroughtheair.Oxygenpromotesfire,sotheseforestsare susceptible toconflagrations that leavebehindquantitiesofcharcoal thatwe find tothisday.162

Offshore in the tropics,whereNorthAmericaandEuropeareat this time,warmseassupportsea-lily-,sponge-,andcoral-reefsfrequentedbyspiralammonites,sharksandotherfish.TheGrandCanyon’sfamousRedwallLimestoneisformingatthistime.

Theworld isnotall tropical,however.Africa,SouthAmerica,Arabia, IndiaandAntarctica are still welded together as theGondwana supercontinent, centered near theSouthPole.Coal formationdropsCO2 levels to roughly halfmodern values (to around220ppm) 166 and this combinedwith the fact that there is a huge continent sitting at theSouthPole,againtriggersglobalcooling.IcecapstakeholdinmountainsandplateausallacrossGondwana,166waxingandwaningincyclesduetovariationsinEarth’sorbitaroundthe Sun. 167 168 169With each cold era, ice caps near the South Pole expand, the tropicalhinterlanddries and thewetlands shrinkas sea levels fall, givingup territory to coniferforests.Ineachwarmandwetphaseicemelts,sealevelsrise,wetlandforestsexpand,andanotherseamofcoalforms.164

At93–TheinventionoftheeggshellAmericaandEuropehavenowencroachedonAfrica,soNewJerseyandMorocco

arenowjoinedasone, 117 sacrificing theJerseyShore tocreate thesupercontinentcalled“Pangea.” Any islands in the way are smeared into the ever-expanding AppalachianMountains.

Tetrapodsarehavingafieldday.

Theydiversifyintoanamazingdiversityofcreaturesadapted to all the permutations from wet to dry habitats. One branch of tetrapods hasinventedtheshelledegg,asinadurablereproductioncapsulethatcansurviveondryland.Amphibiansandprimitivetetrapodsaretiedtothewatertoreproducethewayfishdo,buttheshelled-eggsetsthenewly-evolvedreptilesfreetoreproduceondryland.Ineffecttheegg isaportablepond 170 - adistinctadvantage in thiscapriciousclimate. Justafteryouturn 93 (315 million years ago), reptiles like Hylonomus are running about, chasinginsects 171and leaving theirdistinctive tracks in themudsofseasonallydry riverbanks. 170Alreadywesee2distinctlineagesofreptile,theSynapsids,whichwilleventuallygiverisetomammals,andtheSauropsidswhichwilllatergiverisetodinosaursandbirds.170

Outoftheblue,arandomspacerockthreatenstoannihilateallthis.

Atsomepointwhenyouare93(roughly305millionyearsago)anasteroidbiggerthan10km(6mi)wideslams intoAustralia,creatinganenormous200kmwidecrater. 172Forcomparisonthat’sthesamesizeastheimpactthatwillwipeoutthedinosaurslaterinyourlife.Shockwavessendviolentearthquakesacrosstheplanetasavastamountofrockisvaporizedandsentaroundtheworldtoblockout theSun.Itmaybethis that tips theclimateintoasevereiceagethatlasts4monthsofyourlife(until290millionyearsago)atthecloseoftheCarboniferousandintothePermianPeriod.166172

AsmoreofEarth’swaterislockedawayinice,andasshallowseasareannihilatedbycontinentaldrift,weseeaprogressivedryingofEuropeandAmericaandacollapseofthe rainforests, isolating land animals in shrinking ‘islands’ of forest. This has adevastatingeffectonamphibians,manyofwhichbecomeextinct.173Butreptileswiththeirdurableeggssurvive themorearidconditions,andevolvenewfeedingstrategies (somebecome predators as others specialize as herbivores). 173 Conifer forests take over thedryingcontinent,whiletropicalcoal-formingrainforestspersevereintheislandsthatwilleventually become China, central Asia and Siberia. 167 174 The loss of vegetation causesoxygenlevelstofalltomodernlevels,159leadingtothedisappearanceofthosegiantbugs.

11Theworld300millionyearsago,withglaciation.RedrawnfromTorsviketal,Earth-ScienceReviews(2012),Moderncoastlinesforreference.GlaciationfromIsbell

etal,GondwanaResearch(2012).

6.TheDayYouSetYourselfOnFire

InYourLifeasPlanetEarthyouarenowinyourmid-90s,andlet’s justsayyouaren’tasreliableasyouoncewere.It’sonlytobeexpected,sincemanypeopleyouragesufferfromdementia.175At94,yourself-inflictedaccidentisyourclosestbrushwithdeathsincetheformationoftheMooninyourinfancy.

The omens in the early Permian are not good. You are assaulted by a pair ofasteroids in a double-impact at Clearwater in Canada that leaves two wounds tens ofkilometers wide. 96 Meanwhile the ocean that separated Europe from central Asia andSiberiaisshrinkingtoextinction,andyoumuststillendurechillswhenicetakesholdonthemountainsofAustralia–which isnot far fromtheSouthPole–andexpands to icecaps3times(eachsmallerthanthelast)duringthePermian.166

Justasone’sskindriesinoldage,sodoestheEarth’satthistime.Thegreatdryingof Pangea causes deserts to spread across vast swathes of landwhere once there weretropicalrainforests.DesertDunesblowacrosswesternAmericaandpartsofEurope,whileextremelysaltyseasbuildupthicksaltdepositsinnorthernEuropeandsouthernAmerica.117Incoolerclimes,forestsofconifersandGlossopteristreesfringepolarregions.49

TheEarlyPermian“topdogs”aresynapsidreptiles,withasprawlingkomodo-dragon-likegait.The3meter(10foot) longDimetrodon,forexample,hasafearsomearrayofteethandabizarresailonitsbackforwarmingitscoldbloodinthemornings.49

EventuallybythelatePermian,weseeherdsofpig-likePelanomodonroamingthe

dusty plains, crushing and grinding plants with their bony beak-like jaws, while smallDiictodon burrow for shelter like modern groundhogs. Nearby, herds of thick-skulledMoschops,alsoplanteaters,giveScutosaurus,withitsmedieval-lookingstuddedarmor,awideberth,whilethecat-sizedRobertiascurriesaroundtheherdslookingforrootsofitsown to eat.These are collectively called“dicynodonts,”whichevolve into “cynodonts”(suchasProcynosuchus)whoarebeginningtodevelopthehighermetabolicrateoftheirmammal descendants, and they becoming furry. 49 These early mammal ancestors areeverywhere,buttheirruleisabouttoberudelycutshort.

As Siberia and central Asia finally crumple into Europe, the sediments thatoccupiedthegapbetweenthemgetpinchedintothepeaksoftheUralMountains.Atthesame time, northern China attaches itself to Siberia. 117 There is now a single colossalcontinent –Pangea–which straddles the globe from theSouthPole to theNorthPole,leaving only the remote islands of Southeast Asia, parts of theMiddle East and Tibetunattached.

Catastrophicfire

ThefirstwarningsignsofthecatastrophestartinSouthChina–hundredsofmilesacrosstheoceanfromNorthernChinaatthetime.AplumeofhotmagmafromdeepintheEarth’s mantle surges to the surface 262 million years ago, inundating the area ofEmeishanwith deep lakes of glowing, churning basalt lava covering an area of nearly1,000 km diameter (600mi). 176 The gasses and ash from these eruptions cause climatechange,whichresultsinsomeplantandmarineextinctions.174ThisChineseburnisseriousenough,butthenextislife-threatening.

Verynearlytheend–the“GreatDying”

Halfway through your 94th year, amedium-sized asteroid smashes into southernBrazil,254.7millionyearsago.Its40km(25mi)craterisn’tespeciallylarge,butlandinginoilshalesitignitesalargeamountofcarbonintotheair.Powerfulearthquakesfromtheimpact shake the ground, fracking the oil shale and liberating a large amount ofatmosphere-warmingmethanethatwastrappedthere.177

Thatwouldbebadenough,butitgetsmuch,muchworse.

At around the same time, a searing slug of magma rises from the core-mantleboundaryuntilitmeetstheancientcratonrootsofSiberia.

178

Burningitswaytowardsthesurface, the poisonous plume impregnates any sediments it encounters with manysubterraneansheetsofmagmathatspreadacrossmorethan1.6millionsquarekilometers(618,000 squaremiles), and which swell to hundreds of meters thick. But these aren’tordinary sediments. These contain petroleum reserves created in Ediacaran times, saltdepositscreatedinCambriantimes,andCarboniferouscoal.88

Likeaheatingelement inanelectrickettle, themagmabakes the sediments, theoil, thesaltandthecoal,generatinganoxiouscocktailofCO2,methane,sulfurdioxide,hydrochloricacidandhalocarbons.Thesegassesbursttothesurfacethroughhundredsofundergroundchimneystoventintotheatmosphere,billowingvastquantitiesofcoalfly-ashandsmokelikeWilliamBlake’sdarksatanicmills,251.9millionyearsago.88179180

BoilinglavanowvomitsintotheSiberianlandscapedeluginganareaover4,000

km(2,500mi)indiameter–anareathesizeofEurope–infuming,seething,moltenrock,eventuallybuildingto3km(2mi)thick.

18117888

The explosion in greenhouse gas levelsover perhaps as little as 2,100 years

179 181 causes rapid global warming with averagetemperaturesjumpingbymorethan15°C(27°F),whichbakestheplanettothepointthatequatorial and tropical regions become lethal for most life on land and in the sea.Equatorial ocean temperatures reach 40°C (over 100°F), while land temperaturesconsiderablyexceedthat–toohotforplantstophotosynthesize,andhotenoughtocauseproteindamageinanimals(i.e.tocookthem).182

Butthatisnotall,notnearly.

Thehalocarbonsdestroytheplanet’sozonelayer,exposinglandlifetodamaginglevels of UV radiation, even as sulfuric acid rain burns plants and leaches away soilnutrients.88183

We call it “The Great Dying.” In the ensuing 61,000 years, more than 90% ofspeciesgoextinct. 179 184 Woodlandsaredevastated, ashalfof allplant speciesdieout. 4996%ofinvertebratesandover60%ofreptileandamphibianspeciesarelost. 49Gonearethe trilobites, some corals, eurypterid sea scorpions, 5 entire orders of insect, andGlossopterisforests.49It’samassacreofspecies,theworstinyourlife.

Age94-DawnofthedinosaursYou begin the Triassic on your hospital bed, a comatose world baking in a

swelteringhothouse.Complexlifeisallbutabsent intheequatorialandtropicalregionsfor hundreds of thousands of years. 182 Once-vibrant coral reefs are covered in morbidbacterial slime, 181 once-fecund forests lie desiccated and deserted, 182 andwide areas ofoceansareoxygen-starveddeadzones. 184Thelossofplantscausesoxygenlevelstodropbackdown to15% (comparedwith today’s21%) - a levelnot seen since theDevonianMass Extinction. 159 Low oxygen and lack of vegetation mean that wildfires no longeroccur.185

Ittakessome10millionyearsforyoutorecover.179

And even when you do, for the most part Pangea is a hot and arid land. The

exceptions are the polar regions, now ice-free, which begin to support temperate coal-producing forests after agapof severalmillionyears, 182while the islands thatmakeupmodernTurkey,IranandTibetaremonsoonaltropics.49Graduallythroughyour95thyearthe climate cools to thepoint that a tropicalmonsoonal climate can spread tomainlandPangea,greeningEurope,49andboostingoxygenlevelsagain.159

Inthesea,moderncoralsevolveandsimpleammonitesarereplacedbyoneswithfancier shells. Already a few species of reptiles have returned from land to the water.Some have developed a dolphin-like shape as Ichthyosaurs, some are the ancestors ofturtles (butwithonly the lowerhalfor their shell), and somehaveelongatednecksandtailslikeLariosaurusandTanystropheus.49

TriassiclandlifeonPangeamustbeadaptedtoaridconditionstosurvive.

Thefirstdinosaur,a2-footedcreaturecalledNyasasaurus,appearshalfway throughyour95thyear (around247millionyearsago) inthe near-polar south, scampering across the plains of Tanzania. 186 Dinosaurswill havetheir day, but for now they are minor players compared to crocodile-like crurotarsanreptiles. Primitive plants hanging on from the Permian lose ground to seed plants likeDicroidiumtreesandnewly-evolvedGynkos.Thefewgroupsofmammal-likereptilesthatsurvivedthePermianextinction(suchasthepig-likeLystrosaurus),stilldominateatfirst,but they are gradually out-competed by the swifter bipedal dinosaurs. Mammal-likecynodonts evolve to the point that they are nowalmostmammals, such as the dog-likeCynognathusandthemouse-likeMorganucodon.49

Bythetimeyouare95(215millionyearsago)thefirsttruemammalssuchastherat-like Eozostrodon appear 49 but they are dwarfed by giant reptiles like the vicious-toothedcarnivorePostosuchus,andbulkyherbivoreslikePlateosaurus.Theskyhasnowbeen conquered by reptiles – pterosaurs like Eudimorphodon 49 - leaving reptiles themastersofland,seaandair.

The unified continent gives evolution a boost, because any land animal thatevolves inone locationcanradiate toallareasof theglobewithoutbeingobstructedbyocean.Thisway reptiles andprimitivemammalsget to spreadacross theplanet to takeoverecologicalnicheswheretheycanspecialize.

Backtotheburnunit

But trouble is, oncemore, afoot. Around 228million years ago a profound butshort-livedchange in theweatherdrenchesawideareaof thenorthernhemispherewithmegamonsoontorrents.187TheculpritisahugefloodofbasalteruptinginAlaskaandtheYukonwhichcausesanotherCO2spikeandglobalwarming.188189190

Attheendofthisbizarreclimaticbliptheweatherreturnstoitsregulararidity,butwe now begin to see those tiny chalk-forming algae with a giant name,Coccolithophores, 188 which make their skeletons from calcium carbonate. They start asubtlebutfundamentalchange inoceanchemistrybyraining theircarbon-richskeletonsinto thedeepabyssofoceanswhen theydie.Thisgraduallyexports carbon to thedeepoceansasacalcareousooze,whichwilleventuallyserveasbufferagainstextremeglobalwarmingeventslikethePermianGreatDyinginthefuture.181191

Suddenlyyouareslammedbyanotherasteroid.Itgougesoutahugecrater100km(60mi)wideatManicouaganinQuebec,Canada,214millionyearsago,thatcanbeseenfromspaceeventoday.Devastatingasitis,itisstilljustaforetasteofworsetocome.

12Theworld200millionyearsago,atthebirthoftheAtlanticOcean.Redpatchesarelargeprovincesofvolcanicandigneousactivitythatoccurredataboutthesametime(TheToarcianeruptionsare20millionyearslater).RedrawnfromTorsviketal,Earth-ScienceReviews(2012),

withmagmaticprovincesadded.Moderncoastlinesforreference.

The real trouble is that the enormous supercontinent of Pangea just can’t holdtogetherforlong,andnowitbeginsaslowself-dismemberment.Cracksbegintospread

betweenEuropeandNorthAmerica.Thecracksbecomerifts thatfillwithalgae-chokedlakes surrounded by vegetation, which get continually buried, building ever-thickerdepositsofoily,organic-richsedimentsliketheblackshalesoftheLockatongFormationofNewJersey.192

ThemessybirthoftheAtlanticOcean

Astheriftdilates,fissuresteardeepthrough the crust to the mantle, where they gash into molten rock. Red magma searsthroughthetears,injectingitselfbetweenlayersofsedimentbeforegushingoutenormouslakesoflavaallalongtheeasternedgeofNorthAmerica,WesternEurope,WestAfrica,and into Brazil. At least four hemorrhages ofmagma spew through hundreds of deep-rootedfissuresover600,000years,eachreleasingmassiveamountsofCO2andresultingin some 2 million cubic kilometers (480,000 cubic miles) of basalt lava, forming the“CentralAtlanticMagmaticProvince.”ThisisthemessybirthoftheAtlanticOcean.193194

In a similar process to what happened in the Permian Great Dying, oily lakesediments get baked andbelchoutmassive quantities ofmethane 195 and other pollutioninto the air, including sulfur dioxide and toxic chemicals. 189 192 196 The extra greenhousegassescausea10°C(18°F)jumpinseatemperaturesandrapidsealevelrise.192197194195Lavaflows igniteallvegetation they touch,andevenfar fromthe lava,deadvegetation fuelshuge wildfires. 193 Seas are starved of oxygen, clogged with algae and stink of rotteneggs.198

And,toaddinsulttoinjury,youaremuggedbyanotherasteroidthathitsshallowseasinRochechouart,France,leavinganimpactcrater23km(14mi)wideandsendingatsunamiintoBritain201millionyearsago.199

Not surprisingly all of this triggers another mass extinction at the end of theTriassic,192oneofthe5biggestinYourLifeasPlanetEarth.

Aroundhalfofmarinebiodiversityislost,195includinganumberofammoniteand‘clam’species.Manylandvertebratesincludinglargeamphibiansaregone.Soaremostofthepreviouslydominantcrurotarsans,exceptcrocodylomorphsthatwilllatergiverisetocrocodiles and alligators. Forests collapse and are replaced by ferns 196 causing oxygen

levels,whichwereimprovingintheTriassic,tofallbackto15%.159

It’sdisasterforthoseextinctspecies,butit’sjustthebreakothersneed.

95to96–JurassicgiantsFreed fromdominationby thecrocodile-likecrurotarsans, thedinosaurswehave

growntolovefromthemoviesnowinherittheJurassicEarth.

A bewildering array of land and marine reptiles, including dinosaurs, nowcolonizeseverynicheoftherecoveringJurassicplanet.Inthiswarmandhumidclimate,coal-formingconiferforestsreturntopolarlatitudesandpalm-frondedbenetitalianplantsshadethetropics.Astheyproliferate,oxygenlevelsbegintorecovertolevelsthatwefindtoday.159

ButthedismantlingofPangeacontinuestocausedisruption.

Whenyouhave just turned96(180millionyearsago-inthe“Toarcian”stageoftheJurassic)therestlessmantlebeginstopullSouthAfricaapartfromAntarcticaandSouthAmerica,sendingfloodsoflavafromsouthern Africa as far as South America, 200 and across central Antarctica, even toAustralia. Ina repeatof theend-Triassicevents,CO2andmethanefromcookedorganicsedimentsgetreleasedintotheatmosphere.Theclimatechangesgraduallyoverahundredthousandyearsbut thenwarmssuddenlyinas littleas650years,andstayshotwithseasurface temperatures rising by around 10°C (18°F) for around 300,000 years. Reefsdissolve as the ocean acidifies and sea creatures have trouble growing shells. Oceandepthsbecomeoxygen-lessdeadzonesagain,devoidoftheusual“benthic”fauna,andonlandchemicalweatheringofrockincreasesduetotheheatandacidrain.201202

Many marine species die out 201 but on land it seems to promote a turnover inanimal and plant species.Giant sauropod dinosaurs take off after this event, and so doornithischiandinosaurs–thegroupthatincludesarmored,beakedandhornedherbivores,including iconic species likeStegosaurusand Triceratops.Monkey-puzzles and cypress

treesalsoseemtoprofitfromtheexperience.200

AsthefragmentsofPangeaspreadapart,thegapsbetweenthemformanextensivenetworkofshallowseasteemingwithmarinelife.TheislandchainthatincludesTurkey,IranandTibethasnowbeguntoscrapeoffontoAsia,whileIndiaandMadagascararejustbeginningtobreakawayfromAfrica.117

Theworldisnotalldinosaur.Thelargestfishthateverlived,Leedsicthys,cruisestheoceansfeedingonplankton.Itgrowsto22meters(over70feet)long–thesizeofalargeluxuryyacht-andsharestheoceanwitharangeofreptilianmonstersandachunkyshark that’s been around since the Permian, called Hybodus. The marine reptileLipleurodonstretchesto10meters(30feet)withjawsasbigasamanand30centimeter(1foot)longteeth.Curlyammonitesdiversifyrapidlyandseaurchinsandcoralsthriveinthewarmshallowseas,asdoahostofothermollusksandcrustaceans.49

After an asteroid blasts an 80km (50mile)wide hole into the rocks at Puchezh-KatunkiinRussia199167millionyearsago,just7millionyearslatertheEarthundergoesabigdeclineindinosaurdiversityasanumberofspeciesgoextinct,possiblyasaresultofmoreeruptionsinSouthAmerica.203204

Butafterthattheworldofdinosaursbecomesevenmorespectacular.Itisatimeofgiants and staggering statistics. Brachiosaurus andDiplodocus grow to lengths of 25meters (over 80 feet). 49Allosaurus is a hunter that grows to 12 meters (40 feet) andresembles a T-Rex with exaggerated eyebrows. Barely noticeable in comparison,mammals like the tinymouse-likeMegazostrodon,andSinoconodon,whichresemblesaminiatureweasel,scurryandburrowintheJurassicundergrowth.49

OntheoriginoffeathersWhile all this is going on, the first bird –Aurornis – has evolved, around 160

millionyearsago. 205Feathersareadinosaurinnovationearlyintheirdevelopment,andanumber of different groups of dinosaur already had primitive feathers by the earlyJurassic,longbeforebirds.206

By now the 2main branches of dinosaurs both have species that possess eitherfeather-like filaments (such as Tianyulong, which had a Mohawk) or they have actual(albeitprimitiveandflightless)featherssuchasthepheasant-likeEpidexipteryx.207Somedinoshave a fuzzof filamentswhenyoung, and thengrowwingswhen theymature toimpresspotentialmates,208whileother,smalldinosaursusedownyfeatherstokeepwarm.Overmillions of years feathers become fluffier andmore complex, and eventually – insome species – actual flight feathers. 207At this point inYourLife asPlanetEarth, birdancestorsarelivingintreesandusingtheirwingstoglidefromtreetotree,orasakindofparachutetobreaktheirfalltotheground. 209Othersmayusetheirwingsasaspeed-boostforrunning.210Feathersnextevolveintoairfoilsthroughthedevelopmentoftinybarbsandbarbulesthathelpthefeatherkeepitsshapeagainsttheair,enablingbirdstofly.

EarlybirdslikeAurornisandMicroraptorarestilltransitionalcreatures,notfullybirds.Theyhavefeatheredlegsthatfunctionashind-wings 211sotheyflylikebiplanes. 212Manystillhaveclawsontheirfore-wingsandteethratherthanbeaks.49Bytheendofyour97th year (around150millionyears ago) creatures likeXiaotingia 213 andArchaeopterix,

with fully recognizable feathered wings and larger tails have evolved, 207 and they’realreadybeginningtolosetheirlegfeathers.

Dinocrisis

As you approach your97thbirthday in the latterportionof theJurassicPeriod (around160millionyearsago),Europe and America experience big 3 to 4 million year swings between warm-humidspellsandcoldsnapsinanincreasinglycold-aridclimate,withcorrespondingchangesinglobal sea level. 214 In global warming periods, coral reefs disappear from equatoriallatitudeswhereseatemperaturesaretoohotforthem,andspreadnorthtolatitudeswherewe find Nova Scotia, Brittany and Vladivostok today. 215 In cold snaps CO2 drops andseasonalicecanbefoundinthepoles.216

The causes of these fluctuations are uncertain but the ongoing separation ofcontinentsprovidesplentyofvolcanicsuspectsthroughthistimeperiod,fromSwedenandtheNorthSeatoJapan,WesternAustraliaandthewesternPacific.217218219

Acrash indinosaurdiversity ensues as several sauropod species (agroupwhichincludesmanygiantdinosaurs)andstegosaursdieoutattheendoftheJurassic,probablybecausefoodplantsareaffectedbytheseclimateshifts.203

ThefinalpunctuationmarktotheJurassic isanerrantspacerockthatslamsintoyou, blasting a huge 70 kilometer (43 mile) crater at Morokweng in the Kalahari ofsouthern Africa 96 followed soon after by pair of smaller asteroid impacts, one in theBarentsSeanearNorway,andanotherincentralAustraliaatGossesBluff.

YouendtheJurassicshiveringinanaridice-house.214

7.AnOveragedHippie!

Youmight think thatatyour ripeoldageof97youwouldbe toooldfor flowerpowerandgrass,butinYourLifeasPlanetEarththisistheyearyougohippie.TheageofT-Rexcan’t exactlybecalled the timeofpeace, loveandunderstanding,but just as theoptimismofthesixtiesendedinviolence,sowillyours.

Ittakes4to5monthsofyourlife(15to20millionyears)fortheclimatetowarmanddinosaurdiversitytorecoverfromtheextinctionsattheendoftheJurassic,203andhowitrecovers!Thisistheeraofthemoviestarsofdinosaurs:thefeatheredVelociraptor,themassive-jawed Tyrannosaurus Rex, the sail-backed Spinosaurus and the hornedTriceratops.Hundredsofotherkindsofdinosaursof all shapes, sizes and fluffiness filleveryniche,someactivebynight,somebyday,andsomegrazingbothdayandnight. 220Pterosaurs thesizeofaircraftcruise theskiesabovemigratingherdsof Iguanodon eachthesizeofaschoolbus.Ammonites(thespiral-shelledrelativesofsquid)andbelemnites(akindofsquidwithabullet-shapedinternalshell)dartthroughtheoceans.

Coexisting with this exotic menagerie are creatures we would recognize today.Birdscomeintotheirownandtaketotheskies.Sharks,turtlesandfishcruisecoralreefswhileseaurchinscreepacrosssandyseabeds,dodgingcrabsandmollusks.

FallingApart

In this “Cretaceous”period,SouthAmericaandAfrica turn their trial separationintoafulldivorceasthesouthernAtlanticOceanwidens.NorthAmericaandAfricahavealready drifted far apart, and southern Europe loses touch with Newfoundland by 112million years ago. 221 222 As North America bulldozes its way across the Pacific, itencounters volcanic archipelagoswhich scrape off and accumulate asmountains on itswesternedge.223

Northern Europe, however, maintains a lingering attachment to Canada throughGreenland, 117 and India andMadagascar are inseparable as they start their estrangementwithAntarctica,evenascracksemergebetweenAntarcticaandAustralia.

You are finally enjoying a stable, steamy climate. Tropical sea temperatures arearound 32°C (90°F), 224 which compares with current Caribbean Sea temperatures ofaround27°C(80°F).EvennorthernandsouthernextremeslikethesoutherntipofSouthAmericaarepleasantlywarmwithseatemperaturestomatchthemodernCaribbean.Thepolesareice-freeandAntarcticaisforested,supportingahealthypopulationofdinosaurs.49Oxygen levels,whichwere already healthy at the end of the Jurassic, rise steadily to25%(today’sare21%).159

13Theworld100millionyearsago.RedrawnfromTorsviketal,Earth-ScienceReviews(2012).Moderncoastlinesforreference.

Floweringplants appear right after your 97th birthday (130millionyears ago) 225andby3monthsofyour life later (125millionyearsago) theyhavealreadydiversifiedwidely into landscapes previously been dominated by conifers, ferns, horsetails, cycadsand palm-like bennettitaleans. 226 49 Now flowers like magnolias bloom color into themonotonous greenscape as deciduous trees and palms take over. Today over 250,000differentfloweringplantspeciesdominatemostterrestrialecosystems.226

This era is known as the “Cretaceous Terrestrial Revolution” because animalsundergoarapidincreaseindiversity,probablyinresponseto theexplosioninfloweringplantdiversity.Weseetheriseofsocialinsectslikeants,termitesandbees,andthefirstsnakesevolvefromburrowinglizards. 227Birdslosetheirwing-clawsandlegfeathersanddevelop beaks so that they look like birds today, exploiting varied habitats, from theseagull-likeIcthyornis(whichstillhadteethinitsbeak),tothegoose-likeVegavis,tothedivingHesperornis.49

New dinosaurs also evolve (armored ankylosaurs, hadrosaurs andparacephalosaurs) but it’s not clear that this has anything to dowith the emergence offloweringplants.228

Mammals,whohaveclungdoggedlytoexistencesinceyouwere95(theTriassic),also diversify. By 6 months after your 97th birthday (115 million years ago) the firstmonotremes–egglayingmammalslikethemodernplatypus–appear. 49Mostmammalsare small, furry rat-like creatures (actually rodents are yet to evolve) but some are

different. Volaticotherium climbs trees and glides just like a flying squirrel, 49 andRepenomamusisadog-sizeddevourerofyoungdinosaurs.229

Thefirstsignoftroublestarts4monthsafteryouturn97.

A brief period of cooling, complete with ephemeral ice in the poles, is sharplyreversedbyamajorspikeinvolcanicactivitycoincidinginthesouthernIndianOceanandwestern Pacific 219 which injects a whole lot of extra CO2 into the atmosphere, causinganotherperiodofglobalwarmingknownasthe“AptianAnoxicEvent.” 230Oceanswarmand turnmildlyacidic, leading toextinctionsand the spreadofcyanobacteriaandalgaewherecoralshad thrived. 231 Thewarming oceans expand causing sea levels to rise, andreducingtheiroxygencontent-turninglargeareasintodeadzonesonceagain.230

Aftersome5millionyears,weatheringofrockandburialofsedimentsbringstheCO2andglobaltemperaturesdownagain,returningyoutothecoolerclimesthatyoufeltbefore theAptianAnoxicEvent, and sea levels fallby50meters (160 feet).This“newnormal”ofrelativelylowCO2, temperaturesandsea levels lastsanother5millionyearsbeforethingsgowrongagainasyouapproachyour98thbirthday(94millionyearsago).

232

150

This timethecause isamassiveburstofvolcanismasMadagascarandIndiaripapart, alongwithgianteruptions in theCaribbeanand theArctic. 233 234 219 235 In a carbon-copy(punintended)oftheAptianevent,this“Cenomanian/TuronianAnoxicEvent”seesaspikeinatmosphericCO2andcorrespondingjumpinglobaltemperatures,189leadingtoadie-offandalackofoxygenintheoceans.234Sealevelsrisetoaboutashighastheyhaveeverbeenintheentirehistoryoftheplanet,andfor450,000yearsthepreviouslyhealthyoceansturnsluggishandstagnant,depositingblackmudsthatwilleventuallybecomeoneoftheworld’smostimportantsourcesforoil.236233

At98–grassandchalkAstheworldrecoversandyouturn98,theCretaceousoceansarealivewithspiral

anduncurledammonites,giant50foot long(15meters) reptilescalled“mosasaurs”andlong-necked plesiosaurs almost as big. The warm seas are the perfect environment forCoccolithophores.Theircellwallscompriseplatesofpurewhitecalciumcarbonateandwhen they die they sink to the sea floor to make chalk, which gives its name to the“Cretaceous.”Eventuallygreatthicknessesoftheirskeletonsaredeposited,someofwhichwilleventuallybeexposedintheWhiteCliffsofDoverinEngland.

On land, the first grasses have evolved in India as minor vegetation in forestclearings 237 and are being grazed by dinosaurs andmammals alike. 238 In factmammalsseemtohavecomeoutfromtheCenomanian/TuronianAnoxicEventbetteroffthanmost.Their success tracks the proliferation of flowering plants that they eat, and it’s theseadaptationsthatsetourancestorsuptosurvivetherapidlyapproachingcatastropheforthedinosaurs.239

Butthisidyllicimageofruralgrazingisabouttoend.Theworldbeginsaperiodofpronouncedcoolingandadrop in rainfall2monthsafteryour98th birthday (82millionyears ago), 189 apparently due to changes in ocean circulation brought about by drifting

continents.240Sealevelsfall,exposingtheseabedtoweatheringwhichreducesCO2levelsfurther, and so on in a slow feedback loop of increased cooling. 240 Temperatures in theArctic also decline because of the closure of the “Western Interior Seaway” – a broadshallowseathathaduptonowconnectedtheArctictotheAtlanticthroughtheAmericanMid-West.241

Despite the relative cooling, the Cretaceous climate remainsmuchwarmer thantoday.EventheArcticstayswetandhumid,withsummertemperaturesaveragingaround14°C(57°F),andwinter temperaturesdippingonlytoaround-2°C(28°F),aclimatenotunlike Portland, Maine today. The Arctic margins of North America and Russia areforested and populated with dinosaurs, and the Arctic Ocean remains largely unfrozeneveninwinter,asapersistentcoverofcloudkeepsNorthPoletemperaturesfromgettingtoocoldduringthe5monthsofwinterdarkness.242241

Meanwhile, inapremonitionofthecalamitythatisabouttobefall theplanet,anasteroidblastsa120kilometerwide(75mile)holeatKarainRussia.96243

TheviolentdeathofthedinosaursThe tragic chain of events just 6months after you turn 98 is like a serious car

crash.

Theblow-out…

India’s breakneck racenorthwards takes its toll on the subcontinent. A few degrees south of the equator 117 itpassesoverahotplumeofmagmainthemantle,68millionyearsago.Fissureshundredsofkilometers longopenup, spewing fuming lavaoveranarea1200km(750mi)wide.Justaswehaveseensomanytimesinyourlifebefore,thisexpelsvastquantitiesofCO2,withprofoundeffectsontheglobalenvironment,244raisingaverageglobaltemperaturesbyaround5°C(9°F).245Theeruptionscontinueoverthenext3millionyears,buildingup1.3

million cubic kilometers (312,000 cubicmiles) of lava to a height over 3½km (2mi)thick. That’s like burying the states of Utah, Colorado, Arizona and New Mexico infuming,molten rock, to a height of 8 stacked Empire StateBuildings! Each individualeruption may have lasted a decade or more, sending fountains of fire high into theatmosphere. 244 The resulting rugged landscape in India is known today as the “DeccanTraps.”

Temperatures cool a littlebefore thenext,most intensephaseof eruptions startsjust2daysinyourlife(around300thousandyears)beforethedinosaursarewipedout. 245246 This extra pulse of CO2 sends global temperatures soaring another 7°C (13°F) andbeginstokilloffmanyspecies150,000yearsbeforetheendofthedinosaurs.245

ButtheIndianeruptionsaren’tthekill-shotforthedinosaurs.246

…sideswipe…

An asteroid hits Boltysh in the Ukraine half an hour in your life (2,000 years)beforetheirdemise,247buteventhatisn’tthefatalblow.

…head-oncollision

The coup-de-grace comes in the formof amuch larger asteroid that crosses theorbit of the Earth 66million years ago. 248 At 10 kmwide (6mi) it’s about the size ofManhattan,andmuchtoolargetoburnupintheatmosphere.Travellingatcosmicspeeds,249 it barely registers the atmosphere before it vaporizes on contact with the CaribbeancoastofMexico.

The shock wave excavates a vast hole 200 km (120mi) wide in the limestoneseabed,246generatingearthquakesatleastathousandtimesstrongerthanthemagnitude9

Tōhuku earthquake that hit Japan in 2011. 246Anair-blast levels forests for thousandsofmilesandacataclysmic tsunami 246sweeps across the Atlantic Ocean, engulfing all low-lying land, leaving it strewn withdeathanddebris.Thereboundfromtheshockwaveejectsrockvaporandrubbleatspeeds

fasterthan11kmpersecond(7milespersecond)intospaceandtheupperextremesoftheatmosphere,whereitspreadsaroundtheplanet,raininghotcinders.

Thishotfalloutmakestheskyacrossmuchoftheworldglowred-hotforanhouror two, enough to broil a good portion of land life and set light to dry vegetation, sowildfiresrageacrosstheplanet.246250

Around500billiontonnesofsulfurisinstantlysuckedintotheatmosphere,alongwithfinedustandsoot.Thiscausesacidrainwhichacidifiestheoceansurfaceandinlandwaters.Italsoplungestheworldintodarkness,whichquicklycoolstheplanetsurfaceby10°C(18°F).246Itstayscoldforyears.246

Alreadystressedbyglobalwarming,thisisthefinalstrawformanyspecies.

Allthedinosaurs,allthemarinereptiles(exceptturtlesandcrocodiles),andalltheflying reptiles (pterosaurs) and 83% of snakes and lizard species die out. 251 Theammonites, belemnites and “rudist” mollusks vanish, along with nearly half of allinvertebratespecies.4998%ofnorthernnanoplanktonand73%ofsouthernnanoplanktonspeciesatthebaseofthemarinefoodchaindieout,249somarinecreaturesallthewayupthe foodweb starve.On land, forests chokedbyashanddarknessmolder anddecayas60%ofplantspeciesgoextinct.24649Mostanimalsstarvetodeath.

Youareinintensivecarefor2½daysofyourlife(300,000years).

Creatures that can burrow (like somemammals) and those that live off detritussurvive.Asthedustsettlesandshaftsofsunlightoncemoredapplethemolderingground,ferns take over the landscape 246 and life begins to renew itself. The ocean depths holdenoughheattobeminimallyaffectedbythecoolingandtheywarmtheplanetupagaininafewyears,246butitstilltakes300,000yearsformarineecosystemstorecover.249

Youslowlyheal,buttothisdayyoustillhaveascarfromthiswoundatChicxulubintheYucatanpeninsulaofMexico.

8.HotFlashes

Youhobbleoutofhospitaloncrutchesintoastrangenewdinosaur-freeworld.ThesteadycoolingtrendoftheCretaceoushasbeencompletelyreversedbytheCO2releasedbytheDeccanTrapseruptionsinIndia,sotheclimateishotandhumidoncemore.

India is on a collision course with Asia, and already the seabed sediments inbetweenarebeingcrumpledintoislandsthatbegintoemergefromthesea.AustraliaandPapuaarestartingtoseparatefromAntarctica,whiletheRockyMountainsriseinNorthAmerica.SouthAmericaretainsanarrowattachmenttoAntarctica,andCentralAmericaisstilljustanislandchain,allowingthewatersofthePacificandtheAtlanticOceanstomix. 117 Much of Central Asia is a shallow sea connecting the Arctic Ocean to theMediterraneanandawaterloggedNorthAfrica.MostofEuropeisatropicalarchipelago.117

Just 20,000 years after the impact, mammals begin to recover, 248 indeed somegroups are remarkably unaffected. 239 Seizing their head start, they proliferate now thattheirerstwhilepredatorsandcompetitorshavebeenwipedout.Within400,000yearsoftheimpactthefirstmammalwithaplacentainitswomb–ourancestor–hasevolved.It’sa small insect-eating, shrew-like thing. 252 By 5 million years after the impact, largeflightlessbirds likeDiatryma havealready takenover the roleofkinghunters from thedinosaurs.Earlyanteaters,shrewsandmoles,andeventhedistantancestorofelephants,havenowevolved.252

Astheworldrecovers,itcools,andasitcoolsitbeginstolockawayreservesofcarboninpeat,permafrost,andmethanehydrates.253

AtlanticfizzTheNorthAtlanticOceanistearingopenasfarastheArctic,triggeringaheated

divorceofEuropefromGreenland.Astheteardevelops,cracksripdeepintothecrustallthewaytothemantle,wheretheypunctureamagmaartery–amantleplume–whichwilllaterfuelthevolcanoesofIceland. 254Likebloodrushingintoanopenwound,lavaeruptsinto the tears, clotting as basalt over large areas including the Faroe Islands, NorthernIreland, western Scotland and offshore Norway, emitting large quantities of CO2. Thebasalt landscapes of Northern Ireland and the Scottish Hebrides, including Giant’sCausewayandFingal’sCave,originateintheseeruptions.

Thisisenoughtoturntheclimatefromcoolingtowarmingagain,andasitdoessothose reservesof carbon frompeat,permafrost andmethanehydrates are liberatedbackintotheatmospheretoamplifytheglobalwarming.253

Now some more magma spreads, bruise-like, deep inside the North Atlantic

marinesediments.Moreandmoremagmableeds internally, swelling these subterraneanabscessestoalmost100meters(300feet)thick,forcingitswaythroughmorethan85,000squarekilometers(33,000squaremiles)ofseabednearNorway. 255256Theheatbakesoil-richCretaceous sediments, generating thousands of billions of tonnes of carbon gasses,mostlymethane. 257 3months before you turn 99 (56million years ago), that gas burststhroughtheAtlanticseabed,eruptingfrommorethan700craters,someasmuchas12km(7mi)across255andthenorthAtlanticOceanfizzeslikesoda.

Methane is 25 times more potent as a greenhouse gas than CO2, 255 258 259 so theinjectionofthousandsofbillionsoftonnesofitintotheatmospherecan’thelpbuttriggermoreglobalwarming.Theresult isapronouncedwarmingof theclimate,knownas the“Paleocene-Eocene-Thermal-Maximum”or“PETM,”thatlastsabout170,000years.

Itisthebiggestglobalwarmingeventuntiltheageofhumans.Globalaveragetemperaturesriseby6C(11°F)260andinwhatisnowafamiliar pattern of symptoms, oceans develop dead zones and global sea levels rise. 261Oceansacidifyandcoralreefsdecline.260Wetclimatesbecomemonsoonalanddryareasgetdryer.Wildfires are common 262 as rainfall becomesmore seasonal, eroding the landand choking rivers with sediments. 260 Global ecosystems are profoundly affected as insomeplaces20%oflandplantsgoextinctandplantdiversitydropsbymorethanathird,263and there isalsoa significantextinctionofmarinecreatures. 255Even though there isnovertebrate mass extinction, mammals respond to the environmental stress by gettingsmaller.264

Jungleplanet-thefirstprimatesYourecover,coolalittle,andadjusttoahotandhumidworld–onaverage10°C

(18°F)warmerthantoday. 265Palmtrees fringe theArctic, 266 lowlandAntarctica iswarmand covered in near-tropical vegetation, 267 and London is a mangrove swamp 49 asrainforests spread acrossmuchof theplanet. 268But you experience repeatedhot flashescalled“hyperthermals”asyouapproachthelastyearofyourlife.269

Thesehotflashesarecausedbyvariationsinyourorbit,whichaltertheintensityofsunlight at high latitudes, and are regularly paced at 100,000 year and 405,000 yearcycles.253Astheseorbitalcyclesbringmorehigh-latitudesolarwarmth,evenmorecarbonis released from reserves in permafrost, peat, methane hydrates, and from the oceans,

exacerbatingglobalwarming.270269260Asyourorbitcyclesbringlessintensesunlightatthepoles,yourclimatecoolsalittle,untilthenextwarmcycle,overandoveragain.Yourhotflashes are probably made worse by occasional CO2 additions from ongoing NorthAtlanticvolcanicactivity.271272

Thejungleplanetspurstheevolutionofnewspecies.OntheshoresofIndia,earlywhale species like Ambulocetus – a 3 meter (9 foot) long, walking, furry, web-footedcreature, adapted to underwater hearing 49 - show up. Primitive horses graze in theundergrowth:thetinyProtohippus,whichishardly40cm(15in)tall,evolvesintoseveralversionsofMesohippus,aknee-highhorsewithtoesratherthanhooves. 49Earlyversionsofelephantsandrabbitshaveappeared,whilebatsflitthroughthenightairanddugongsnose along sandy sea beds. Multi-horned rhinos with tusks (Uintatherium), face offagainst hyena-like predators bigger than amodern lion (Andrewsarchus), 49 while smalldeer-likeruminantscalledLeptomeryxgingerlypicktheirwaythroughtheundergrowth-theancestorsofmoderncows,deerandcamels.

Many modern birds can now be found flitting through the rainforest canopy,including songbirds, parrots, swifts, and woodpeckers. 49 Flowers are becomingincreasingly adapted to pollination by bees,while fruits and seeds are getting larger totemptbirdsandmammalsintospreadingtheirseedsforthem.49

Cruciallyforus,thesultryforestsprovideidealhabitatsforsmalltree-scamperingmammals,andby55millionyearsagothefirstprimateshaveevolvedinAsia,includingArchicebus, amouse-sized, long-tailed insectivore,which is very closely related to ourancestor.273

Trainwreck1monthbeforeyou turn99 (50millionyearsago 274) India finallycrumples into

southernAsia,but itdoesn’t justslideunderAsiaanddisappearquietly into themantle.Insteaditramsintothelargercontinent,forcinggreatslabsofAsianrocktorideupandoverthecollisionzonelikecarriagesinatrainwreck,squeezingmillionsofyears’worthofsedimentsandbedrockintoloftypeaksthatwecalltheHimalayas. 275Therisingrangeofmountainsisravagedbyerosionwhich,aswesawbefore,slowlydepletesatmosphericCO2.Thisprocesswillintensifyoverthecoming20millionyears(5monthsofyourlife)anditcontinuestothepresentday.276

Ataroundthesametimethereisaseriesofminorasteroidimpacts,atMontagnaisinNovaScotia,andadouble impactatKamenskandGusev inRussia 96but thesedon’tseemtohavehadmucheffectonbiodiversity.

9.YourLastYear-AChiller

You turn 99 in themiddle of themuggyEocene epoch.We’ll pretend that yourbirthdayisatmidnightonNewYear’sEve,sothatwecancountthemonthsofftowardsyourcentury.Anyway,happybirthday!Youdon’tknowityetbutitwillbeyourlast…

CentralheatingfailureAs people age,we find it harder to regulate body temperature, and our average

temperatureprogressivelydecreases.277TheEarthhasthesametrouble.

Lush swamp forests, inhabited by turtles, alligators, primates, tapirs, and thehippo-like Coryphodon, now inhabit your Arctic shores. The Arctic Ocean is rain-drenchedandalmostland-locked,totheextentthatit’spracticallyafreshwaterlake.Theseare perfect conditions for a kind of bright-green floating fern called Azolla, whichpracticallychokestheoceansurface.Overabout800,000yearsthisAzollabloomcapturesCO2fromtheatmosphere,lockingitawayinsedimentsasthefernsdieandsink.278Theyare soprolific that theymake a significant dent inglobalCO2 levels, 279which togetherwith the cooling caused by the erodingHimalayas, cools the global climate enough forglaciers toadvanceonhigherelevationsofAntarcticaaweekbeforeyour99thbirthday(49millionyearsago).280

OutofAsia

Africa’s northward drift has brought it tantalizingly close to the archipelago ofEurope,andtoAsiathroughArabia(whichisstillpartofAfrica).281TheclimatecoolingcausessealevelstodropjustenoughtoallowthefirstlandconnectionsbetweenAsiaandAfrica since they broke apart in the Jurassic. 282 Many kinds of lizards and mammalsemigrate from Asia to Africa, including mole rats, porcupines, and anthracotheres – agrouprelatedtohippos.283284

This is also the time when Africa imports primates from Asia – our distantancestors.283

ThefecundforestsofAfricaarenowhometoadiversecollectionofnewprimatesincluding tarsiers (big-eyed nocturnal tree climbers with long-fingered hands),Parapithecus(resemblingamonkey)andOligopithecidae(smallinsectivorousprimates).285

Butthedaysofthejungleplanetarenumbered.

Heatingbreakdown

Itallstartswhenyourcentralheatingsystemfails…

Eversinceyouwere93(intheTriassic),landbridgeshaveconnectedAntarcticatoSouthAmericaandAustralia. 117Thesehaveblockedoceancurrents, forcingnorth-southcirculationofoceans,whichcarries tropicalheat topolarareas,keeping themalmostaswarmasthetropics286injustthesamewaythatacentralheatingsystemcarriesheatfromafurnacetoroomsinahouse.

South America and Australia continue to driftnorth, placing unbearable strain on their connectionswithAntarctica. Theworld keepscooling,andnowseasonaliceraftsappearintheArcticOceanbytheendofFebruary(38millionyearsago). 287 InearlyApril (33.5millionyearsago) the linkbetweenTasmaniaand Antarctica is breached like a broken levee, spilling cold currents from the IndianOcean into the Pacific, and precipitating a steep drop in global temperatures andatmosphericCO2.288289Soonafter,inearlyMay(30millionyearsago),theDrakePassagebetweenSouthAmerica andAntarctica finallybreaks,opening the floodgates forSouthPacific water to surge into the South Atlantic. 290 Antarctica is finally marooned in theSouthernOcean.

Antarctica’s isolation is like a breakdown of your heating system, starting yourfitful descent into an ice-house, caused by a “perfect storm” of tectonic and climacticcoincidences.287280

First,chillgalesblastingtheSouthernOceandriveanewandpowerfulcurrentthatencirclesAntarctica,isolatingitfromtropicalwarmthandplungingthesoutherncontinentinto cold. 286 This new current, which persists today, is known as the “AntarcticCircumpolarCurrent”or“ACC,”anditistheonlycurrentthatextendsfromthesurfaceallthewaytotheabyssaldepths. 290Beingsodeepandpowerful,itislikeablenderthatmixes surface anddeepwaters, increasing thenutrient supply toplankton that consumeCO2. 291 Colder water also dissolves more atmospheric CO2 than warmer water, 292

increasingtheoceans’capacitytoreduceatmosphericCO2,furthercoolingtheclimate.

Second, theTethysOcean,whichup tonowhadallowed IndianOceanwater toflow through the Mediterranean region into the Atlantic, closes as Arabia begins tobulldozeintoTurkeyandIran.281Thisactionrebootsglobaloceancurrents,enhancingtheeffect of theAntarctic current. It also closes off the largeCentralAsian Sea called the“Paratethys.” Isolated, this stagnant sea now accumulates black, carbon-rich muds thatwillformCaspianSeaoil,tobeextractedlaterbyhumans.281

Third, the Himalayas undergo intense uplift, 276 almost matched by powerful

erosion and weathering that absorbs even more CO2 from the atmosphere, 281 furthercoolingyou.

Consequently,likearoominwinterwhoseheaterhasbroken,Antarcticabeginstofreeze.AstheCO2levelsdrop,288theSouthPolefirstdevelopsisolatedicesheetsathigherelevations,whichcoalesceintoalargeicecapinearlyApril(33.7millionyearsago). 293Antarcticforestsrecedeandarereplacedbymossesandfernsas theice-freeportionsofAntarcticaturntotundra. 294ThefirstpersistentglaciersdeveloponuplandGreenlandatthe same time. 295Asmore andmorewater gets locked up as ice, sea levels around theglobedropby50to60meters(160-200feet).296

Thefourthdriverinthiscoolingtrendistheiceitself.Likeashinybaldhead,itreflectswhat little sunlight reaches the polar ground back into space,where previouslyrock and vegetation would have absorbed it and warmed up. This “albedo effect”exacerbates the cooling further, contributing to a feedback loop of cooling and CO2reduction.

The result is plunging global temperatures - average air temperatures in midlatitudesdropby4-6°C(7-11°F),asdooceantemperatures.297

SeasofgrassIt’s hard to imagine aworldwithout grass. Today grass dominatesmuch of the

world, from the lush lawns of England to the brassy stalks of the Serengeti, from thebillowing prairies to the Pampas of South America, to the sea of grass on the AsianSteppesthatgaveGenghisKahnhisname. 298Around20%ofthemodernlandsurfaceiscovered ingrass 299 and thestaplesofhumandietacross theglobe:corn (maize),wheat,barley,rice,sorghum,millet,ryeandoatsareallgrasses.

UptothispointinYourLifeasPlanetEarth,grasseshavebeenshy,retiringplantsinforestclearings.237299Butinthenewcolderanddryer300climateofunreliablerainfall,therainforestsrecede.Clearingsenlarge intopatchesofopengrassland,growingever largerastimepassesandtheclimatedriesfurther.237

In thenewdry, lower-CO2 environment, shallow-rootedgrasseshaveaproblem.TogettheCO2theyneedforphotosynthesistheymustopenupmoreporesintheirleaves,becauseCO2 levels in the air are lower. But more open pores mean more water-loss,risking desiccation and death. At this point some grasses invent new kind ofphotosynthesis that solves thisproblem. 301Called“C4 photosynthesis,” it appearsby theendofFebruaryinyour100thyear(35millionyearsago),237andismuchmoreefficientinhot, sunny conditions than the older C3 photosynthesis, and it also allows the new C4grasses to conserve water. 299 In other words, it’s a great adaptation for sunbaked, drygrasslands.

Grasslandsalsobringwith themanewkindofsoilknownas“mollisols,”whichhaveadeeporganicrichlayer–whereCO2 isburied.It’spossiblethatthespreadofC4grasseshasasideeffectoflockingawayevenmoreCO2.302

It takes a while, but C4 grasses come to dominate increasingly arid grassland

habitatswhileC3grasses remainprevalent inmore temperateclimates.OpengrasslandshadalreadyappearedinSouthAmericainmid-February(around40millionyearsago),237andnowtheyspreadtoNorthAmerica,AsiaandAfricaandeventuallyAustralia. 237Thenew savannahs, prairies and chaparrals favor new creatures: giraffes, pigs, gazelles andgoats all appear at this time. 49 Small horses like Merychippus roam the plains. 49

Elephantine creatures like Deinotherium (with tusks curving from its lower jaw) orGomphotherium(withtusksonbothjaws)competewithgianthornlessrhinoceroseslikeParaceratheriumandTeloceras,ortheirminiaturecousinsMenoceras.49Feedingontheseherbivoresisthemarsupialsaber-toothedcatThylacosmilus.49

But before you get used to this idyllic scene of grazers and grasses, the worldchanges–explosively!

Volcanoesandinstantdiamonds

On the spring equinox of your 100th year (35.7 million years ago) an asteroidcollideswithEarthataplacecalledPopigai inRussia, leavingacrater100km(62mi)wide,96instantlytransformingnaturalgraphiteintherocksintodiamonds! 303Then,just3daysinyourlifelater(35.3millionyearsago), 96thereisadoubleasteroidimpactontheeastcoastofAmericaatChesapeakeBay96andoffshoreNewJersey.TheChesapeakeBayimpactblastsacraternearlytwicethesizeofthestateofRhodeIsland,andnearlyasdeepas the Grand Canyon, sending a giant tsunami clear over the 3,500 foot high (1,000meters) Blue Ridge Mountains. 304 305 The crater affects the shape of the bay and thedistributionofaquiferstheretothepresentday.

April to June (around 33-23million years ago) seesAfrica pushing further intoEuropeandtheMiddleEast,squashingtheoldTethysmarinesedimentsintoAlpineandPyreneespeaks.306ButnowArabiastartstofracturefromAfrica.Thesplitreleasesfloodsofbasalt lavaacrossEthiopiaandtheYemenaround30millionyearsago. 307Bychance,unusuallyviolenteruptionsareburyingalargeareaofMexicoinashandlavaatthesametime. 272Consequently the atmosphere sees a spike inCO2

308 and a correspondingglobalwarmingbyabout2°C.309

WithCO2at roughlypresentday levelsofaround400ppm(partspermillion), 310

the Antarctic ice sheet is now very sensitive to such shifts in climate, and it partiallythaws,leadingtosealevelrise. 308Goingforward,yourclimate issofinelybalancedthatvariationsinsolarwarmthcausedbyEarth’sorbitalwobblestriggermanylargeadvancesandretreatsof theAntarctic ice,withcorresponding large(around20metersor66feet)seesawsofglobalsealeveleveryfewtensofthousandsofyears.311310

Thefirstapes

Like the slow unzipping of a dress,East Africa begins to part from the rest of the continent, and the Great Rift Valleydevelops progressively from Ethiopia into Kenya and into Tanzania, 312 and the easternedge of theCongo, in June of your last year (25million years ago). 313 The rift zone isbuoyed up by hot mantle and rises thousands of feet in altitude, raising savannah toplateaussusceptibletotheoccasionalnightfrost,alteringthelocalclimatetothepresentday.313Thisdividesformerlycontinuoushabitatsandalsoputstheareaeastoftheriftintoarainshadow.

Asaresult,apes(ourancestors)begintoevolveseparatelyfrommonkeys,314andsoweoweourancestrytothisgeologicalmarvel.TheoldestapeiscalledRukwapithecus 314anditissoonjoinedbyotherapespecieslikethewaist-highProconsul.315

Yellowstoneblow-torch

MeanwhilemountainsarealsorisingatthewesternedgesoftheAmericas,astheseabedof theever-shrinkingPacificOcean is consumedunder them,melting intogassymagmasthatfuelviolentvolcanoes.InColorado,magmaworksitswaythrougholdflawsinthecrustleftbytheweldingtogetherofancientcontinentsbackinyouryouth,causinghugeexplosiveeruptionsthatburythesurroundinglandinvolcanicash. 316OneeruptionaloneintheSanJuanarea28millionyearsagospraysout5,000cubickilometers(1,200cubicmiles)ofash.317TheMexicaneruptionssetoffagainaround24millionyearsago, 272and over time there are many such eruptions all around the Pacific, a situation thatcontinuestomoderntimes.

NorthAmericaisdriftingwestwardsatarateofaround2cm(1inch)ayearoverthePacificplates.316DeepinthePacificmantlethereisaplumeofhotmagmaacting likeablow-torch that searsa trailofburnmarks into theover-ridingAmericancrust,whichsizzlesasitpassesoverit,spurtingoutmassiveflowsofbasaltlavabetweenAugust21andSeptember3(16.6-15millionyearsago).Hundredsof eruptionsbuildup toover 4km (2½mi) thick, coveringmuchofWashingtonState,northern Oregon, and western Idaho in a few hundred thousand years. 318 This lava isknowntodayasthe“ColumbiaRiverBasalts” 316andthemantleplumewilllaterfueltheYellowstoneSupervolcano.ButrightnowitsheatmeltsbitsofcrustthathadscrapedoffontotheundersideofNorthAmericasincetheCretaceous,releasingtheirtrappedCO2toaddtotheCO2thatwouldnormallyaccompanysucheruptions.223319178

Theresult–asyouhaveexperiencedmanytimesbefore–isariseinatmosphericCO2(tolevelssimilartotoday’s),andcorrespondingglobalwarmingby2to4°C, 310andtheAntarctic icesheet reduces toaround10 to25%of itscurrentsizeassea levels riseroughly50m(160ft).308310Butintime,asallthatfreshbasaltweathers,alongwithongoingweatheringinalltheactivemountainranges(theRockies,Andes,AlpsandHimalayas),itabsorbsCO2fromtheatmosphere,soglobaltemperaturesdropmarkedly.Antarctica’sicecapstabilizesinearlySeptember(15millionyearsago),Greenland’siceexpands,andtheArctic Ocean develops perennial sea ice at around the same time. 295 As the ice capsexpand,theylockinmoreandmorewater,andglobalsealevelsdropagain,bybetween55and75m(180to250feet).310290

ThedaytheseadriesupAsIndiawedges itselfeverfurtherunderTibet,andAfrica’sadvances toEurope

continue,theyforcetheParatethysSeatoretreatfurthertowardsthemodernshoresoftheBlackandCaspianseas.ArabiacontinuesitsseparationfromAfricawiththeopeningofthe Red Sea, pushing its way into Iran which responds by throwing up the ZagrosMountains.117

ApeshavenowspreadfromAfricatoEuropewherewefindDryopithecus,andtoAsiawherewe findSivapithecus (theancestoroforangutans). 320Butback inAfricawefindapeslikeKenyapithecus,whoseteeth,faceandelbowmorecloselyresemblemodernapesandourpre-humanancestors. 320KenyapithecusbecomesverysuccessfulandspreadsasfarasTurkey,andit’spossiblethatitisourancestor.320[A]

Theninmid-October(10millionyearsago)theAndesundergorapidupliftofover2km(1.2miorabouthalftheheightoftheRockyMountains)inunder2millionyears.321That’sthegeologicalequivalentoftakingtherapidelevator,andprovidesyetmoreerodedrocktoweatherandcooltheclimate.AtthesametimeweatherpatternsaroundIndiaandEastAfricachangetemporarily,turningdryAfricansavannaintowetlakelands,asIndia’smonsoon intensifies. 322 As ice sheets grow, global ocean currents switch over to themodern“conveyor”patternatthistime,changingthedistributionofnutrientsbetweenthePacificandAtlantic–aneventknownas“TheGreatSilicaSwitch.”323

ByNovember (7.3million years ago) your climate has now cooled enough formostofGreenlandtobecoveredinice.295

Toumaï,Ardiandthedawnofhumankind

Coincidentally, a new ape calledSahelanthropus (but nicknamed “Toumaï”) appears on the tranquil shores of a vastSaharanlake,inthe1stweekofNovember(7millionyearsago).WhatsetsToumaïapartis hismoreupright stance.And even thoughhis brain is small (a chimp-like370 cubiccentimeters(cc),or27%ofmodernhumanbrainsize),itsshapeisnew.Ithasthedistinctbrainstem,occipitallobes,andprefrontalcortexthatmarkhimoutasourearliestupright-walking“hominin”ancestor.324325SoToumaïstands–literally–atthedawnofhumankind.

He is soon joined by another ape-like creature called “Orrorin,” who is alsoadaptedtospendingatleastsometimeon2legs.325

By the middle of November (5.8 million years ago) the lush forests and riverbanks326oftheAfarregioninEthiopiaareechoingtoanewcall.Thescreechescomefromatroopofnewupright-walkingapes 327at thewater’sedge,vigilant forpredators,beforethey retreat into the branches of fig trees and palms as a herd of Kudu antelopeapproach.327Amongthemisyourancestor,“Ardi.”Ardiisayoungmotherstandingabout4feet(120cm)tall,withlargehandsandthumb-likebigtoesonherfeet,andabrainlargerthan Toumaï’s at around 450cc. 49 She is anArdipithecus, a group of primitive humanancestorsthatsurviveuntilabout4.3millionyearsago(theendofNovember).

CostaDelSaltInthecontinuinggameofcontinental“Sardines,”MoroccoisnowtouchingSpain

atGibraltar, constricting the connection between theMediterranean and theAtlantic.Asharpcoolingphasenowtriggersadropinsealevels,leavingtheStraitsofGibraltarhighanddryonNovember14(5.96millionyearsago).328329Theland-lockedMediterraneanSeabegins to evaporate, and over 360,000 years the level falls until it is two small seasseparatedby Italy, and it keepson falling.Eventually it dropsby5km (3mi) (that’s3timesdeeperthantheGrandCanyon)depositinga2.5km(1.5mi)thicklayerofsaltandgypsumontheseabed. 330Infact,somuchsaltandsedimentisdepositedsoquicklythatits weight depresses the crust of the Mediterranean into the mantle a little, sending atopographicbulgeripplingoutwardsacrossEurope.330

Whilethisishappeningthepolarlatitudesendtheirlatestglacialphase, 329raisingtemperaturesandrainfallaroundtheworld.Asicemelts,sosealevelsrise,anditmaybethat or the ongoing seismic jostling between Morocco and Spain, which releases thedeluge.

Itisadelugeonacolossalscale!

On November 19th of your life (5.33 million years ago) water bursts into theMediterraneanfromtheAtlanticOceanthrougha200kilometer(124mile)longchannel.Likeafirehose,watercannonsontothedesiccatedsaltpansat100millioncubicmeterseverysecond(130millioncubicyardspersecond),withthepowerofathousandAmazonrivers.331TheMediterraneanSealevelrisesasmuchas10metersadayforayearortwoinrealtime,untiltheAtlanticandtheMediterraneanareleveloncemore.331

OficeandmanNorthAmericareachesforthecompanionshipofSouthAmerica,couplingthrough

theIsthmusofPanamaaroundNovember23(4.8millionyearsago).332Theirunionblocksocean currents from crossing between the equatorial Pacific and theAtlantic, rebootingdeep water currents in the Pacific Ocean. 332 Consequently, eastern Pacific watertemperaturescoolsubstantiallyandsoglobaltemperaturesdropanother1.5°C(3°F)overthenextmillionyearsorso.332333

LucyInto this capricious climate a new creature is born in Africa, at the end of

November(4.2millionyearsago). 325SheisahumanancestorcalledAustralopithecusbutshe goes by the easier nickname: “Lucy.” 49 She is your great, great… (280,000 times)grandmother!Sheismorecomfortablewalkingon2legsthanArdiwas334327andhasabraincapacity slightly larger than Ardi at 380-485cc, which is 28-36% of that of modernhumans.49Butsheisstillfairlyape-like,spendingagoodamountoftimeclimbingintreesto avoid predators and gathering food. 335 Lucy is one of several different varieties ofAustralopithecusthatspreadacrossthecontinentofAfricaduringthenext2millionyears,sometimesleavingtheirfootprintsinfresh-fallenvolcanicash325oftheGreatRiftValley.

It isn’t long before the environmental rug is pulled from under ourAustralopithecusancestors.Asmallasteroidblastsan18km(11mi)holeatEl’gygytgyninSiberia 3.6 million years ago, 96 336 and you experience another warm period betweenDecember2ndand3rd (3.6 to 3.4millionyears ago)whose cause is unclear. 337 336 In thisPlioceneepoch,summerArctictemperaturesare8°C(14°F)warmerthantoday,andCO2

levelsaresimilartotoday, 336as rain-drenchedforests fringe theArcticOceanagain,andice-aversemollusksliveinArcticwaters.295Buttheever-driftingcontinentsstillhavesomemoves to pull. This time it’sAustralia that shoves itsway towardsChina, bullying theIndonesian islandson itsway.BetweenNovember29andDecember7 (4and3millionyearsago),therearrangementofislandschangestheoceanconnectionbetweenthePacificandtheIndianOceans,turningoffawarmflowtotheIndianOceanandswitchingitovertoacoldcurrentfromtheNorthernPacificthatstillflowstoday.338

The change in heat distributionsubstantiallycoolshigherlatitudes,soicebeginsitsgripontheArcticwhileGreenland’sicecapexpandsandthickens.339Antarcticacoolssubstantiallyanddevelopsaskirtofsea-icethatlaststhroughthesummer.340Coolingandmixingthedeepoceanwaters,alongwitherosion andweathering in theHimalayas, theAndes, and elsewhere, draws downCO2further.291Thissendsyouyo-yoingdownacoolingtrajectory 332339 336untilaverageglobaltemperaturesfallbelowtoday’saroundDecember7(3millionyearsago).341

Andyoukeepgettingcolder.

IceageLikeadoseofthe‘flu,yourtemperaturegoesthroughtheclimateequivalentofa

“buckingbronco.”341339Yousufferatleast15temperatureswingsjustbetweenDecember2andDecember10 alone (3.6-2.6millionyears ago).Then the fluctuationsbecomeevenmore extreme, with each cold swing colder than the last. 342 Ice now envelopes thenorthern latitudes of America, Europe and Asia, advancing and retreating with eachclimate swing. In the tropics, especially Africa, cold swings bring drought and warmswingsbringrain.

Theicyharmonyoforbitalwobbles

Theseclimateswingshavea regularpattern.Up toDecember26 (700,000yearsago)theicycyclesareevery41,000yearspunctuatedbywarmrespiteslastingfrom500toafewthousandyears.342Somewarmepisodes,liketheone2.4millionyearsago,resultinsubstantialmeltingoftheGreenlandIceCap.295

These swings are mainly causedbyEarth’sorbitalwobbles.AsourEarthcirclestheSun,theorbitisn’taperfectcircle,sotheplanetisfractionallycloserandfurtherfromtheSunatdifferenttimesoftheyear.This“eccentricity”changesona100,000yearcycle.Also,theaxisaboutwhichwespingivingusdaysandnightsisonaslant(ahangoverfromtheMoon-formingimpact).Butthereisaslightwobbleinthatslant,called“obliquity,”whichvariesona41,000yearcycle.Thereisanotherorbitalcyclecalled“precession,”whichisthewobbleofourrotationaxisaboutevery22,000years-likeatoyspinningtoptracingoutever-wideningcirclesbeforeitfallsover.Justasaguitarchordismadefromthecombinationofseveraldifferent individualnotes,sotheharmoniccombinationoftheseorbitalcyclesdrivesthewaxing-waningsongoftheiceages.

Theicesheetsspreadandthickenduringcoldswings.Thethickertheygrow,themoretheirweightdepressesthecrustbeneaththem,likeaboatloadedwithcargo(thisiscalled “isostacy”). The more the ice sheets thicken, the lower the crust sinks, and thecolder the climatemust remain to preserve them.Eventually, as your orbit bringsmoreintensesolarwarmth to thenorthernhemisphere, it’s justnotcoldenoughtosustain thelow-altitude ice.So the icesheetsmeltawayrapidly, retreating to thefarnorth in justafewthousandyears,beforethecyclebeginsalloveragain.343

Butthereismoretoiceagecyclesthanorbitalwobblesalone. 342CoolingoceansabsorbmoreCO2butthiseffectlagsbehindtheorbitcyclebyathousandyearsorso, 344

anditiscounteredbychangesinoceansalinity,sealevels,andreductioninvegetatedlandarea.344Thereisalsothealbedoeffect,wheretheincreasingareaofbrighticereflectsmoreof theSun’sheatback intospace,amplifyingcooling(theshinybaldheadanalogy, likeyouexperiencedinthePermian).

Ocean currents such as theNorthAtlanticConveyor System slowor shut downaltogether during cold stages, changing the heat distribution around the globe, 342 whilevariationsinsealevelhaveknock-oneffectsthemselves.WhensealevelsarelowenoughtoturntheBeringStraitintoalandbridge,theArcticOceaniscutofffromthePacific,butinwarmingcycles the twooceans reconnect.This is like throwingaswitch in theheat-

distributionpatternfortheoceans,whichmayberesponsibleforthesudden,extremeflip-flopsinclimateknownas“Dansgaard-Oeschger”and“Heinrich”events.345

Volcaniceruptionsdisrupt thecycleswithrandomtiming.Largeeruptionsofashand sulfur create a temporary cooling by screening out solar radiation, amuch shorter-termeffectthanwarmingfromtheirCO2output.342

So the climate cycles are not at all like a classical guitar chord of clean orbitalnotes.Rathertheyarenoisyanddistorted,morelikeaJimiHendrixelectricguitarchord.

Droughtandtsunami

AsicesheetsspreadacrossNorthAmerica,EuropeandAsia,theypickupbillionsofrocksandboulderstogrindthelandunderthem.Eachsummerretreatingiceexposesthefinedustproducedbythisgrinding,andwindwhipsitintoduststorms,depositingitinthick“loess”deposits inAsia. In fact there’s100 timesmoredust in theairat this timethaninthemodernera.346

Astemperaturesfallsodoesthehumidityoftheatmosphere,sotropicalmonsoonsweaken and conditions become evermore arid.EvenC4 grasses can’t cope, so tropicalgrasslandsnowgiveupterritorytoaridscrublandandthendesert.347Largeswathesofmid-latitudelandalternatebetweenaridsteppesincoldcyclesandtemperateforestinwarmerinterglacials.348Globalsealevelsfallwitheachiceage,exposingcontinentalshelvesandproviding land bridges between former islands, reconnecting Arabia and Africa, andacross theBeringStraitbetweenAsiaandNorthAmerica.There isnoEnglishChannel,and theThamesandRhine rivers join in a single estuary in theNorthSea. 49With eachinterglacial,sealevelsrise,monsoonrainsreturntotheSahara,turningdesertintopasture,and filling lakes and numerous rivers that wind their way across the Sahara to theMediterranean.349

Out of the blue you are slammed by another asteroid, which hits in thesoutheasterncornerof thePacificOcean2.51millionyearsago.Knownas the“EltaninImpact,” it vaporizes water and rock into the atmosphere, and sends a giant tsunamithroughoutthePacificandthesouthAtlanticwhereitdevastatescoastallandandburiesitinmetersofdebris.Anditmaybetheoceansaltandrockdustfromtheimpact,spreadingthroughthehighatmosphere,whichamplifiesanalreadycoldclimateintoapowerfuliceage.350

Largeanimalscanconservebodyheatmoreeasilythansmalleranimals,sogiantmammals begin to dominate. 5meter high (16 feet) woolymammoths evolve, bearinghugetuskstoclearsnowfromthegroundsothattheycaneat. 49InSouthAmericagiantgroundslothsasbigasanelephantandbrandishingclawsaslongasaman’sarmfightoffpacksofsaber-toothedtigers. 49Woollyrhinosandherdsofmodern-sizedhorsesviewithgiantelkforgrazing.49

Yellowstone

AllthistimeNorthAmericahascontinuedtocreepwestwardsovertheblowtorch-likemantleplumethatcausedtheeruptionoftheColumbiaRiverBasaltsbackinAugust.Ithasburnedan800kilometer(500mile)chainofvolcanoesthatstretchesfromnorthernNevadaallthewaytoYellowstoneinWyoming,316whereitnowitcreateshavoc.

On December 15 of Your Life as Planet Earth (2.1 million years ago) theYellowstone Supervolcano explodeswith titanic force, depositing a blanket of volcanicash over more than half of the present United States. 351 The eruption sprays so muchmoltenrockintotheairthattheroofofitsmagmachambercollapses,creatingacaldera100km(60mi)wide.ItwilldoitagainonaslightlysmallerscaleonDecember21(1.3million years ago), and again onDecember 26 (640,000 years ago). That last eruptionagain covers about half of theUnitedStates in ash. 351 Shortly after that, an eruption atLong Valley in California blankets about a third of the United States with ash onDecember26(630,000yearsago),351anderuptionsliketheseoccursporadicallyallaroundthePacific.

Butwearegettingaheadofourselves.

BecominghumanYour see-sawing temperature means the environment is constantly changing. In

EastAfricawoodlandgiveswaytosavannah, thenreturnstowoodlandinpacewiththeclimate cycles, placing great environmental stress on our ancestors. This winnows thefittestandmostadaptablefromtherest,forcingtheirrapidevolution.352

Thefirststonetools

During the reign of Lucy and herAustralopithecus relatives, someone invents simple stone tools. 353 It’s not clear exactlywhich species is responsible, but it starts humankind’s obsessionwith technology. It isnowaroundDecember15 (1.98millionyears ago), andAustralopithecus has evolvedamorehumancreature,calledAustralopithecussediba.Sedibawalksuprightonarchedfeetlikemodernhumans, 354andherhands,pelvisandfeetaremorehuman-like thanLucy’s.Herhandsareevendexterousenoughtofashiontools, 355andherbrain,thoughstillaboutthe same size as earlierAustralopithecus varieties, is shaped like a human brain. 356 Herbrain is different because a crucial gene for brain development called “SRGAP2” hasrecently undergone its second bigmutation (around time the first tools were invented)resultinginanextracopyofthegene,whichwehaveinheritedtothisday.357

She coexists with a variety of other early humans spread across Africa.Paranthropushas evolved from Australopithecus into thickset creatures with powerfuljawsandthickteethforchewinggrassesinthesavanna.358Theirbrainsarearound30-41%

49ofmodernhumanbrainsbuttheydon’tappeartomaketools.ButSedibaissooneclipsedbyherdescendants.

Thefirstcook

December16 (1.8millionyears ago)unveils a tall, slenderEastAfricanhuman,similarinheight,gaitandstaturetoyouandme.Homoerectus’physiqueisadaptedtohot,dry conditions, and he can run and walk as effectively as modern humans. 325 49Homoerectus is suchadiversegroup that severalvarieties lookalmost likedifferent species -ergaster, habilis, rudolfensis, antecessor and another as-yet-unnamed creature currentlylabeled“KNM-ER1802.” 359Hisbrainenlargesfrom44-67%to95%ofours49inthefirstfewhundredthousandyears.Hisfaceisdistinctlyhumanwithapointednose,buthehasalowforeheadandamassivemonobrow,andlacksachin.325

Whatsetserectusapart fromhisancestors ishowsmarthe is.Hesoon learns tomake fire and use it for cooking,which provides the nutrition to support his enlargingbrain.360361Healsomakesavarietyofsophisticated“Acheulean”stonetools,sometimesindedicatedfactories,362andhecanthrow,enablinghimtokillfromasaferdistance.363

Armedwith tools,huntinganddefendingbythrowing,nourishedbycookedfoodandsupportedbycooperativesocialskills (possibly including some form of language), 364 erectus wins out against largecarnivores inAfrica for carrion and prey. As erectus takes over at the top of the foodchain,dethronedcarnivoreslikethe“bear-otter”goextinct.365Henowbeginstotraveltheworld, spreading out of Africa into Europe and Asia, reaching as far as China andIndonesia,366325evenashisParanthropuscousinsbegintodieout.

ChristmasinEnglandEarlyhumans–probablyavarietyoferectus -havenowarrived inEastAnglia,

England,justintimeforChristmas(between970,000and814,000yearsago).366Theyare

camping on a grassy plain by the Thames estuary (which is in Norfolk at this time),leaving footprints in estuarinemuds and fending off the occasional hyena. 366 367 As theychipflintsintotoolsnearaconiferforest,theyaresurroundedbyhorsesandantelope.Anicywindblowsoff theNorthSea from theadvancing ice sheet a fewdays’walkaway,signaling the onset of another cold phase. 366 They will leave England soon, and theirdescendantswillreturnwhentheicerecedesagainonBoxingDay(700,000yearsago).366

14Thefirstcook:Homoerectus(byCiceroMoraes-viaWikimediaCommons)

Bynowtheclimatehascooledsomuchthaticesheetsnolongermeltawayinthewarmpartsofthe41,000yearcycle,sotheynowlastforentire100,000yearorbitalcycles(now responding more to orbital eccentricity than orbital obliquity). 342 295 Also, thescouring effect of the icehas cleared awayall the loose sedimentbeneath it, so the icechanges from a flattish quick-responding sheet to a high plateau of persistent thick ice,weldedtobedrock.295

It’s also around this time that another mutation in the genetic code for humanbrains occurs, creatingyet another copyof theSRGAP2gene. 357 Thismay be a crucialfactorinthegradualemergenceofhumanswithbrainsaboutthesizeofourownbrains. 325KnownasHomoheidelbergensis, theymaketheirwayallacrossAfricaandintoEuropebetweenDecember27and29(600,000to250,000yearsago).Theyarearchaicversionsofourselves, and they look likeus except theyhave a low forehead and large eyebrow

ridges,andlackachin.Thesepeoplealsobehavelikeusinmanyways,efficientlyusingfire368andhuntingwithspears,somewithstonetipsthattheyhaftedontowoodenshafts, 369andothersthattheyhavewhittledtoasharppointandthenhardenedwithfire.Theyarealsocreatingandusingarangeofsophisticatedstonetools,49andhavelanguage.364

heidelbergensisisoneofseveralregionalvarietiesofhumanspreadacrossAfrica,EuropeandAsia.AsheidelbergensisemergesintoEuropeandAfrica,tribesoferectusarenowevolvinginAsia.SomespreadfromSoutheastAsiatoSiberia,eventuallybecoming“Denisovans.” 370 Others become isolated on an island in Indonesia, where limitedresourcesdrivethemtoevolveintotheminiaturefloresiensis,325nicknamed“TheHobbit.”

Each time the ice advances, it isolateshumanandanimalpopulations in refugeslike Spain, Italy, and the Balkans where they begin to evolve in response to localconditions, differentiating from the original population. 371 Brown bears, bison,musk oxandevenhedgehogsalsoundergodifferentiationatthistime.371

ManOur species,Homo sapiens, gradually emerges in Africa around December 29

(around208,000yearsago).325372Theyareyourgreat,great…(13,000times)grandparents!

We seem to be the product of thecontinuing extreme climate fluctuations. Soon after our lineage appears, the world isgrippedbyanextremelycoldandhyper-aridphaseknownasMIS6. 373AterribledroughtgripsAfricaatthistime:lakesandriversdryup,andtheparchedsavannaswirlswithdustdevils. 374 Desiccated corpses and bleached bones are all that remain of the herds ofantelopeandgazelle.Ourancestorssurvivebytheirwitsandmigratetoisolatedrefuges, 375such as the caves atMossel Bay in South Africa, where they survive on shellfish andfynbosplants.373374

Tostartwith, thesehumans lookmore likeheidelbergensis, 325butover time theycome to look like themodern humans of today. They use fire not just forwarmth andcooking,but theyhavealsofiguredouthowtoheat-treatstone to improve their tools, 376resulting in advanced stone tools like “bladelets.” 373 They hurl spears rather than juststabbingwith them,making themmuchmoreeffectivehunters. 377Theyalsohavea richculture,usingochrepigmentforpainting.Theyclearlyhavethe“smarts”fortechnological

andculturalinnovation,cooperationandcommunicationskills-language-toteachthemtoothers.

Aroundthesametime,Europeanheidelbergensisdevelopintothemuscular,thick-bonedandbig-eyedNeandertals. 325378Neandertalanatomymakessenseforacold-adaptedpeople that range the fringes of the ice sheets. Far from being the brutes of popularculture,theyarepeoplewithbrainsactuallylargerthanmine(andmaybeevenyours)at1412ccor105%ofmodernhumans.’49Theyprobablyspeakalanguage,36449andtheyhaveaculturewhichhasa special regard forblackbird feathersandeagleclaws. 379 Fromaninitiallysmallpopulation, 325 theyspreadacrossEurope,NorthAfrica, 380 theMiddleEast,andasfaraswesternAsia,huntinginthefluctuatingforeststhatfringetheicesheetsandglaciers,andlivinginlargehutsconstructedfrommammothbones.381

Our ancestors eke out an existence in their separate refuges, evolvingindependently,untiltheclimatebeginstoimproveroughly140,000yearsago.382Whenthehyper-aridMIS6periodisover,therainsreturntoAfricatheybringwiththemtheherdsofgameandmankindcanmultiplyandwander throughAfricaonceagain.Monsoon rainsrefill Saharan lakes and rivers, tempting game and humans alike towards theMediterranean.349

Andyet there isa“suckerpunch.”AnewcolonyaroundtheoasisatDakhlehinEgypt is decimated by a meteorite impact on December 30 (around 145,000 thousandyearsago).Theblastsnaps trees for thousandsofmiles, injuringandkillingpeopleandanimalsalike,whilemoltenejectasetswildfiresacrossthearea.383FortunatelythedamageislocaltoEgypt,andhumanskeepspreading.

10.Your100thBirthday!

Youstartyourbirthday–yourlastday-onanoptimisticnote.

At midnight (126,000 years ago) you begin to feel the lovely warmth of aninterglacialcalledthe“Eemian,”withArctictemperaturesrisingto8°C 384warmerthaninthe mid-20th century, but globally it will only reach about 1°C warmer than moderntimes. 295 It’swarmbecauseourorbit isdeliveringabout11%moresolar radiation to theArcticthanthepresentday.295Sealevelswillsoonriseupto9meters(30feet)higherthanmodern times, as a result of a drastic reduction of ice sheets in Greenland andAntarctica.384385

But before the sea levels rise, ourancestorsarealreadyonthemove.ArrivingatthesoutherntipoftheRedSea,theyfindthewatershavebeenmiraculouslyandbiblicallypartedduetolowsealevels.TheycrossintoArabiabyaround125,000yearsago 386 and fanout,wandering thewildernessuntiltheyreachtheStraitsofHormuz.TheygazedownfromtheOmaniheightsatariver(thePersianGulfisallbutemptyduetolowsealevels)insearchofaford.FromtheretheymoveintoIranandfurtherintoAsia, 386387wheretheyencountervestigialbandsofHomoerectusandDenisovans.

Further waves of human tribes emerge from Africa through the easternMediterranean,encounteringNeandertals in theprocess.Someofus takea fancy to theNeandertals(ortheotherwayaround)andthereissomeinterbreeding,totheextentthatmodernEuropeans,NorthAfricans 380andAsiansaregeneticallyabout2%Neandertal, 388whereas Africans south of the Sahara are mostly pure sapiens. 389 Those of us with

Neandertalgenesinherit theirgenesforhair,skin,andenhancedriskofvariousdiseaseslike lupusandChron’sdisease. 388Theoffspringof theseunionsarealso less fertile thanpurebredsapiens.388

AswespreadintoAsiawecolonizenewareasandmixwiththelocalpopulations,includingDenisovans.Somecloseencountersareofthesexualkind,withtheresult thatMelanesians,PolynesiansandAustralianAboriginals 390have inheritedup to5%of theirDNAfromDenisovans.391StilllaterpopulationsemergefromAfricaandspreadandbreed,maskingthegeneticsofearlierinterbreeding.

But you’re already plunging into another icy chill that will last all day.Occasionally,warmeroceancurrentsfromthetropicseatawayatnorthernseaiceuntilalarge area collapses. 392 This generates a warm flash (known as a “Dansgaard-OeschgerEvent”) thatcomesonsuddenly(in justdecades in real time),warming theNorthby9-15°C(16-27°F),butbarelyaffecting theSouth.Within30minutes(3,000years) theseaiceregrowsandyouarebacktonumbingcoldinhigherlatitudes,anddrasticdroughtinthetropics.Thishappensatleast25timesonyourbirthdayalone!342

Change forces innovation. It drives our ancestors to develop new technology,craftsmanship and adornments in fits and starts, coinciding with periods of increasedrainfallthataccompanyeachwarmswingintheglobalclimate.393

The Asian front of our exodus receives a small setback when the TobasupervolcanointheislandofSumatraexplodesaround9:55AMonyourbirthday(73,880yearsago). 394Theeruptionisextremelyexplosive,andithurlsimmensequantitiesofashand sulfur into the upper atmosphere to circle the globe. Ash blankets a 6,500km(4,000mi)wide area that extends fromArabia toChina and the southern IndianOcean,covering India in ash 15 cm (6 in) thick. Amazingly it has very little effect on ourancestors,whoquicklyreturntotheaffectedareas.395

WorldtourWehumanscontinueourperipatetichabits,multiplyingandexpandingourrange.

At 2PM, as an asteroidblasts theBarringerMeteorCrater inArizona 96 (49,000 yearsago), we have already reached Australia. 396 Even in these times of low sea level thisrequirescrossingabout70km(40mi)ofopenoceanonsomekindofraftorboat,andit’sthereforenosurprisethatfishermenaresoonharvestingtunaandotherfishfromthedeepoceanby42,000yearsago.397

By3:30PM(45,000yearsago), 398 takingadvantageofa temporary respite fromtheextremecoldandaridity,wehavespreadthroughtheMiddleEastandEuropeasfarasEngland,399andthroughAsiatosoutheastAsia.400By4PM(40,000yearsago)someofushavemadeittoChina.401

Along the way we adopt man’sbest friend – domesticated dogs - by 33,000 years ago, 402 and by 20,000 years ago inChina,someofushavemovedfromheat-treatingstonetoolstoheat-treatingclay.Thisistheinventionofpottery(a.k.a.China)andwe’realreadycookingfoodinthosepots. 403404Byaround17,000yearsagoweareanglingusingfishhooksmadefromshell.397

Fratricide

As we go, either by genocide or by technical superiority or by sheer superiornumbers,HomosapiensgraduallyeliminatesallotherclaimstotheHomothrone.Attea-timeonyourbirthdaywhenyouaresnuffingthesolitarycandlethatrepresentsyour100years,thelastoftheNeandertalsfinallydiesout(around40,000yearsago),some2,500to5,000 years after our arrival in Europe. 398 TheDenisovans, the Hobbit-like floresiensispeople,andamysteriousChinesegroupknownonlyasthe“RedDeerCavePeople”alsodisappear.405

YourlonggoodnightItisnow8:30PM(18,000yearsago)andyouareascoldasyouhaveeverbeen.

YouhaveathickicesheetallacrossNorthAmerica,fromNewJerseyandLongIslandintheeast,asfarasWashingtonStateinthewest.342IcehasalsooverrunGreenland,Iceland,most ofBritain, Scandinavia, northernEurope andRussia.Neandertals have long sincediedoutbuttherestofourancestorsaredoingOK.

By 8:45PM (17,000 years ago), as your orbit brings you warmer northernsummers, thereis thefirsthintofa thaw.Meltwaterpoolsbehindlobesofrecedingicesheet in western Montana 406 and builds to a vast lake called Glacial Lake Missoula,covering 7800 square kilometers (3,000 squaremiles) to a depth of 610meters (2,000feet). 406Aswarmingcontinues, the icedamweakensuntil it ruptures, sendingawallofwater 240meters (800 feet) high surging acrosswestern Idaho and easternWashingtonstate, travelling at 85 kph (50mph), ripping away the ground in its path to the PacificOcean,leavingawide,barrenlandscapeof“scablands”initswake. 406Amazinglythisisjustthelatestofmanysimilarcatastrophesduringtheiceages.406

IcehasnowrecededenoughfromtheAleutianArc 407toexposealandconnectionbetween eastern Siberia and Alaska, and by 9PM (15,000 years ago) some of us havealreadyspreadacrossbothAmericancontinents.49

Whereverwego,animalsbegintodisappear.NotlongafterwearriveinAustraliawehuntthegiantkangaroo,a2-tonnewombatandotherlargeanimalsintoextinction,causingapermanentchangeinvegetationpatternsandpromoting wildfires. 396 408 409 Before we arrive, the Americas are crammed with largeherbivoreslikemammoths,camelsandgiantgroundsloths,butwithinabout1,000yearsofourarrival,80%ofthelarge-bodiedmammalsareextinct.410Overtheplanetasawhole,ourarrivalcoincideswith theextinctionof⅔ofmammalgeneraand½of largeanimalspecies.411Someofthestressonthesecreaturesmaybeduetoclimatechange,butwearethemainculprits.411

By 9:15 PM (14,000 years ago) the thaw turns to lasting warmth and the icecontinues to retreat all across theworld.As it departs it abandons its rocky luggage aspilesofsandandbouldersthatmarkitsfurthestextent,creatingLongIslandinNewYork,412andstrandingimprobablylargebouldersonhilltopsacrossthehinterlandastheysettlethroughmeltingice.Globalsealevelsarerisingfast–byasmuchas5m(16ft)percenturyforawhile,thenslowingto40cm(16in)acentury.258413Theywilleventuallyrisebyover120m(400ft)fromtheirlowestlevelsintheiceage.

Inventingagriculture

By9:30PM(about13,000yearsago)humanstakecontroloftheirfoodsupplybyturningtoagriculture,andtheybegintosettleinvillages.414

Butjustasyouarestartingtogetcomfortable,somethingtipsyoubackintoice-at9:33PM(12,890yearsago). 415 416 It’spossiblyanasteroid impact thathitssomewhere inQuebec,Canada,sprayingplatinum-richejectaacrossEuropeandtheAmericas,andintothe upper atmosphere, darkening the Sun and returning you to an ice age climate foranother1,200years(“TheYoungerDryascoldperiod”).415417416418

TheendThecoldsnapisrelativelyshort-lived,andat9:45PM(11,700yearsago)youfeel

anotherhotflashcomeon…andthistimeyoustaywarm-theiceageisover!

Butyoudon’tfeelmuchlikecelebrating.Youfeeltheendapproaching.

Temperaturesaroundtheworldriseuntilaround10,000yearsago,whentheyleveloffcreatingastableandbenignclimate,coolingslowly,thatlastsuntilthemodernera.419

By10:15(9,000yearsago)MiddleEasternpeoplearemakingpottery.

In the continuing thaw,meltwater has pooledbetween the dwindling ice cap innorthernCanadaand theGreatLakesofNorthAmerica.Called“LakeAgassiz,” itnowcoversalargeswatheofCanadainupto250m(820ft)ofwater.At10:24PM(8,330yearsago)thelastoftheiceinHudsonBaycollapses,sendingtheicywatersofLakeAgassizand an armada of icebergs flooding through theHudson Strait into theNorthAtlantic,chilling ocean temperatures, and cooling the climate inEurope andAmerica for 3 or 4centuries.420

And itmay be this that promptsMiddle Eastern people to invent irrigation andcheesemakingby10:30PM(8,000yearsago). 421 414 10:50PM (6,000years ago) sees theadoptionofmetaltoolstoreplacethestonetoolsusedovertheprevious4millionyears.414AtaroundthistimetheEgyptiancivilizationbegins.

…onehourtogo…

Ageofempires

At11PM (3,400BC) theMesopotamians inventwriting as the earliest city-statestakeholdontheshoresofthePersianGulf. 414Soonafter, theBritsbeginconstructionofStonehenge (3,100BC)and thenEgyptgoesonapyramidbinge, constructing theGreatPyramidatGizaandtheSphinxaround11:06PM(2,560BC).422Nottobeoutdone,SargonofAkkadbeginstheworld’sfirstempireinMesopotamiaat11:10PM(2334BC)414aroundthesametimeassomeIndianscolonizeAustralia,bringingdingoesandnewtechnology.423At 11:30 PMCyrus theGreat of Persia creates an empire 50% larger than the RomanEmpire, gives theworld its first bill of rights and a “Suez” canal. 414 Then at 11:33PMAlexandertheGreatdefeatsthePersiansin331BC.414

11:36PM-JuliusCaesarisappointeddictatorofRome(44BC)…11:37-thebirthof Jesus…11:44 -ThebirthofMohammed (570AD)…11:49 -LeifEricsondiscoversAmerica in the year 1003… 11:54 - Columbus discovers America again in 1492…Between 11:54 and 11:58 Europe goes through a “Little Ice Age” in the 14th to 19thcenturiescausedbyacolossaleruptionoftheSamalasvolcanoinIndonesia424425…11:58-The Wright Brothers make the first powered flight in 1903 and in 1908 a meteoriteexplodes over Tunguska, Siberia, flattening over 2,000 square kilometers of forest andshoweringthegroundinironanddiamonddust.426

…1minute(70heartbeats)togo…

55heartbeats – the end ofWorldWar 2…39heartbeats -TheBeatles arrive inAmerica…35heartbeats–thefirsthumanontheMoon…8heartbeats–terroristsattacktheUSon9/11…

Yourlifehasflashedbeforeyoureyes,andyouarebarelyawareofyourlastbreathleavingyourbody…

11.BottomLineYourplanetisvery,veryold.

Measuredinthelifespanofacentenarian,simplelifeemergesinyourteens.Youarewell intoyourmid-lifecrisisbefore lifestartspollutingtheplanetwithoxygen.Youareinyourlate70sbeforeprimitiveplantsandfungibegintocolonizeland.Youare86before complex animals show up, 88 by the timeworms alter ocean chemistry and theseashellisinvented,and90whenfishevolvejawsandlivebirth.Inyour90s4-leggedlifeevolves from fish, and then dinosaurs,mammals, birds and flowering plants (includinggrasses)emerge.Thedinosaursareextinguishedwhenyouare98,leavingthefieldclearformammalsandbirds.Primatesevolveinyourlastyearoflife,andinyourlastmonthprimitive pre-humans appear. Early humans spread fromAfrica to Europe andAsia byChristmasinyourlastyear,andthenmodernmanevolves2daysbeforeyouturn100andspreadsacrosstheglobeonlyduringyourlastday.

Throughallthis,theplanetshapedlife,lifeshapedtheplanet,andcosmiceventsshapedboth.

Cosmic events shaped the Earth. The Moon-forming impact changed ouratmosphereandthecore,whichgeneratesthemagneticforce-fieldthatprotectsusfromsomuchharmfulsolarradiation.TheMoon’stidesstirtheoceans,andtheoff-balancetiltofour planet gives us seasons and orbital cycles ofwarmth and cold that even drove theevolution of humans. Comets and asteroids have pelted us from the dawn of time,sometimesbringinglife-enablingchemicals,sometimesdevastation.

Geologic events shaped life. Shifting tectonic plates gave us CO2-producingvolcanoes,mountainsthaterodeandtrapCO2intheprocess,andoceandepthswhereoldcrustandcarbonisconsumedforrecyclinginmoreeruptions.Shiftingcontinentsattimesallowed the global spread of creatures like dinosaurs and mammals, and at timesmaroonedlifetoevolveindependently,likethekangarooandapes.Itreconfiguredoceancurrentsthatchangedglobalclimate,tippingtheplanetintotheiceagesthatgrippedtheworldoverthelast3millionyears,drivingtheevolutionofman.Evenlifeitselfseemstohavestartedasarock-lifehybridinmagma-poweredunderwatersprings.

LifeshapedtheEarth.Fromthebeginning,lifestartedtomoldtheEarthinitsownimage as “stromatolites.”Sunlight-harvesting germs gave theworld oxygen that causedthe world to freeze, but they also made oxygenated minerals and ores, and saved ouroceans. Life colonizing land changed weathering, the carbon balance, and climate. Itgeneratedmoreoxygentosupportbiggeranimalsthatgrewlegstowalkonthegreenland.Newplantsthatgrewtallonthestrengthofwoodbecamecoal,whilealgaediedtogiveusoil.

Rare,copiousvolcanicoutpouringscalled“Large IgneousProvinces”punctuatedevolution by producing global warming, ocean acidification, and oxygen-starved seas.TheyinducedtheseclimatechangesbyemittingvastquantitiesofCO2andMethanefasterthan the planet could safely absorb it, with devastating consequences for life at thosetimes.

Nowweliveinanewgeologicalepoch-the“Anthropocene.” 427HumansarejustasmuchofaninnovationtotheEarth-climate-lifesystemasthecyanobacteriathatgavetheworldoxygen,orthewormsthatchurneduptheCambrianseabedandchangedoceanchemistry.We are generating CO2 at rates and quantities to rival those Large IgneousProvinces,dwarfingtheCO2producedbyvolcanoesinrecentmillennia.428Wecan’texpectthebalancebetweenclimate,lifeandEarthtoremaininerttothishuman-inducedchange.Rather,wecanexpectittobehaveasithasdonesomanytimesbefore.

Part2:TechniquesoftimetravelWhenyouthinkabout it, it isanactofbreathtakingaudacitytoplot thestoryof

Earthdeepintothepast,beforehistory,beforepeople,evenbeforelife.YetitisthesameintrepidlyinquisitiveinstinctthatputmanontheMoon,sailedtheunexploredoceans,orinventedtheairplane.Theforcethatdroveourancestorstoreplacestonetoolswithmetalones,ortotakecontroloftheirfoodsupplybyfarming,isthesameself-improvingforcethatdrivesscientificinquiry.ItisashumanasourDNA.

“Your Life as Planet Earth” is a story revealed by some amazingly smartdeductions from evidence gleaned through all branches of science, across all scales ofobservation from the microscopic to the cosmic. It is not fiction, and neither is it anembroideredcreationoverathinarmatureofevidence.Everyfactisbackedbyscientificevidencereferencedtoitssource,andmostofthosesourcesarefromrespectedscientificjournalsthatemployapeer-reviewcheckingprocessbeforetheypublishtheinformationtotheworld.

Someofthatevidenceisstrong,someismorecontroversial.ThedeeperrecessesofEarth’shistory–thefirst2billionyearsorso–arefraughtwithdifficulty,forinstance.Ofcourse,todescribeeverypieceofevidenceandtoflesh-outeveryargumentwouldtakeas much space as a large university library, so I have focused on a few key areas ofevidence:cosmicorigins,adynamicplanet,evolutionoflife,clockingtime,andsamplingancientclimates.

New research is performed all the time, so fresh insights are likely, and someconclusionsarebound tobeprovenwrongat somepoint. That’s anessential aspectofscience– its conclusionscanbe testedandeither confirmedordisproved.Yet thebasicskeleton of facts tends to remain over time, being fleshed-out and brought into ever-sharperfocuswitheachnewresearchresult,evenifthefinerdetailschange.

1.OriginWhatsouvenirsremainofthedarkbeginningofourSolarSystem?

TheoldestEarth-formedspecks(zirconcrystalsfromJackHills,Australia)aretooyoung-only4.4billionyearsold–asare theoldestcompleterocksat4.2billionyearsold.That leaves the first400millionyears (oryour first8yearsofYourLifeasPlanetEarth)without Earth-rock evidence. Fortunately, we do have rocks older than that.Wehavespacerocks–meteoritesandlunarsamples-andtheytellusagreatdealabouttheoriginoftheSolarSystemandtheEarth.

CosmicsedimentsDatingthebirthoftheSolarSystem

The date for the birth of the Solar System ismeasured from a kind ofmottledmeteorite called a “CI chondrite” and on basalt-like meteorites called “angrites.”Chondritesareessentiallycosmic sediments 6 composedofgrainsand sooty specks thatclumpedtogether inspaceat thebirthof theSolarSystem.TheproportionsofchemicalelementsinCIchondritescloselymatchtheproportionsinourSun6sotheyarethoughttorepresenttheprimitivecompositionofourSolarSystemasawhole.Theyalsocontainakind of granule called a CAI (calcium-aluminum-rich inclusion) which, due to theirchemistryandhighmeltingpoint,wereprobablythefirstminerals tocondensefromthesolarnebula.TheageofCAIsisthereforetakenasthedefactoageofourSolarSystem,andthey,togetherwithangritemeteorites,yielddatesof4.5682billionyearsgiveortake400,000years.4296

Planetformation–fromEnlightenmenttoSpaceAge

AnyplausibleexplanationoftheformationofourSolarSystemmustbeconsistentwith some present-day observable facts: (1) the planets contain most of the ‘angularmomentum’ (rotating force) in theSolarSystem, (2) all theplanetsorbit theSun in thesame plane, which is also the Sun’s equator, and (3) most planets rotate in the samedirection,andmosthavetheirequatorparalleltotheSun’s.6

The“NebularHypothesis”for theformationofourSolarSystemactuallypredatesthespace-agebyalongway.Originallysuggestedbythe German Enlightenment-era philosopher Immanuel Kant in 1755, then refined by

Laplace in 1796, 6 it has since been refined by space-age scrutiny. Telescopes like theHubble,SpitzerandHerschelSpaceTelescopeshaverevealed“stellarnurseries”inhugecloudsofgasanddust(nebulae),with thousandsofyoungsunsburningwithin them.Insome, like theOrionandTrifidNebulae,astronomershavephotographedyoungstarsattheirearlieststageofformation,inthecenterofdisksofdust,justlikethatinferredforourearlySolarSystem.430431Thelightemittedbythesedisksrevealstheircompositionofwaterice,silicaterock,methanolandcarbondioxide,432similartoourownSolarSystem.

NASA’s space probe visit to the asteroid “Vesta” backs this up. The spacecraftmeasurementsmatchedVesta withmeteorites found on Earth, and revealed it to be anembryonicplanetformedfromchondritemeteorites,preservedtodayasitwasintheearlydaysoftheSolarSystem.433EarthmusthavebeenlikeVestainitsinfancy.

Thelawsofphysics(specificallythe‘conservationofangularmomentum’)dictatethatasthesediscsofdustaredrawninwardstowardstheirembryonicstarsbygravity,thecontractingmass is forced to spin faster (just thewayan ice skater spins faster as theycontract theirarms)andso theparticlescollectively flatten into toadisk 6 rather thanasphereoranyothershape.InthiswayplanetsforminthesameplaneastheSun’sequator,spinningthesamedirectionasthesun.

15Embryonicsolarsystems:HubbleSpaceTelescopeimageoftheOrionNebulashowing5youngstarssurroundedbygasanddustdisksthatmaygoontodevelopplanets.Inset:close-upofayoungstarunderamillionyearsoldwithinaprotoplanetarydiskabout7timeslargerthanoursolarsystem.Credit:C.R.O’Dell/RiceUniversity;NASA

MoonformationThe“GiantImpactHypothesis”isoneofseveralideasthathavebeenputforward

toexplainourMoon,butitmatchestheevidencebestandabroadconsensusofscientistsacceptsit. 6Theevidencefor theGiantImpactcomesfromthechemicalcompositionoftheMoonandEarth, theirphysicalstructure,andtheconstraints imposedbythepresentorbitoftheMoon.

ChemicaltwinsLunar rock brought back by the US Apollo and Soviet Luna missions are

remarkably similar to the Earth rocks anorthosite and basalt. Of course there aredifferencesduetotheharshlunarenvironment–lunarsamplesarepittedbymicroscopicmeteoriteimpacts,alteredbycosmicrays,andshowinternalsignsofshocksfromasteroidimpacts.434Theyarealsototallydryandhavenotbeenoxidized.434

These samples reveal that the Moon and Earth are compositional twins.Proportionsofisotopes(meaningthesameelementsbutwithfractionallydifferentmasses,suchasthoseoftitanium,435tungsten,8oxygen68andhydrogen436)intheserocksareexactlythesame in theEarthand theMoon,yet thereare largechemicaldifferenceswithotherSolar System objects. 8 This tells us the Moon and the Earth formed together from alargelysharedsource.

Tyingdownthedateofthatimpactishelpedbythefactthaturanium-to-leadratioscan be graphed back to an event of major loss of lead from Earth’s crust and mantleroughly100millionyearsaftertheformationoftheSolarSystem,whenitandotherheavyelements sank to the core through amagma ocean created by theGiant Impact. 437 Thistimingcoincidesneatlywitharubidium-strontiumradiometricdatefortheMoon,whichis70-100millionyearsafterSolarSystemformation.8TheconversionofiodinetoxenonbyradioactivedecayalsopointstoahugelossofxenonfromtheEarthataround100millionyearsafterformation.82

Interestingly,theMoon’srelativelackofvolatileelements,likesodium,potassium,6andlightisotopesofzinc,438dovetailswiththemissingxenon.Theirlossisexplainedbythesearingheatof theGiant Impactwhichdroveourprimordialatmosphereand lighterelements of rock vapor into space. 8 2 It also makes sense of Earth’s relatively thinatmosphere todaycompared toVenus. 8 4Without theGiant Impact, theEarthmaywellhave retained itsprimordialatmosphereandsuffered thesterilizing runawaygreenhouseconditionsthatVenussuffersfromnow.

TrojansandorbitsThe Earth-Moon orbit, 8 Earth’s tilted daily-rotation axis, and theMoon’s slow

recessionawayfromus,areallexplainedbytheGiantImpact.MoreevidencecomesfromourMoon’srelativelylargesizecomparedtoEarth,unliketherelativelysmallmoonsofmostotherplanetsintheSolarSystem,8whichmakessenseifitformedfromanimpact-mixture of another planet (Theia) and Earth debris. TheMoon’s core is proportionallymuch smaller than the cores of other terrestrial planets, as we would expect from itsformationfromimpactdebrisaroundthemoremassiveEarth,whichwouldhavecapturedmoreoftheheavierparticlesaftertheimpact.6

Computercalculationsofgiantimpacts,usingthelawsofphysicsgoverningmass,gravity and angular momentum, show how a Moon-forming Giant Impact is a very

feasible way to create our Moon. 6 But these simulations also suggest an intriguingpossibility–thattheEarthhadmorethanonemooninitschildhood.

Evidencefor2moons

Companionmoonsareacommonoutcomefromsuchsimulations,althoughmostoftheirorbitsareunstable,sotheyquicklyimpactorflyoffintospace. 10However,ifacompanionmoonformsata“TrojanPoint”(apointabout 60degrees aheador behindourMoonbut in the sameorbit as ourMoon) it cansurvivefortensofmillionsofyearsbeforecrumplingslowlyintothelargermoonatsub-sonicspeeds.10Bythattimethesmallermoonwouldhavesolidified,butthelargermoonwouldstillbepartlymolten.Theslowimpactleavesarubblepileoverthelargermoon,rather than blasting a high-energy,mantle-melting crater, and the addedweight of thatdebrissquishesthestill-moltenlunarmantletotheotherside,thesideweseefromEarth.

Although not universally accepted, this conjecture neatly explains the lunarhighlandsonthefarsideoftheMooncomparedwiththeflatterlandscapeofbasaltlavasthatweseefromEarth.RecentgravitysurveysoftheMoonsupportthisidea.Theyfoundthehighlands tobe12%lessdense thanexpectedfromtherestof theMoon(consistentwith the rubble pile from a smaller moon), and found giant lava-filled cracks in theMoon’scrustshowingtheMoonadded10km(6mi)toitscircumferenceatatimeafteritwas partly cooled. 17 18 Satellitemeasurements of the composition of the far side of theMoonalsoconfirm its rockshavenotgone throughasmuchchemicaldifferentiationasthe near side of theMoon, as would be expected from a smaller moon that solidifiedquickly.19

16The“BigSplat”collisionof2moonstocreateourpresentsinglemoonwithremnantsofthesmallermoonformingthemountainousfarside.1to3redrawnfromcomputersimulationsbyJutzi&Asphaug2011,

photofromNASALRO

Otherideasandwhytheydon’twork:AtonetimetherewerethreemaincompetingideasfortheMoon’sformation.The

firstwasthattheEarthandtheMoonformednexttoeachotheratthesametime.ButthedifferentdensitiesoftheMoonandEartharenotfeasibleiftheycongealedfromthesamepartof thenebula, and it’sveryunlikely theneighborswouldhaveavoidedcollision. 6 8ThesecondideawasthattheMoonspunofffromtheEarth,butforthistoworktheMoonwouldhavetoorbitEartharounditsequator–whichitdoesn’t. 6ThethirdideawasthattheMoonwas a passing planet that got captured into orbit aroundEarth.But this ideacan’t account for the chemical similarities between the Earth and theMoon, 6 and thevanishingly small chance that theMoon could approachEarthwithout either slammingintousorjustdeflectingintospaceagain.

Thelostelements–cluestoEarth’schildhoodSincetheEarthismadeuporiginallyfromasteroidsandcomets,youmightexpect

theproportionsofelementsinEarthrocksamplestobethesameasthoseformeteorites(asteroids that fell to Earth). They aren’t, and this tells us a great deal about whathappenedintheearlychildhoodoftheEarth.

Iron-loversImentioned the lossof lead from theEarth’s crust andmantle that helps tie the

Giant Impact to about 100 million years after the formation of the Solar System. Butevidencefor thewholesalemeltingofa largeproportionofEarth’smantlecomes in theformof theabundance(or lackof it)ofothermetalssuchasmanganese,nickel,cobalt,goldandplatinum.Theseso-called“siderophile”(“iron-lover”)elementsdissolveinironatthetemperaturesandpressureswithintheEarth.Whenyouplottheirabundanceagainsttheir concentrations inmeteorites, you see that they are strangelydepleted compared toothermetals. This is explained by iron, alongwith those othermetals, sinking into theEarth’s core through a “Magma Ocean” while the mantle was mostly molten after theGiantImpact.Thequantitiesoftheseiron-loversthatremaininrocksnearthesurface(thatwemineforpreciousmetals)areparadoxicallygreaterthanyouwouldexpectaftercoreformation.Infactthereissomeevidence(basedontungstenisotopesinsomeoftheoldestcrustalrocks)thatourcrustwas‘topped-up’withthesepreciousmetalsbyasteroidsthatbombarded Earth after the core was already formed. 35 This “late veneer” of asteroidmaterialseemstohavehappenedeverywhereinourSolarSystem.439

Rejecteddaughters

A clue for how quickly theEarth cooledafter theGiant Impact canbe found in isotopesof thegasxenon,which ImentionedaboveindatingtheGiantImpact.Xenonisrejectedduringmineralformation,soitescapesintotheatmospherethroughvolcanoesoramagmaocean.Isotopes,youmayrecall, are the same element but with fractionally different mass. The lighter isotopexenon-129 was produced by radioactive decay of short-lived iodine-129 which wentextinctinthenaturalworldaverylongtimeago. 30Theheavierisotopesxenon-131to136were produced by a different, longer-lived parent (Plutonium-244), which is also long-extinct. 30Andthat’sthecleverbit–wecanusethesexenondaughterstoinferwhatwashappeningtotheirparentswhiletheywerestillaroundinthosefirstfewtens-to-hundredsofmillionsofyearsaftertheformationoftheSolarSystem!

They show that more than 98% of the xenon offspring from the shorter-livedparentiodine-129waslosttospacefromthehotatmosphere,butonlyaround70%ofthe

xenonoffspringfromthelonger-livedparentPlutonium-244waslost.Inotherwords,welost much more xenon earlier, and much less later, 30 so the Earth’s mantle must havecooledquicklyinfirst200to400hundredmillionyearsaftertheGiantImpact.Infactitcooled by about 300°C,which ismore than it has cooled in the entire time since then,muchfasterthanthepresentrateofmantlecoolingofabout40°Cperbillionyears.30Suchrapid coolingmeans that heatwas lost from themantle very efficiently through strongchurningofamagmaocean,andprecludesathick,insulatingcrust.

ThispaintsapictureoftheEarthsoonaftertheGiantImpact,coveredbyamosaicofblackandyellow,patchesofbrightliquidseethingtothesurface,crustingtoblackandsinkingbeneaththeyellowglowoffreshlava,resurfacingtheentireplaneteverymillionyears or so, 30 in a scene resembling the surface of lava lakes in active volcanoes today(albeitonamuchlargerscale).

Freezingthemantle

Thereisconsiderableuncertaintyinhowlongthemantle(asopposedtothecrust)took tosolidifyafter theGiant Impactandbefore theJackHillszircons.Estimatesvarybetweenaround1 to5million 30 7 to 50million 4 years to freeze themantle, and thenanother100

7

to2004millionyearsbeforeradioactiveheatremeltedthemantleenoughtoallowit tomoveasitdoestoday.ButthesestudiesdoagreethattheEarthcooledmuchmoreslowlyonceitwascoveredbyasolidcrust,justasthelidonacookingpotkeepsitscontentswarm.

Flaccidearlycrust-beforeplatetectonicsIn“yourfirstmemories”Imentionedatypeofveryresilientmineralcalledzircon

thatconstitutestheearliesttraceofourEarth,from4.4billionyearsago.Withinzirconslikethese,extractedfromtheoldestrocksonEarth,therearemeasurableimpuritiesofachemicalelementcalled“hafnium”(Hf).Oneisotopeofhafnium(176Hf)isaproductofradioactivedecay, so its ratio to another isotope (177Hf) changes over time. When thatratio is plotted against the sample’s formation date, most rock samples plot along thestraight linewewould expect from continualmelting and differentiating of the Earth’smantle fromanoriginal asteroid composition.But themost ancient rock samples (fromJackHillsinAustraliaandtheAcastaGneissfromCanada)plotalonganentirelydifferentline, 4showingthattheyformedindependentlyincrustthatstayedstableforhundredsofmillionsofyears.Thatisfarlongerthanmodernoceaniccrustsurvives,andsoittellsusthat the world worked differently back then, without the plate tectonics of themodernplanet.

Infact,somerocksfromthat timeareofa typecalled“komatites”whichhaveahigh level ofmagnesium oxide in them. These only form at temperatures about 350°Chotter than even the hottest basalt lavas of today. Scientists plugged these temperaturesand rock properties into computer models that run equations for the physics of fluiddynamics,viscosity,buoyancy,stress,strain,andheat flow.Themodelsshowedthat thecrustwas tooweak back then to sustain subduction - the process of one tectonic platedescendingintothemantlebeneathanother,whichisnecessarytosustainplatetectonics.31It took a long time before the crust cooled enough to gain the strength needed forsubductionandthesteadymantlecirculationandcoolingthatgoesalongwithit.Careful

analysisofover70,000 rock samples fromacross theglobebacks thisup. It shows theclear geochemical signal of long-term, gradual cooling in themantle and crust startingroughly 3.2 billion years ago, consistent with the steady loss of heat through mantleconvection440aftersubductionandplatetectonicswasinitiatedatthattime.

LasttangoofSaturnandJupiterDatingMoonrocksamplesfromtheApolloandLunamissionsyieldedacurious

gapinimpactcratersolderthanabout4billionyears,andalsoshowedthatafterabout3.5billionyearsagocraterformationwasatleast100timeslessfrequent. 44132Sincemostofthe samples were collected from impact craters on the near side of the Moon, it wasreasonable to suspect the samples were dominated by a few major impacts, obscuringolderdates.InfactsamplesfromApollo16and17werelaterfoundtobedominatedbyasingleimpact3.9billionyearsago. 441SoscientiststurnedtobitsoftheMoonthatfelltoEarthasmeteoritesinplaceslikeLibyaandAntarcticatosupplementtheirdata,butsofarthesehave justconfirmed thegap. 441 32Lunargeologistshavealsomapped thecraters inlunar reconnaissance photos, building up a detailed sequence of crater formation. Thisallows themuse theages fromthe few lunarsamples tobracket theagesofmanyothercraters, and again shows that 80% of the Moon’s surface was resurfaced by impactsbetween the formationof theNectarisandOrientalebasins 32 (3.92 to around3.8billionyears ago, a duration of 120 million years). In other words there was a Late HeavyBombardment of theMoon during the teen years inYourLife as PlanetEarth.But thepaucityofsamplesanddatinguncertaintiesdoallowforamoreprotractedbombardmentpotentiallylastingmorethan400millionyears.441

The Moon retains its ancientcrateredface,butall thecratersfromthebombardmentonEarthhavelongsinceerodedaway. Fortunately we can estimate what the bombardment was like on Earth byextrapolatingfromtheMoon.TheEarthwasbothabiggertargetandmoremassive(withmoregravitationalpull) than theMoon, so itwouldhavebeenhit about20 timesmoreoftenthantheMoon.11

Since thecratersonEarthhavebeenerased,directevidence insteadcomes frommicroscopicglass spheres that formfromcondensing rockvaporafteran impact.These“spherules”havebeenrecoveredfromdatedsediments,revealingthefrequencyandsizeof ancient asteroid impacts. It shows that impacts big enough to spread molten rockdropletsacrosstheentireglobewerefrequentwellafterthetraditional“end”oftheLateHeavyBombardment,onlytailingoffbyaround2.5billionyearsago.36

The clues to the cause of this cosmic hailstorm came from peculiar orbits ofobjects in theKuiper Belt, the distant cloud of asteroids that includes Pluto. Scientistsfound that some asteroids in the Kuiper Belt orbited in a regular dance with Neptune,orbiting theSuneither twice forevery three,oronce forevery twoofNeptune’sorbits.Otherobjects in theKuiperBelthavestrangelyelongateorbits,ororbits that take themwelloutoftheSolarSystemplane,442andsomehavecompositionsthatmusthaveformedmuch closer to the Sun before finding themselves at the outer boondocks of the Solarsystem.443

SolvinghowtheSolarSystemcouldendupinitscurrentconfiguration,includingtheKuiperBeltoddities,startingfromadiskofdustandplanetaryembryos,hasinlargepartreliedoncomputermodelscontrolledbythelawsofphysicsconcerninggravityandorbits. It turns out that to get to the current configuration from the earliest days of theSolar System, you have to startwith amore compact distribution of the planets – likerewindingavideoofagameofbilliards.Fast-forwardingtheorbitsrevealsacriticalpointat the beginning of the Late Heavy Bombardment when Jupiter and Saturn begin toresonatewith eachother in a kindof cosmic tango.This instability yanksNeptune andUranusoutof their stableorbits,which in turnscatterscometsandasteroids throughouttheSolarSystem,includingtowardsEarth.444Thisideaisknownasthe“NiceModel”(notbecauseit’sespeciallypleasing,butaftertheFrenchcitywhereitwasdiscussed).

Armageddon?Ever since we discovered that the dinosaurs were wiped off the planet by an

asteroid, we have had a cultural obsession with impacts, resulting in more than a fewdisastermovies.Butonly recentlyhavewebegun toassess thereal threatofan impactcatastrophe. Tens of thousands of centimeter-sized objects burn up in our atmosphereyearly, as do a few meter-scale objects. 445 The fireballs they make are detected bymonitoringsatellitesandground-basedsoundrecorders,orobservedas“shootingstars.”

It’sthebiggerobjects,50-100m(150-300ft)orlarger,inorbitsthatcomenearus,whichmightposeadanger.NASA’sestimateforthenumberofthoseisunsettling:roughly5,000,ofwhichonly30percenthaveactuallybeenlocated. 446NASAmaintainsawatchonNear-EarthObjects ithas identified, includinga325-meter-wide(1,000foot)asteroidnamedApophiswhichhasaverysmallchanceofanimpactwithEarthin2068. 447Infactweseemtobe“buzzed”byasizeableasteroideveryfewmonths,like“2012DA14”thatmissedusbyjust15minutesonFebruary152013,onthesamedaythata13,000tonasteroidexplodedintheskyaboveChelyabinskinRussia,injuringmorethan1,000people.448

To date some 185 impact craters have been positively identified on Earth, withnewonesloggedeveryyear, 96andestimatessuggest thereareanother700orso thatwehaven’tfoundyet. 445Todetermineifacraterwascausedbyanimpactratherthananothercause(likeavolcano,saltmovement,orfolding),scientistslookforthefingerprintsofahigh velocity impact and its resulting shock wave. These include characteristic shockfracturesinquartzgrains,beadsofglasscooledfromvaporizedrock(impact“spherules”),mineralslikestishovitethatonlyformunderextremepressures,abnormallyhighlevelsoftheelementiridium,evidenceofmelting,andcrater-sizedcone-shapedfractures.

Andifthedinosaurswerewipedoutbyanasteroidimpact,maybetheothermassextinctionslikethePermian“GreatDying”werealsocausedbyasteroids?

Notsofast.

AlthoughtherewasanimpactinBrazilabout3millionyearsbeforethePermianextinction(almostcoincidentgiventhedateuncertainty),itwascomparativelysmallsoitcan’t explain the extinction and all the other documented environmental effects of thattime by itself. Itmay have released a large amount ofmethane into the atmosphere bybreakingupoilshales(“fracking”),whichwouldhaveaddedgreenhousegassestothoseemanatingfromtheSiberianTrapseruptions.177AnextensivesearchforalargerimpactatthetimeofthePermianmassextinctionfailedtofindthehighlevelsofiridium,shockedquartz and impact spherules that would be expected, 449 and no “smoking gun” impactcraterhasbeenfound–anditwouldhavetobebig!Instead,allindicatorsforthecauseofthePermianGreatDyingpointtohugerisesinatmosphericgreenhousegassescausedbyimmensevolcaniceruptions.176174180181178

EventheCretaceousChicxulubimpact,which iswidelycreditedwith theendof

the dinosaurs,was not the only killer at the time. Indian volcanic eruptions around thesame time undoubtedly did affect the climate, and began the mass extinction 150,000yearsbeforetheimpactthatfinishedoffthedinosaurs.245450

Rapidclimatechangesinducedbygreenhousegasemissionsfromthesameclassof immense volcanic eruptions that caused the Permian “Great Dying,” have beenconsistentlyfarmoredeadlytolifeonEarththanasteroidorcometimpacts.116

2.TheTerraFirmaIllusion“Terra firma,” “bedrock,” “rock solid,” “rockof ages”– they all convey solidity,

constancy, reliability. For that reason rock is the brand image for many a bank andinsurancecompany.Andforthemostpart,onthescaleofournormalexperience,nothingisassolidasarock,aspermanentasamountain,orasfirmasthegroundbeneathourfeet.But just as the stability of financial institutions sometimes proves illusory, so is theconstancyofourEarth.

The“solid”Earthmovesinmanywaysbeyondournormalperception.

Driftingcontinents

Itwasn’tuntiltheearly20thcenturythatcontinentaldriftwassurmised,andeventhen it took decades to be widely accepted, but these days it’s not so hard to provecontinentaldrift–wecanjustmeasureit.

The same GPS (Global Positioning System) that you use in your car or yoursmartphone is also used tomeasure continental drift, andNASAdisplays these data ontheirwebsite. 451GPSmeasurements reveal ongoingplatemovements today, such as thecollisionofIndiaintoAsiamovingat4cmayear(1½in)ontheIndianside,taperingoffto very slowchanges north ofTibet.They also show theHimalayas herniating towardsChinaataround2cm(1in)ayear.274

17GPSmeasurementsshowtectonicmovement.RedrawnfromSearle,etal,J.Geol.Soc.2011overMODISimagebyNASA

Scientists supplement GPS measurements with “Very Long BaselineInterferometry” (using natural radio signals from quasars in deep space) and SatelliteLaserRanging(usingpulsesof laser lightbouncedoffsatellites).Together, thesesurveytechniques have been crucial in plotting continental drift over the last fewdecades, buttheyarelittlehelpinworkingoutwhatisgoingonbelowthesurfacetodrivethisprocess.For that geologists use a combination of field geology, calculations from earthquakesignals,andhighlysensitivemeasurementsoftheEarth’smagneticfield.

DiscoveringdriftInfact,continentaldriftwasdiscoveredthroughjustthosetechniques.TheGerman

meteorologist and explorer Alfred Wegner, when he first described the concept ofcontinental drift back in 1912,

452

was convinced by theway continents fit together likejigsaw puzzle pieces, and by the matching distribution of plant fossils and rock typesacross now widely-separated continents. Wegner’s ideas didn’t gain widespreadacceptanceuntildecadeslater–itwascounterintuitivethatsolidlandmassescoulddrift,or that continents couldplow theirway through solid rock. And besides,what possiblemechanismcoulddrivesuchanepicprocess?

In1962HarryHess,anAmericangeologist,madethepivotalinsightintoboththeprocess of continental drift and the mechanism that drives it. He knew that since the1950s,surveyshadidentifiedunderwatermountainchainsallacrosstheglobe,forminga“GreatGlobalRift.”Henoticed that thesemid-ocean ridgeswerealwaysat themedianline between continents, had very high heat-flow, and conducted earthquake wavesstrangelyslowly,showingtheywerepartlyliquid.Hededucedtheymustbezoneswherethe mantle wells-up hot fresh magma. Noticing that ocean floors have relatively littlesediment accumulation nearmid-ocean ridges, he concluded that oceanic crustmust beyoung.Putting all the evidence together he concluded that themantlewas convecting -circulating likeaheat-drivenconveyorbelt -ataround1cm(½inch)ayear,withmidoceanridgesmarkingthelocationof therisingpartof theconvectioncycle.Herealizedthatthismantleconvectionwasmovingcontinentspassivelyaroundthegloberatherthan“plowingthrough”themantle,andherealizedthattheremustbedescendingpartsoftheconvectioncyclewhereoceaniccrustgetsdestroyedina“jaw-crusher”-ashecalledit-whenitisshovedbeneathcontinents.453SoHesshadproposedthemechanismthatdrivescontinentaldrift,andfurtherproofwassoontofollow.

Magneticzebra

Inthe1920saJapanesescientistnamedMotonoriMatuyamaestablishedthat theEarthhadundergoneswitchesinitsmagneticfield,flippingnorthtosouthandbackagain.ThisinformationbecamepivotalforourstorytheyearHesspublishedhistheoriesofseafloorspreading,astheshipHMSOwenconductedamagneticsurveyoftheIndianOcean.When British scientists Drummond Matthews and Fred Vine graphed their magneticmeasurements, they showed a regular pattern of alternating strong and weak magneticfield.TheyrealizedthatthesewerearecordingofthemagneticfieldflipsthatMatuyamahaddescribed, fossilized into theocean floor likezebra stripes,whosepattern formeda

mirrorimageeithersideofmid-oceanridges.OtherscientistsfoundsimilarpatternsintheNorthAtlanticandthePacificOcean,soithadtobeaglobalphenomenon.

VineandMatthews realized that their stripeswereproofofHess’s theoryof thecontinuous production of new crust along mid-ocean ridges. Fresh lava at the ridge isconstantly cooling and freezing-in the magnetic field of the time, and is constantlyseparating as the sea floor spreads, with the gap filled by fresh lava in a continuousprocess.454

Jaw-crusher

The missing part of the puzzle was whathappenedat theotherendof thesea floor tomake roomforall that spreading–Hess’s“jawcrusher.”That’swhereseismologists(earthquakescientists)camein.ForsometimetheyhadnoticedadistinctconcentrationofearthquakesaroundthePacificOcean.TheyfoundthatthezoneofearthquakestendedtoplungedeeperintotheEarththefurtherawayfromtheoceantheyoccurred.TheyrealizedthatthequakesaretherattleofHess’s“jaw-crusher”asoceaniccrustplunges into themantle inazoneofcrustdestructioncalleda“subductionzone.”

The Canadian geologist Tuzo Wilson realized that these processes must havedriven oceans towiden and shrink, and caused to continents to break apart and collidethrough geological time. He and many others refined the idea of continental drift into“plate tectonics” through the sixties and seventies, 455 and since that pioneeringwork, ahostofnewevidence, includingvideosofunderwatereruptionsandgeothermalventsatmid-oceanridgesandgeophysicalsurveys,hasconfirmedthetheory.AndtocirclebacktothoseGPSmeasurements, theydo showHess’s deduced spreading atmid-ocean ridges,andhis“jawcrusher”collisionofplatesatsubductionzones.451

Realizing that our Earth is made of tectonic plates, dynamic at the rate of afingernail’sgrowth,hasbeen thecrucial epiphany forunderstanding somuchaboutourplanet.Itexplainsearthquakesandvolcanoes,buildingmountainsandsubsidingbasins.Itenablesscientiststopredictthelocationsofoilreservesandmineraldeposits.Inshort,itisgeology’sunifyingtheory.

CATscanresultsSince then,advances ingeophysicshaveenabledus to“see” into theEarthwith

increasingclaritytorevealstructureswithinthecrustandbeneath,allthewaytothecore.This is an exciting time as satellite-based gravity 456 and magnetic surveys 457 vie withseismicimaging458forevercleareranddeeperviewsofourplanet’stectonicengine-room.We’vecomealongwaysinceWegner.

Scientists measuring the electrical conductivity through mid-ocean ridges haveshownthattheseparationoftectonicplatesisapassiveprocess.Platesarepulledapartbymantle movement and sinking of the subducting edge of the plate. The pulling-apartlowersthepressurebetweenplates,whichcausesthemantleinbetweentomeltandrise,filling fissures that form as the plates tear apart. 459 Geothermal heat powers seawaterthrough the newly-solidified, cracked crust until it jets into the ocean as hydrothermalventsor“blacksmokers,”andthisspreadingzoneisrelativelybuoyant,ridinghighasthemountainouschainofmid-oceanridges.

Seismic imaging, using echoes from artificial shockwaves or from naturalearthquakes,hasrevealedevermoreinformationaboutthestructureandprocessesofthedeep crust, the mantle and even the core. As computing power and storage capacityincreased, they permitted seismologists to record and process ever larger quantities ofdata.Seismologistsnowusehundredsofthousandsofearthquaketracesfromhundredsofnaturalearthquakesacrosstheglobe,detectedinmonitoringstationsdistributedacrosslarge sections of the world. 316 They analyze the spectra of the earthquake waves andcompareminisculedifferences in thewaveshapesrecordedatdifferent locationsfor thesame earthquake, in order to assess the properties of the rocks that the waves passedthrough on their journey across the interior of Earth. In this way they build a 3-dimensionalimage,justlikeamedicalCAT-scanorMRI,ofthecrustandmantle.

Seismic tomography shows the Pacific crust plunging to annihilation beneathWashingtonStateandCalifornia.Itrevealsgraveyardsofbrokenpiecesofsubductedplateburiedunder thePacificNorthwest of theUSA; aplumeofmagmaextending from thehadean depths of the mantle feeding the Yellowstone Supervolcano; ancient crust thatunderpins the Great Plains and parts of the RockyMountains; and even gobs of crust“dripping” off into themantle fromunder theColoradoPlateau. 316 It reveals finger-likeplumesofhotmantledottedallacross thePacific thataresmeared intochannelsby themotion of the overlying tectonic plate. 458 These plumes also show up in satellite-basedgravitymeasurements,correspondingwithhotspotsallovertheglobe.456

MantlePlumesseemtoprotrudefromenigmatic“PlumeGenerationZones”-beltsnear Earth’s core that appear to leak hot fingers of core-mantle magma towards thesurface.Thesezonesarelikeskirtsenclosingtwolargemushyregionsatthecore-mantleboundary,onebeneathAfricaandthenorthernAtlanticnicknamed“Tuzo,”andtheotherbeneath the western Pacific, nicknamed “Jason.” Both these regions, and the PlumeGenerationZonesaroundthem,seemtohaveremainedremarkablystaticoveratleastthelast 500million years, even as continents drifted high above them theway clouds driftoveralandscape.455

MantlePlumesare also linked to those catastrophic eruptions that causedglobalwarmingandextinctionsinEarth’spast,knownasLargeIgneousProvinces(LIPs).178

18SeismictomographyrevealingamantleplumebeneathYellowstone.Reddercolorsaremolten/semi-moltenzonesthatslowdown

earthquakewaves.Redrawnin3Dfrom2DsectionsinObrebskietal,Geophys.J.Int.(2011).

LargeIgneousProvinces(LIPs)vsVolcanoesWearesomewayfrombeingabletopredictvolcaniceruptionsreliably,butthere

have been some successes. In one example, US and Philippine scientists successfullyforecastthe1991eruptionofMountPinatubo.Theirforecastsavedatleast5,000livesandat least$250millioninpropertydamage. 460Hopefullywewillseemanymoreexampleslikethisasnewtechnologyandresearchisbroughttobear.Forinstance,satellitesurveysusingasophisticatedradartechniquecalled“InSAR”cannowrevealvolcanoesswellingwith magma buildup before eruptions, 461 and geophysicists have found characteristic“long-period”seismicwavescansignalanimminenteruption.462

Volcanoes–adouble-edgedswordforourclimate

ExplosivevolcanoeslikePinatuboandElChichonusuallyoccurabovesubduction

zones where waterlogged plates plunge into white-hot mantle.

They release CO2 when they erupt, whichcombineswithCO2 fromeruptions all over theworld overmanymillennia to keep theplanetfromfreezing.Butviolenteruptionsalsoinjectsulfurdioxideintothestratospherewhereitconvertstosulfate,whichactslikesunscreenintheshortterm.Consequentlyweseecoldperiods lastinga fewyearsafter largeeruptions likeLaki in1783,Tambora in1815, andCosiguina in1835,ElChichon in1982andMountPinatubo in1991. 463 Lakicausedaseverewinterin1783/4andarangeofextremeweatheracrosstheglobe,whileTamboracausedayear“withoutasummer”in1816.Earliervolcanic-inducedcoldperiodsareevendocumentedinmedievalIrishchronicles.464

Butother volcanoesoccur away from subduction zones and spreading ridges, inthemiddleofplates.Theyseemtobeassociatedwithmantleplumes,liketheonebeneathYellowstone.Theplumesburn“hotspot”volcanoes throughthesolidcrustabove them,sometimesmaking long island chains, like the Hawaiian-Emperor chain. Usually thesevolcanoes are non-explosive, with hot, runny lava like at Hawaii. But sometimes localconditions canmake even these volcanoes explosive, for example if the volcano eruptsthroughaglacier(aswasthecasefortheEyjafjallajökulleruptionthatgroundedEuropeanflightsin2010).Andsometimestheplumemagmaismixedwithmoreexplosivemagmameltedfromcontinentalcrustaroundit–likeatYellowstone.

LargeIgneousProvincesarenotyourregularvolcanoes

Even Hawaii, Pinatubo, Vesuvius and Tambora are dwarfed by the epicoutpourings of lava known as “Large Igneous Provinces” (LIPs). Every year there aresomething like 50 to 65 volcanic eruptions, but we haven’t had a LIP eruption in 16million years. 465 272 LIPs are about 40 times larger than even our largest modern-dayvolcano (Hawaii) and seem to be caused bymantle plumes interactingwith continentalcrust,injectingsheetsofsuper-hot,veryliquidmagmathroughsedimentsunderground,aswellassmotheringenormousareasoftheEarth’ssurfaceunderlakesoflava.Theyareathrowbacktotheeruptiveincontinenceofthepre-teenEarth,conductingheatandmagmafromthelowermantletothesurface.LIPsareconcentratedattimeswhencontinentsarefragmenting, and at placeswhich occur over deepmantle PlumeGenerationZones.As

moreandmoreLIPsareanalyzedacrosstheworld,it’sclearthatthecombinationoftheirextraordinary scale (some cover 1% of the planet’s surface) erupted over short timeperiods (geologically speaking) and their ability to release prodigious quantities ofgreenhousegasses(directly,andalsobybakingthecrusttheyeruptthrough),makesthemsodisruptivetotheclimate.272178

19ALargeIgneousProvince(LIP)eruptioncomparedtotheworld’slargestactivevolcanotoday.LooselybasedonSvensenetalEPSL2009,HowarthetalLithos2014,Elkins-TantonGSASpecPub2005,Kelleretal

JGeolSocIndia2011,LietalNatureGeoscience2014.

WanderingpolesButwhatofthewanderingsofcontinentsinthedeeppast?

The evidence for that comesfrom the same kind ofmagnetic phenomenon thatVine andMatthewsmeasured in theIndianOcean.Butratherthansurveyingvastswathesofcrust,scientistsmeasurethefossilmagneticfieldfrozenintoindividualrocksamplesofdifferentages.Asmoltenrockscool,some crystals record the direction of Earth’smagnetic fieldwithin them. In fact, rocksdon’tevenhavetobemolten,theyjusthavetobeheatedabovetheir“curietemperature”whichismuchcooler(inthecaseofironoxideit’slessthanhalfthemeltingtemperature).Sothemagneticdirectionofthesemineralsissetandresetanytimetheyareheatedabovetheircurietemperature.

Asanyschoolkidknows,theEarth’smagneticfieldpointsoutwardsatthesouthmagnetic pole and curves until it points inwards again at the northmagnetic pole. So,knowinghowsteeplythemagneticfielddipsinto(oroutof)thegroundtellsyouhowfarnorthorsouthyouare. In thisway, rockscan record their latitudeof formationand it’soften far fromtheirpresent latitude.Unfortunately thisdoesn’thelpwith theireast-westlocation (longitude) but with enough samples these studies of fossil magnetism (or“paleomagnetics”)plotacontinent’sjourneythroughtime,andshowwhendifferentpartsof a continent came together. 466 There are now sufficient data to plot the wanderingcontinentsbackhundredsofmillions,evenbillionsofyearsago.466117

GhostsofcontinentspastIt seems improbable that therecouldbeany trace leftofapieceofEarth’scrust

afterithasbeenthroughHess’sjaw-crusher,descendedintothehellishtemperaturesandpressures of the mantle, melted and conveyed through the planet’s interior, and theneruptedinsomenewlocationonthesurface.Andyet,likeasuspect’sDNAleftatacrimescene,chemicaltracesoflong-ago-lostcrustcanberevealedthroughexquisitelysensitivegeochemicalmeasurements.

They say “diamonds are forever” and perhaps they are right. The timing of thebeginning of subduction was hotly debated until a recent study of some remarkably

resilient diamondswaspublished.The scientists datedmicroscopic impurities locked indiamonds that had been dragged from deep beneath ancient continental roots by morerecentvolcaniceruptions.Impuritiesinthediamondsthatweremorethan3billionyearsoldweremadeofperidotite,akindofolivine-richrockthatmakesuptheupperpartoftheEarth’s mantle. But a different impurity – eclogite – only showed up in youngerdiamonds.55Eclogiteiscreatedbytheultra-highpressurecookingofslabsofoceaniccrustduring subduction. The switch in the type of impurity told scientists that the “WilsonCycle” - rock recycling through subduction - started roughly 3 billion years ago,consistentwiththechangeinmantlecoolingrateatthattime,thatImentionedinthelastchapter(those“rejecteddaughters”andthat“flaccidcrust”).

Evenoxygen-freetracesofsediments thatweresubductedintothemantlebeforetheGreatOxidationEvent(Earth’smid-lifecrisis)canbedetectedinvolcanicglassfromrecent eruptions at somemid-ocean ridges today – a recycling that tookmore than 2.5billion years to complete. 467 Subducted sediments can also leave tell-tale variations inisotopes of hafnium trapped in resilient little zircon crystals like those we spoke of asEarth’sfirstmemories. 468Thesehafniumisotopesare“ghostsofsedimentspast”becausethey indicate the kinds of sediments that lay on long-vanished seabeds before theydisappeared during an ancient continental collision,millions or billions of years beforetheywererecycledintothecrustagainasfreshigneousrocks.

Startingsubduction

There are chemical fingerprints for both the onset of subduction and the steadycoolingofthemantlesincethen.ThosehafniumisotopesIjustmentionedalsorevealthebeginningofsubductionthroughamarkedchangeinthegrowthrateofcontinentalcrustaround 3.2 billion years ago, in line with the appearance of those subduction-flaggingdiamonds.565755

Ontopofthatevidence,statisticalanalysisofover70,000rocksampleshasalsofoundthe tell-talechemicalsignatureof longtermcrust recycling,mantlechurning,andprogressivecooling.Lavaatmidoceanridgesandothervolcanoesfedbymantlemagmashowadeclineinconcentrationsofnickel,chromiumandmagnesium,butanincreaseinsodium and potassium, over the last 3 billion years or so. This change reflects thereduction in the proportion of mantle that’s actually molten from about 30% 3 billionyearsago,toabout10%intoday’scoolermantle. 440Continentalrockshavechangedtoo.Ratios of indicator elements like sodium/potassium, lanthanum/ytterbium and theproportionofeuropiumtoother“Rare-EarthElements,”haveprogressivelyreducedoverthe last 3 billion years or so, reflecting the continued evolution of a repeatedlymelted,maturingcontinentalcrust.440

Amasia–thenextsupercontinent?

So,inthecenturysinceContinentalDriftwasfirstposited,agreataccumulationofevidence has confirmed and refined the theory into Plate Tectonics. Multiple lines ofevidence narrate the story of wandering ancient continents, their collisions and thebuilding of mountain chains, their inevitable break-up and renewed journeys until ourpresent-daygeographyisarrivedat.Andinknowingthis,itisclearthattoday’sglobeofseemingpermanenceisjustasnapshot,amomentofillusorystillnessonacontinuumof

change,whichwill eventually lead to the assembly of a new supercontinent - dubbed“Amasia”-probablyneartheNorthPoleinroughly200millionyears’time,469whichwillin turn break up, its parts destined to drift in cycles of supercontinent assembly andbreakupinperpetuity,oratleastuntilourplanetisengulfedbyourdyingSun,whichisaverylongwayoff.

TheplumbingofHadesThechemicalsymbolforleadis“Pb”fromtheLatin“plumbum”whichisalsothe

originfor theEnglishword“plumbing.”That’s ironicbecause leadisat theheartof thechemical evidence for the origin,movement and connections ofmagma throughout themantle. It beginswith a problemknown to geochemists as the “lead paradox,” and thesolutiontothatproblemrevealsfundamentalinformationaboutthewaytheinnardsoftheplanetwork.437

Experimentshaveshownusthatasthemantlemeltsandrecrystallizesovertime,itshouldexpelmoreuraniumthanleadintocrustalrocks,leadingtoadecreaseinuraniumlevelsinthemantle.Andsinceuraniumdecaysradioactivelytobecomelead,themantleshouldhavemoreof the leadfromtheoriginof theSolarSystem,andmuchlessof thetype of lead that ismade by the decay of uranium.But samples of lavas atmid-oceanridgesandoceanicislandslikeHawaiishowtheopposite.437Andthat’sthe“leadparadox.”

Plumpuddingandmarblecake

Thesolutionwasexpected to lie in thefactthatslabsofuranium-enrichedcrustareconstantlysubducting,returninguraniumanditsdecay-product-leadtothemantle.Butthere’sahitch–therecycledcrustcanonlyexplaintheleadlevelsfoundinoceanislandbasaltsifthesubductedslabsarekeptisolatedinthemantleforbetween1and2billionyears(likeplumsinplumpudding),fortheiruraniumtodecaytomorelead,beforebeingmixedinwitheruptingbasalts.Butconvectioninthemantle (that drives plate tectonics) should smear andmix the recycled crust with low-uraniummantle(likethechocolatelayersinmarblecake,andnotlikeplums)toanextentthatconflictswiththoselavasamples.437Theparadoxremainedunresolved.

A rare piece of lower crust that got transported to the surface in theHimalayanmountain-buildingtrain-wreckfinallytippedthebalanceinfavorofbothmarblecakeandplums! The Pakistani rocks showed that as fast as melting over a subduction zone

generatesnew,buoyantcontinentalcrust, it leavesaresidueofveryheavyminerals(theanti-crust, if youwill).Thesedense, low-uranium ‘plums’ofmatter preferentially lock-awaythekindofleadthatwasformedattheoriginoftheSolarSystem(asdistinctfromleadcreatedbythedecayofuranium).Theydripslowlytothelowermostmantlenearthecore,wheretheyresistmixingwiththemantle.Thatleavestherestofthemantle‘cake’–withitsmarblingofrecycledoldcrust–toeruptasbasaltinvolcanicislandsormid-oceanridges, enriched in uranium-derived-lead. 470 This elegant combination of geology andchemistryshowedustheconnectednessofcrustformation,subduction,mantlecirculation,andmid-oceanvolcanicspredictedbyHess’scontinentaldriftallthoseyearsago.

Asourcenearthecore

Evidence for ocean island volcanoes being plumbed into a layer near the corecomes from isotopes of osmium. Osmium-186 is the product of radioactive decay ofplatinum-190,whichwasstronglyconcentratedintothecoreearlyinEarth’sdevelopment.Sohighlevelsofosmium-186inHawaiianlavasandinPermianbasaltsfromtheNoril’skareaofSiberiastronglyhintatmagmainteractionwiththecorebeforeeitheroftheselavaserupted.Ocean-islands’magmaoriginfromanear-corereservoirofmagmaisreinforcedbytheirhighlevelsofnickel,helium-3andneon,allindicatorsofa“primordial”ornear-Earth’s-coresource.471437472

Seismicproofofthepudding

Thisgeochemicalevidencehassincebeenbackedupbyseismicimagesthatshowsemi-liquidareasat thecore-mantleboundary,andrevealing that thevolcanic islandsofHawaii,KerguelanandTristandaCunhalieattheedgesofthesezones. 473ThechemicalvariationinHawaii’slavasshowsthatsomeoftheHawaiianvolcanoes(MaunaLoaandothers on the southern side of the chain) aremore plumbed-inwith this core-boundarysource ofmagma, and some (MaunaKea and others on the northern side of the chain)havemorerecycledcrust.473SeismicstudiesshowthateventheYellowstonesupervolcanois fed by a mantle plume originating from near the core, which fits with high core-associatedhelium-3valuesmeasuredinYellowstone’slavas.316

DidtheEarthmoveforyoutoo?OfcoursethemotionsofEarth’splateshaveafarmoreimmediaterelevancethan

justbeing theconveyorofplanetarychange throughdeep time.According to theWorldHealthOrganization,platetectonicsisresponsibleforalmostasmanydeathsasallothernaturaldisasters(storms,floods,drought,wildfires,heatwavesandcold)put together. 474TheHaitiearthquakein2010alonecausedthedeathof222,500people,andonaveragetectonicsclaimsover100,000livesayearthroughearthquakes,volcanoesandtsunamis.474

EarthquakesandinstantgoldIf you have never experienced an earthquake, nothing can fully convey the

stomach-icingpanicyoufeelwhenthesolidgroundshakes,whenbuildingsthatshouldberigid and protective sway and crumble. Since infancy your body has understood these

things to be hard, constant and unmoving. An earthquake feels as if the laws of physics were just revoked, and thrusts thoseunconscious assumptions into vivid fallacy. As the walls shake and gaps appear, youexperiencerawanimalterror.

Plate tectonics makes sense of the pattern of earthquakes around the world. Itexplainswhyquakesaregenerallyconcentratedalongplateboundariesbythejuddering,stuck-unstuck scraping of one plate against another, or by one plate bulldozing overanother,sendingittoannihilationinthemantle.Eachquake(eventhosetoosmallbefeltbypeople)ispickedupbyseismometersandaccelerometerswhichmeasureshakinginalldirections,andarenetworkedtogetheracrosstheglobetoallowaprecisetriangulationofthe locationof thequake.Seismologistscanevencalculate the3Dorientationofa faultplanedeepundergroundfromtheprecisewaveattributes.

Forecastingquakes

And if you look at the patterns of quakes on theUSGSwebsite, it immediatelybecomes apparent that a real-world plate edge such as the one inCalifornia is amuchmore complicated affair than a simple fault “line.” In fact it is composed of manyindividualfaultsandfracturesinacomplicated3-dimensionalhoneycomb.Acloserlookatanyoneof thosefracturesonthescaleofaroadcutting,for instance,reveals itsowninternalfracturestructure,andsoondowntothemicroscopicscaleofindividualfracturedgrainsofrock.Sometimesrockconditionslubricatefractures,allowingthemtocreeppasteachotherwithoutgeneratingearthquakes,but thiscreatesknock-onstressesandstrainson all the nearby fractures.Add-invaried rock-types, fluctuatinggroundwater pressuresand temperature changes atdepth, and it isnowonder thatpredictingearthquakes is sohard!

Westillarealongwayfromforecastingearthquakesthewaywedotheweather,but seismologists are able to generate maps of earthquake risk and predicted shakingstrengths, to aid the design of quake-resilient buildings.Monitoring before, during andaftermajorearthquakeshelpsidentifywherestresseshavemovedandthereforewherethenext large earthquakemight reasonably be expected. This has revealed some surprises,such as the way plates go through a slow-motion rebound lasting years after major

earthquakes475andhowearthquakesononefaultcanraisethechanceofanearthquakeinsome areas, and yet cast “stress shadows” that reduce the chance of quakes in otherareas. 476 It has even revealed how earthquakes can create gold veins in an instant bycausingsuddenpressuredropsinundergroundfluids!477

Theformationofanewplateboundaryisn’tthatfrequentongeologicaltimescales,so it’s all the more remarkable that we can witness the start of one today. 2 hugeearthquakes in April 2012 were the earliest cracks heralding the breakup of the Indo-Australian plate, beginning a newplate boundary in the IndianOcean, triggered by theTsunami-producingBanda-Acehearthquakein2004.478

Earthquakesandvolcanoesaresuchwell-worntopics,coveredinamyriadofotherbooksandTVprograms,soIwon’t takeupmorespacewiththemhere.However, thereareother,moresurprisingways theEarthmoves that fewpeopleknowabout.ThemostcommonbutleastknownisthephenomenonofEarthtides.

EarthtidesandSQUIDsEveryoneknowsthesealevelrisesandfallstwiceadayintides.Butfewpeople

knowthat thesolidEarth itselfrisesandfallswith theorbitof theMoon, imperceptiblybutmeasurably.Thegroundbeneathyourfeetmovesupanddownbyasmuchas30cm(12in) twiceaday.Youdon’tfeel itbecausetheeffect isevenlydistributedoverawidearea.

GPSsystemshave toaccount forEarth tideswhencalculatingpositions 479 as doastronomicalmeasurements,andlargephysicsexperimentsliketheLargeHadronCollideratCERN 480havetobedesignedtoallowfor them.ScientistsmeasureEarthTidesusingexquisitely sensitive instruments like “Quartz Tube Extensometers” (to measure thestretching of Earth’s surface), liquids in tubes (to measure tilt), and “SQUIDs”(“SuperconductingQuantumInterferenceDevices”)tomeasurethetinygravityvariations.

Earth Tides are a rare example of Earth movement that is not caused by platetectonics,but theydohavean influenceon tectonics.Theycanbe sufficient topushanalreadyexpectantvolcanointoerupting481482andmaybethe‘strawthatbreaksthecamel’sback’ in triggering some earthquakes. 483Tidal kneading of our planet’s interior createsheat.Currentlythisamountstoabout0.2Terawattsofenergy484-about150timeslessthanheatgenerationbynaturalradioactivedecay(about30Terawatts)-butintheearlyEarthwhen theMoonwascloser, itmusthavebeenamore significant contributorof internalheat.11

Howfastdoesamountainmove?When Mount Everest was first surveyed, its height was calculated by Babu

RadhanathSikdartobe29,002feet(8840m).Thisfigurewasrevisedassurveyingbecamemore accurate and stands at somewhere between 29,017ft (8844.33m) 485 and 29,035ft(8,850m)486atrocklevel.Orratheritdidin1999–rightnowitisprobablyalittlehigher–itisrisingabout4mm(about1/8in)ayear486-andhasmovednortheastwardsbyabout10timesasmuch.274

Mountains everywhere, not just Everest, are on the move, victims of threecompetingforces:platetectonics,erosion,andaneffectknownas“isostacy.”

Pressure-cooker

Thehigheramountainrangegrows,thedeeperitscrustalrootssagintothemantlebeneath,likeaheavilyladenshipsitslowerinthesea.That’scalled“isostacy.”Therootsofmountain chains are pushed to such depths and temperatures that the rocks they aremadefromarepressure-cooked intonewkindsof rock:mudbecomesslate; slatecooksintoflakeymicaschist;sandstonesbecomequartzite,andeventuallygranite-likegneiss.Ifthecookingcontinueslongenough,therocksbecomeovercookedandmeltcompletelytoformgranite.

Ifyouhavedriven throughamountainousareayoumayhavenoticed loopsandswirlsof folded rocks in roadcuttings. It takes the immensepressuresand temperaturesfound in the roots ofmountains to fold rock, play-dough-like, into such forms of fluidbeauty.

The flip-side to all thismachismomountain-building and forceful folding is theslowslaughterofmountainsbyerosion,andthedumpingoftheirbodies,grain-by-grain,assedimentsinriverbeds,lakesandoceans.Almostasfastasmountainsbuildskywardstheyareassaultedbyabarrageof frostandrain toerode themdown,andonce tectonicpressurerelentsmillionsofyearslater,erosionwins,diminishingmightymountainstonomorethanarangeofhillsmadefromthecookedremnantsofmountainroots.

Erosioncanbeassessedbymonitoring thesediment load inrivers,and itcanbefaster than 10mm (½ in) a year or as slow as 0.5mm (1/64in) a year. 487 488 But erosiondoesn’thave itall itsownway–as thegroundwashesaway, it lightens the loadonthemountain roots,which slowly rise like a shipbeingunloaded (isostacy again),which iswhymountainchainsformedevenhundredsofmillionsofyearsagoarestillhillytoday(liketheScottishHighlandsandtheAppalachians).

LandslidesItturnsoutthatmountainerosionisfarmoredramaticthanjusttheZen-likerain-

erodes-mountain-one-grain-at-a-time. Landslides are the most potent mountain-killingforce.489

Theword“landslide”probablyrecallsTVfootageofanunfortunatepropertyslowlydepartingdownahillside.Andofcoursethose

dohappen from time to time,but theyare at the extremelywimpyendof the landslidescale.

To get an idea of the real power of landslides, imagine miles of mountainscollapsinginaninstant,dropping50billiontonsofrockintothevalleybelow,whichracesuptheoppositesideofthevalleybeforesoaring,airborne,clearovertheridgeandintothenextvalley!Aflyingwallofrock14km(9mi)wide,travellingatsuchspeeditcanglideinto the next valley sounds impossible, but it happened in the ZagrosMountains, Iran,some10,000yearsago,490injustoneexampleofmanyallacrosstheworld.OthersincludetheFlimsrockslideinSwitzerland,whichdumped12cubickilometers(2.9cubicmiles)inthepathoftheRiverRhine,alandslideinAlaskain1958thattriggeredatsunamitallerthan the Empire State Building, and it was a landslide that triggered theMt StHelensvolcaniceruptionin1980.

Thespeedofthesephenomenaisastonishing.In1970anearthquakeintheAncashregion of Peru triggered an avalanche that turned into an 80million cubic meter (105millioncubicyard)slideofwater,mudandrocksthatmovedat340kph(about200mph)-as fast as a Formula 1 racing car! The speedwas enough for it to ride over hills140m(460ft) high, hurling huge boulders into the air that landed up to 1km (½mi) away! 491Tragically,itburiedacityandpartofanotherwithalossofover18,000lives.

Fast-movingslideslikethesehaveaspecialname–“Sturzstrom.”Theymovewithsuch velocity that they can melt the ground they pass over, leaving a distinctive rockcalled“frictionite,”whichhasbeenfoundacrosstheglobe,includingintheHimalayas.492

But perhaps we should be more concerned about the mortal threat posed bylandslidesbeneaththewaves.The“StoreggaSlide”wasanundersealandslidethatstartedoffthecoastofNorwayabout8,000yearsago.Thescaleofthismassmigrationofrockisstaggering – it spread across a large area of the Atlantic seabed, reaching halfway toIceland!IndoingsoittriggeredatsunamithatdelugedNorwegianandNorthSeacoastswithwavesupto20m(65feet)highintheShetlandsandupto3meters(10feet)onthecoasts of the Netherlands and eastern England. This devastated stone-age populationsliving therewhowerealreadyat themercyof risingsea levelsat theendof the last iceage, probablywipingout any remaining inhabitantsof “DoggerLand” (anowdrownedislandintheNorthSea)andfinallyseparatingBritainfromcontinentalEurope.493Similargiant submarine landslides have been identified across the world, for example off thewesterncoastofAfrica(whichflowed1500km,(900mi)orthesameasthedistancefromNewYorktoOrlando),494andinthedeepwatersoffHawaii.495

Theseeventsarenotconsignedtoancienthistory,either.The1929GrandBankslandslideoffthecoastofNovaScotiakilled28peopleandleft10,000homeless,anda6m(18ft)tsunamiofmysteriousoriginaffectedtheBritishIslesandDenmarkonJune51858. 496Today,somescientistssuggest that theeastern seaboard of America is at risk from a tsunami that would result from anunderwater landslide in theCanaryIslands.Studiesof theCumbreViejavolcanoon theislandofLaPalmasuggestitsflankisprimedtocollapseatsomepoint–nobodyknowswhen.Ifitdoes,somecalculationspredictitcouldgenerateatrans-AtlantictsunamithatwouldarriveontheAmericancoast7hourslaterwithwavesashighas3to8meters(10to20feet). 497ThiscompareswiththestormsurgethataccompaniedHurricaneKatrinainNewOrleansin2005,andcouldcauseextensivedamagetomajorcoastalcitieslikeNewYork, Boston, Washington DC and Miami. Many scientists consider this disastrousscenariounlikely, 498

499butresearchcontinues inanattempt togetabetterhandleonthethreat 500 501 and similar-scale submarine slides have been found elsewhere in theCanaryIslands,theCapeVerdeIslands,andtheCaribbean,suggestingthethreatisplausible.

VolcanoesofmudandsaltLavaandvolcanoesgotogetherlike…well,theyjustdo.Butthereare‘volcanoes’

thatspewmudorevensaltinsteadoflava.

OnMay282006adrillingrigwasgoingaboutitsroutinebusinessofexploringforoil in the islandof Java, Indonesia.Thedrillbitwasgrinding itsway toa targetnearly3,000m(9,000ft)belowthesurfacewhenwater,steam,andthesmellofrotteneggsstartederuptingfromtheground150m(500ft)awayfromtherig.Soonhotstinkingmudbegantovomit from the orifice and flow all around it, spreading further and further with eachpassingday.180,000cubicmetersofmuderupteddaily,swampingsurroundingfarmland,homesandvillages leaving13,000peoplehomeless. 502Dubbed“Lusi,” it’s still eruptingandislikelytocontinueforyearstocome!502

Lusiandmudvolcanoeslikeitarenolessaconsequenceoftectonicpressuresinthe Earth than their fiery counterparts. Waterlogged clays buried deep underground as

platescollideareunderintensepressure,readytoescapethroughanyfissuretheycanfind.Anearthquake(orinthecaseofLusi,adrillingrig)canbethekicktheyneedtoopenachanneltothesurface.Aroundathousandnaturalmudvolcanoeshavebeenidentifiedonland,andasimilarnumberhasbeenidentifiedoffshore,spreadallacrosstheglobefromtheNigerandAmazondeltastotheCaspianSeatotheMediterranean,BlackSea,BurmaandtheislandsofIndonesia.Alongwithmud,theyalsoventlargequantitiesmethane(apowerfulgreenhousegas)intotheatmosphere.503

Salt can make its own ‘volcanoes’ in a very slow-motion equivalent of mudvolcanoes. Layers of salt from long-buried salt pans become more buoyant than thesediments above them, so they flow.They rise as blobs that disrupt the sediments theypushthrough,sometimesreachingthesurfacelikehugepus-filledboils,kilometerswideand hundreds of meters high. 504 Such salt mountains can even spawn salt glaciers(“namakiers”) thatslowlydescendtheirbrinyslopes.Therearehundredsof these in theZagrosMountainsofIran(“namak”meanssaltinFarsi).

Saltmovement isaboonfor industry.Undergrounddisruptionbysaltmovementoftencreates trapswhereoil andnaturalgascancollect in sufficientquantities tomakeextracting themworthwhile, forexample in theNorthSeaand theGulfofMexico.Andthetendencyofundergroundsalttoflowmayaiddisposalofradioactivewaste.SimulatedcanistersofradioactivewastewereplacedinaGermansaltcavernandleftthere.Injust8years the seemingly-solid salt crept until it had completely entombed the wastecontainers.505

IntouchwithyourinnermantleThemantle shows itself through the crust in subtle ways, theway a lost toy is

betrayedbyalumpinarug.

Wearenowbeginningtoappreciatethatthemotions of the mantle show themselves in the broadest scales of our continentallandscapes.Where hot mantle mushrooms up under the crust it elevates the land overthousandsofkilometers,likethehighplateausoftheAmericanWest,316andthehighlandsofEastAfrica(the“AfricanSuperswellStructure”).313Asakidinahigh-altitudeschoolinequatorial Kenya, I never imagined that the thin air and occasional night frosts on thesoccerfieldswereadirectresultofaswollenmantle!ButtheAfricanSuperswellissmallcomparedtothelargestinflammationsontheplanet.

AskidswearetaughtthatEarthisasphere,andtoagrossapproximationitis.Butifyouexaggeratethehighsandlowsofourplanet’sshape(the“Geoid”)itismorelikeapotatothanaball.TwobroadinflammationsoftheGeoidatoppositepointsontheglobehave beenmapped by satellites, one corresponding toAfrica and the northernAtlantic,and theotheracross thewesternPacific.Theseswellingscorrespond to those regionsatthe core-mantle boundary (nicknamed “Jason” and “Tuzo”) associated with PlumeGenerationZones.455

20“Jason”and“Tuzo”superimposedonthe“Geoid”(Earth’sshape).ColorscaleshowshowtheactualShapeoftheglobediffersfroma

slightlyflattenedsphere(inmeters).JasonandTuzoatthecore-mantleboundarycorrespondtoswellingsinEarth’sshapeatoppositesidesoftheglobe.Source:Burke,Annu.Rev.EarthPlanet.Sci.2011;Geoid©ESA

–GOCEHighLevelProcessingFacility

Themantleisfartoodeepbeneaththecrusttodrillinto.It’saround5to10km(3-6mi) beneath the deep ocean floor, and 30 to 60km (20-40mi) beneath the surface ofcontinents,whereasthedeepestholeeverdrilledwasinRussiaanditreachedjust12km(7½mi) 506 after nearly 2 decades of drilling! So scientists have relied on lumps of itbroughtupinvolcaniceruptions,orindirectgeophysicalorgeochemicalanalysestolearnabout it. But recently scientists discovered an area where the mantle actually pokesthroughthecrustattheMarionRiseintheIndianOcean.Thisunusuallybuoyantpieceofmantle is probably left over from the Karoo volcanic eruptions associated with theToarcian extinction event in the Jurassic, and the Cretaceous eruptions as MadagascarseparatedfromIndia.507

Reboundingfromtheiceage

During the last ice ageScandinavia andNorthAmericawere covered in a greatthicknessofice–morethan2.5km(1.5mi)deep.508509Thisicemasswasheavy,soheavyinfactthatitdepressedthecrustintothemantle(isostacyagain),andthedisplacedmantleebbedawayfromthedent,causinglandforhundredsofmilesaroundtheedgesoftheice

tobulge.Theice-depressedcrustloweredthealtitudeoftheice,makingiteasiertomeltwhensolarwarmthreturned,generatingrapidcollapsesoftheicesheetsmanytimesinthelast400,000years. 343Whentheicemelted,thatweightwaslifted,allowingthemantletoflowbackagain.But themantle flowsveryslowly,so thecrust is still reboundingevennow,some8,000yearsafterthelasticesheetmelted.

Evidenceforthisliesinshorelinedepositswellabovetoday’ssealevel,andindatapainstakingly gleaned from plant and animal remains in Scandinavian lakes that wereoverrunbyseaand thenreemerged.Dating theseremainsshowsthat inScandinavia thelandroseatbetween1and1.6cm(3/8to5/8in)peryearat theendofthelast iceage.Thenrisingsealevelsovertooktherisingland,drowningfreshwaterlakeswithsaltwatercontainingmarinemollusksandalgae. 510Whensealevelsstabilized,thereboundinglandre-emerged and its lakes contained fresh water again. Elsewhere in Scandinavia thepresenceofmarinesedimentsmilesinland,andfreshwaterlakesthatoncewerepartoftheBalticSea, andancientseasidesettlements thatarenowmiles fromshore,areallvividsignsofthereboundafterthelasticeage.

GPSmeasurementsconfirmthatScandinaviaisstillrisingbyupto1.2cm(½in)ayear,andsurroundingareasaresinkingby2mm(1/16in)peryear.Computermodelingofthesemovements shows that it is putting the crust under considerable strain 511 and thissometimestriggersnewearthquakesonancientfaults. 512Italsoopensthoseoldfracturesupforfluidsandgassestoleakthroughthem.513

Of course, the extreme example of ice depressing a large area of land isAntarctica. Airborne radar, seismic surveys and satellite altimetry allow us to “seethrough”theiceandplotmapsofthesub-glaciallandscape.TheyshowthatinsomepartstheAntarcticicehasdepressedthegroundupto2.5km(1.5mi)belowsealevel,andentiremountainrangeswith3km(2mi)highpeaksaresubmergedunderakilometerofice!514

Bottomline:The Earth is dynamic across scales and timeframes beyond our everyday

experience.Ourlandscapesandouratmospheretodayareaconsequenceoftheseongoingchanges, which have been revealed by amyriad of scientific techniques, ranging fromCAT scans through the interior of the Earth using earthquake waves, to exquisitelysensitivemeasurementsofchemicalvariations.Thesedatahaveshownusthattheseemingpermanenceof ourEarth is just an illusion.Ourplanet has beenmoving, changing andevolvingsinceitsbirth,anditcontinuestodosotodayanditwillintothefuture.

3.ProofofLifeWhenDarwinfirstsuggestedtheevolutionofspeciesthroughnaturalselection,he

wastroubledbyadilemma:

“… it is indisputable that before the lowest Cambrian stratum wasdepositedlongperiodselapsed,aslongas,orprobablyfarlongerthan,thewhole interval fromtheCambrianage to thepresentday;and thatduringthese vast periods the world swarmed with living creatures… To thequestion why we do not find rich fossiliferous deposits belonging tothese assumed earliest periods prior to the Cambrian system, I cangivenosatisfactoryanswer.”515

Inthemidtolate1800s,intheveryearlydaysofpaleontology,itappearedasifamyriadofspecies justappeared,spontaneously,at thebeginningof theCambrianperiod(541millionyears ago, or aged88 inYourLife asPlanetEarth).That, and the lackoffossilscatchingcreatures“intheact”ofevolvingfromprimitivespeciesintomodernones(birds, amphibians, humans),were serious obstacles to the acceptance of evolution.AsDarwinsaid:

“Whythenisnoteverygeologicalformationandeverystratumfullofsuchintermediate links? Geology assuredly does not reveal any such finely-graduatedorganicchain;andthis,perhaps,isthemostobviousandseriousobjectionwhichcanbeurgedagainstthetheory.Theexplanationlies,asIbelieve,intheextremeimperfectionofthegeologicalrecord.”515

He was right, of course. The geological record is imperfect. But this was theinfancy of Earth science, a time when Darwin himself grappled with the span ofgeologicaltime. 515HeandhispeerswereapparentlystillunawareofMendel’stheoriesofgenetics (published at around the same time as later editions of his “On the Origin ofSpecies”),andofcoursethiswaswellbeforeradiometricdatingandacenturybeforeplatetectonicswasunderstood.ThiswasthetimeoftheAmericanCivilWarandtheCrimeanWar,beforethesourceoftheNilehadevenbeendiscovered.

Thingshavemovedonovertheinervening150+years,andDarwin’sdilemmahasbeenresolved.Notonlyhavefossilsbeenfoundgoingback3billionyears(longbeforethe Cambrian), but also fossils of intermediates for many species have been found,includingprecursorstowhales,horses,birds,amphibians,andevenhumans.

Aswestandtoday,theevidenceforevolutionisoverwhelming.ItisasprovenasaroundEarthandtheSun-centeredSolarSystem. Yet a surprising number of people just do not believe it. 516 517 So it’s worthexploring the fascinating and powerful evidence for evolution, which flows from 3differentareasofscience–thefossilrecord,thebiologicalrecord,andlaboratoryandfieldobservationsofevolutionhappeningbeforeoureyes.

BeforeIdo,it’sworthpointingoutthatevolutionasfactisacceptedbypracticallyall scientists, and is the stated position of scientific institutions like The AmericanGeophysicalUnion,theGeologicalSocietyofAmerica,theGeologicalSocietyofLondon(theoldestlearnedsocietyfortheEarthsciencesintheworld),andTheNationalAcademyofSciencesoftheUnitedStatesofAmerica(includingmanyNobelPrizewinners).

TheGeologicalSocietyofLondonstatesthatevolutionisafact“longestablishedbeyonddoubt.” 518Similarly theAmericanGeophysicalUnioncallsevolution“oneof themost important foundations of the science enterprise,” 519 and theGeological Society ofAmerica calls evolution “now well established as a well-tested fact.” 520 The NationalAcademyofSciencesstatesthat:“Biologicalevolutionisoneofthemostimportantideasof modern science. Evolution is supported by abundant evidence from many differentfieldsofscientificinvestigation.”521

EvolutionbeforeoureyesFarmersandanimalbreederstheworldoverhavelongusedselectivebreedingto

produceallthedifferentkindsofdomesticateddogs,catsandfarmanimalswehavetoday.They used animal husbandry (human selection) to evolve shapes as diverse as theDachshundandtheGreatDane,largecowswithmilk-productionthatwouldbeconsideredmiraculousafewhundredyearsago,bullsforbeeftwiceaslargeastheirancestors,largerwoolliersheep, 522andpigs tuned to thedifferentbaconmarkets inEuropeandAmerica.Formanyyears farmershaveusedknowledgeofgenetics to improvecrop resistance todisease, pests and drought through plant hybridization. These same aims are now thedrivingforcebehindtheproductionofgeneticallymodifiedcrops.It’sinconceivablethat

naturalselectionpressures(likechangesinavailablevegetationorcompetitionfromotherspecies),appliedtogenerationaftergenerationovermillionsofyears,wouldhaveanylesseffect.

21:Humanselectioncausingevolutioninfarmanimals:HistoricalYieldsperAnimalinBritainandHolland.BasedonApostolides,etal,

Univ.Warwick,2008.

Evolution has been directly observed in the laboratory, for instancewithin fruitflies, 523524bacteria, 525wheat, 526andevenBirdFlu. 527 Ithasalsobeenobservedoutside thelab, for example the increasing prevalence of drug-resistantmalaria, antibiotic-resistant“Methicillin-resistant Staphylococcus aureus” (MRSA), and the SARS microbe. Theprocessofnaturalselectioncausingevolution issurprisinglyrapid:scientistsworking inTrinidad watched fish evolve in response to environmental change in as few as 20generations(aguppygenerationis3-4months,sothat’s5-6years).521

Thatdoesn’tmeanthere’snothinglefttolearnaboutevolution–farfromit.

Asthecompletegeneticcodes(genomes)ofmoreandmorespeciesaremapped,wearelearningsomesurprisingthings.Atthetimeofwritingover3,000specieshavehadtheirgenomesmapped,includinghumans,bonoboapes,tomatoes,grapevines,mice,rice,yeast, corn, sorghum, some viruses and bacteria, some fungi, the turkey and even thepuffer fish, 528 529 and over 12,000more species are in theworks. These genome “maps”enablesectionsofgeneticcodethatarethesameindifferentspeciestobeidentified,andsoancestraltreescanbedefinedfrommatchedgenes.

Intheearlystagesofmappingthehumangenome,itappearedasifmost–98%-ofourgeneticmaterialservednopurpose,soitwasdubbed“junkDNA.”Butsincethenthis “junk” has been found to havemany functions, from defining proteins tomarking

landing spots for proteins that influence genes, to switches that turn off bits of ourchromosomes. 530Aportionof the “junk” isnothumanbut actuallyviralDNAmixed inwithourgenes,creatingpopulationsof“endogenousretroviruses”thatwepassontoourkidsinjustthesamewaywepassonthecoloroftheireyes!Itturnsoutthathumans-andmostmammals-carryDNAfromvirusesinheritedovermillionsofyears,goingbacktotheageofthedinosaurs! 531Someofourgeneshavealsobeenfoundtobe“fossilgenes”thatgaveusimmunitytolong-forgottendiseasesdeepinourevolutionarypast,butwhichwestillcarrywithus.532

The technological revolution in gene sequencing that made genome-mapping apractical endeavorhasalso triggerednewmovements inbiology like“epigenetics,”andtheoverlappingfieldof“evo-devo.”Theyarestartingtoilluminatehowourenvironmentmayswitchonorswitchoffgenesthatweinherit,andthewayageneisexpressedinanembryoalterstheshapeofananimal,likeabirdhavingalongerbeakthannormal.Thisrefiningunderstandingofgeneticsholdspromiseforimprovedtreatmentsforcancerandotherdiseases.

Soevolution throughnatural selection isproven in the laboratories, field studiesandmedical experience of today.But that’s notmuch help in narrating the play of lifefromitsearliestorigins.Forthatweturntothefossilrecord,aswellasthetrailofcluescontainedwithinthecellsoflivingorganismstoday.

Notyourgrandfather’sfossilsWhenIsay“fossil”youareprobablypicturingadustypieceofrockwithacurly

seashell protruding from it. That’s the kind that Darwin and his contemporaries werefamiliarwith, but the fossilswe can detect today are beyond thewildest imaginings ofthose 19th century collectors. Paleontologists now study fossils ranging from themicroscopictothesizeofanentireforest.Usingtechniquesasdiverseasrock-dissolvingacidsandCATscans.Theycanevenlearnaboutancientlifewhenthere’snothingleftbutastain,oraburrow,orevenjustanimbalancebetweenatomicisotopes.

Bearinmindthatit’sactuallyquitehardtopreserve a dead creature as a fossil. It requires a series of happy accidents, like beingcoveredbylow-oxygensedimentssoonafterdeathtoavoidrottingaway,compactionofthose sediments into rock, tectonic upheaval, exhumation millions of years later, and

finallyachance findbyapaleontologist.Thismeans that theoldest fossilof aplantoranimalisunlikelytobethefirstindividualofthatspecies,andthelastrecordedfossilmaywell predate the actual extinction of the species (this is known as the “Signor-LippsEffect”).

ElusiveearliestlifeItwouldbegreatifwecouldfindtangiblefossiltracesofLUCA–thefirstlifeon

Earth-butit’shardtoseehowwecouldtellafossilLUCAventfromanyotherancientsubmarine vent. In the absence of fossil evidence for LUCA, we have to resort tobiologicalcluestoherexistencelaterinthischapter.

Forhard,fossilevidenceoftheearliestlifeweturntomicroscopictracesinsomeofEarth’smostancientrocks.Claimsweremadeinthe1990sforfossil tracesof lifeasoldas3.8billionyears, inrockslocatedintheinhospitablewildsofwesternGreenland,but they have since been refuted. Other scientists painstakingly picked-apart thecomplicated rock structure and found that, sadly, the rockswith the putative life-tracesweren’t as old as first thought, and that they were formed as underground magmaintrusions“whollyunsuitableforthediscoveryofprimordialtracesoflife.”533

It’s hard to overstate how difficult it is to find and date unambiguous traces ofmicroscopiclifeover3billionyearsold,inrocksthathavebeensqueezedandfoldedandheated many times until even establishing the original rock type is not trivial. Unlikeseashells,skeletons, teethoreventhe imprintofafernfrond, the tracesofearly lifeareminisculeandbarelydifferentfromtheirsurroundingrock.Itisnotevenenoughtofindtracesofcarbonassociatedwithmicroscopicfuzzybits.Thecarbonmustbederivedfromkerogen (which is also found in petroleum and coal deposits) and must have reducedlevels of the isotope carbon-13 which is depleted by biologic processes. 534 In fact theearliesttracesoflifeonEartharejustsuchcarbondeposits.37

We begin to see clearer traces of early life in theBarbertonGreenstoneBelt inSouthAfrica, in the formofmicroscopic tubeson the rimsof3.5-billion-year-old lavasthat bear uncanny resemblance to microbe-formed structures on the skin of modernsubmarine lavas.The clincher is that traces of carbonwithin these tubes have the rightproportionofcarbonisotopestoindicatetheywerecreatedbylife.48

In the first part of this book I described the humble stromatolite – basically amound created by layers of slimy bacteria trapping and precipitating sediments. Theancient rocks of Western Australia contain an entire 3.43-billion-year-old reef ofstromatolites.46Thedistinctivedomes,conesandegg-crateshapesareproofofearlyslime-life,furtherconfirmedbythesuperbpreservationofclustersofmicroscopiccells, tubes,threads, films and spindle-shapes. The diversity of fossil microbe shapes suggests thatevenbackthentherealreadywasaveritablecommunityofmicrobiallife.Betterthanthat,scientistsfoundtinycrystalsof“fools’gold”(pyrite–orironsulfide)associatedwiththefossil cells, which have a sulfur isotope ratio that is a dead giveaway that they wereformedbymicrobesextractingenergyfromsulfurchemicalsinthewater.47

Thosesulfurisotopesareanexampleofanotherkindof“fossil”thatDarwincouldnothaveimagined:chemicalfossils.

ChemicalfossilsAmazingly, scientists today can detect signs of ancient life even when there is

nothingleftbutaslightstainbetweengrainsofancientrock.

Forinstance, thefirstevidencefor themorecomplexkindofcell that ledtoyouandme shows up as littlemore than a chemical trace. 24 Our cells, like the cells of alleukaryotes (animals, plants, fungi, amoebas and someother single-celled critters) use aclass of chemicals called “sterols,” an important difference from non-eukaryotes likebacteriaandarchaea. 24Sterolsarenecessaryfor thebiochemistryofourcellmembranes(cholesterol is a sterol). So finding sterols in ancient rocks would be a great clue thateukaryotes rather than more primitive bacteria or archaea were present. Unfortunatelysterolsdegradeovertimebutthegoodnewsisthattheydegradeintosteranes,whicharemuchmoreresilientchemicalscapableofhangingaroundforacoupleofbillionyearsormore.Infact,scientistsjokethatafteryouaredeadandburied,thelasttraceofyouwillbeyourcholesterol!535

Traces of steranes have been found in rocks approximately 2.5 billion years oldfrom Canada, Gabon and South Africa. In some locations there is uncertainty that thesteranes date from the formation of the original sediments rather than from fluids thatseeped in over eons since, but for now the balance of opinion is that they remain theearliest,iftenuousproofofeukaryotelife–ourdistantancestors.24

Similarly, the first traces ofcyanobacteria – those first producers of oxygen that gaveEarth itsmid-life crisis – arechemicals.Asmallgroupofchemicalsknownas“2-methylhopanes”areallthatremainofthe first cyanobacteria.Aswas the casewith steranes, therehasbeen somedebateoverhowreliabletheseoilyhintsare,butagainrecentworktendstoconfirmthemasavalidtraceforthosespecialoxygen-producingcreaturesaround2.7billionyearsago.24536

Interestingly,eukaryotesneedoxygen tomakesterols 535– something theyhad torelyoncyanobacteria toprovide,and itmaybenocoincidence thatboth innovations inlife show up at broadly the same time. But the steranes show us that eukaryotes werearoundsome300millionyearsbeforeoxygenhadbuiltupsignificantlyinairandwater,

so scientists were scratching their heads about that for a while. Thankfully, recentexperiments have shown that primitive eukaryotes don’t needmuch oxygen to producesterols – trace amounts are enough, 535 and that’s all that was available from those firstcyanobacteriauntiltherocksandoceansweredoneabsorbingalltheoxygentheycould.

Attheotherendofthetimescale,sterolsinsewagehavebeenputtoworktotrackthe spread of human settlements after the ice-ages. Some sterols (5β-stanols) are onlyfoundinhumanandlivestockfeces,andabovecertainlevelstheyareasureindicatorofhumansettlement.ByanalyzingthefinelayersofsedimentsforthesefecalmarkersinalakeinnorthernNorway,scientistswereabletodatethefirsthumansettlementsthereto2,300yearsago.537

Chemical traces alsoprovide important clues to the first colonizationof landbyearlyplantsandfungi in theCambrianperiod.Analysisoffossilizedsoil fromthat timerevealsapatternofaluminum,siliconandphosphorouslevelsthatshowitwastreatedbyorganic acids – which are naturally produced by plant roots in soils to this day. Aphosphorousmineralcalledapatiteismissingfromthetoplayerofthesoil(plantsabsorbphosphorous, depleting it from soil, which is why you add it to lawns and gardens infertilizer). Inaddition, theclays in thesoilshowthatcalciumionswereplentiful.Thesecluestogetherareatell-tale“biosignature”ofspecializedfungithatliveinsymbiosiswithplantroots,called“mycorrhizalfungi.” 538Becausethesefungirelyonplantsforessentialnutrients,evidenceforthemisalsoevidenceforplantsonlandatthattime,bolsteringthefirsttenuousevidenceforliverwort-likeplantsinthefossilrecord.81

Chemicalfossilsarefarmorethanarcanetoolsforspecializedpaleontologists.Theoilindustryhasbeenusingchemicalfossils(“biomarkers”)fordecadestoappraiserocksthatmayhavebeenthesourceforoil.Theycanchoosefromamenuofseveralhundreddifferentchemicalswithexoticnamestodiscoverwhatkindoflifeledtotheformationofasampleofoil,andinmanycasesthekindofenvironmentthatitlivedin.539Forinstance,a class of chemicals made of a chain of carbon atoms, called “alkanes,” betrays theirorigins in the number of carbon atoms they contain.Alkaneswith 15, 17 or 19 carbonatoms come from lake ormarine algae,whereas thosewith 27, 29 or 31 carbon atomsoriginated as land plants. 539 “Pristane” and “Phytane” are all that remain of long-decomposedchlorophyll fromplants, and“Bicadinane” tellsus the rockoncecontainedthe resins from conifer trees. 539 Still others like “Oleananes” and “lupanes” originatedfrom flowering plants, 539 whereas “dibenzothiophene,” “dibenzofuran” and “biphenyl”,canindicatewherewoodyplantswereburiedbuthavelongsincedecomposed.Theselastthreetellusabouttheamountofvegetationwashingfromlandintooceans.Abigdropinthesechemicalstellsustherewasprobablyabigreductioninvegetationonland,suchashappenedatthePermian“GreatDying.”540

One of the most important traces of life, and yet maybe the subtlest, relies onsubatomic differences in carbon isotopes. Most carbon in the world – in the air youbreathe, the food you eat and the fuel you burn – ismade up of the isotope carbon-12(12C).Only around 1% ismade up of the fractionally heavier carbon-13 isotope (13C).Anditisthisminisculedifferenceinmassthatcantellusiflifeproducedacarbonsample.Life generally prefers the “regular” 12C variety of carbon. This means organismsconcentrate12Cwithin themwhile 13C is shunned andbuilds up in the atmosphere and

oceans.Eventheearliesttracesoflifeshowupas3.7billionyearoldcarbondepositsthataredepletedin13C.37

Fossilfuelslikecoal,oilandmethanehavethesamehigh12Cproportionsoftheirplantorigins.Ifavastamountoffossilfuelisburnedornaturalmethaneisreleasedintotheair,thisshowsupasabigincreasein12Clevels(andacorrespondingnegativeshiftin13C),suchashappenedatthePermianGreatDying,181theTriassic,195andthe“PETM.”262

Soinmyriadwayschemicaltracescanserveas“fossils”oflifeforwhichnootherevidence remains. But subtle variations in chemicals can tell us other surprising thingsaboutancientlife,evenancientbehaviors.

Toothtales:ancientmigrationsanddiets

The idea that dinosaurs underwent seasonal migrations over several hundredkilometers,likemodernwildebeestinEastAfrica,isbeguiling.Buthowcanwepossiblyknow?Wecan’ttracktheirfootprintsveryfar,andthefossilbonesliewherethecreaturedied– likeadeath-mask thatcan’t tellof its living travels.Establishing thebehaviorofextinct creatures is fraught with difficulty, so it would seem impossible to prove thewanderingsofindividualssome150millionyearsago.Yetscientistshavedonejustthatby some ingenious work using fossil teeth from giant, long-necked Camarasaurusdinosaurs.

They analyzed the growth layers in the fossil dinosaur teeth for variations inisotopesofoxygen.Theywerethenabletolinkthosechemicaltracestothewaterthatthecreaturesdrank,usingtheoxygenisotopesinsedimentsatdifferentplacesacrossUtahandWyoming.InthiswaytheyshowedthatseveralCamarasaurusindividualsmigratedfromlowlands in the east to spend the summer dry months in western highlands beforereturningtothelowlands.541

A similar study on our pre-humanancestorswasjustasrevealing.Scientistsusedfossilteethfrom2-million-year-old

South African Australopithecus africanus and Paranthropus robustus specimens. Bycomparing the strontium isotope variations within the teeth ofmales and females withnaturalstrontiumlevelsinlocationsnearwheretheywerefound,theywereabletoshowthatmalestendedtoliveinoneareaalltheirlives,whereasfemalesweregenerallybornelsewhere.Inotherwords,itwasthecustomofourveryearlyancestorsforbridestoleavetheirhomesandlivewiththeirhusband’sfamily!542

Oral hygiene in our primitive ancestors was not up to our modern standards –whichisagoodthingforscience.BypickingatthedentalplaquepreservedontheteethofAustralopithecus sediba (a 2-million-year-old pre-human) scientists were able to findmicroscopic traces of what they had been eating still stuck there. Specifically theycollected“phytoliths”whicharemicroscopicsilicacrystalsformednaturallyinplantcells.Thesehavecharacteristicshapesdependingonthespeciesofplant,sothescientistswereable to tell that sediba atemainlywoodland leaves, fruits and bark. The scientists alsousedlaserstovaporizetinypitsintotheenameloftheteeth(“laserablation”)andanalyzethe isotopes of carbon in the vapor. This showed that throughout their growth, theseindividualshadpreferredaforestdiet.543Thankgoodnesstheydidn’tfloss!

RealmofthetinyAlthough Darwin was aware of microscopic fossils (he referred to them as

“infusoria” and “rhizopods”) 515 advances inmicroscopy since his time have revealed acornucopia of tiny fossils that have played an important part in our understanding ofchanges in Earth through time. Even when rock might appear barren of fossils to thenakedeye,modernmicroscopy-especiallytheelectronmicroscope-allowsustozoomintotherealmofthetinywherewecanfindacastofthousandswaitingtobereadlikeabooktorevealancientclimates,waterconditionsandbiodiversity.

Phytoliths, for instance, are not just used to reveal the dietary choices of ourancestorsandotherextinctcreatures.238Theyareresilientenoughtosurviveinrockswhenallothertracesoftheplantthatformedthemhavedisappeared,andtheyevensurvivethejourneythroughananimal’sguttobepreservedinfossilpoop(moredelicatelyreferredtoas“coprolites”).238Theseallowustotracethespreadofgrasslandsacrosstheglobe,237andtobuildapictureoftheflorathatinhabitedanareaataparticulartime.543

Sporesandpollen–heraldsforlandplants

Crucial evidence for the evolution and spread of land plants comes from fossilpollenandspores.These tiny reproductiveparticleshavedistinctiveshapesand texturesthat are characteristic of the types of plant that they originated from, so in finding thesporeorpollenyouhaveevidencefortheplantthatproducedit.Sporesorpollenaremorelikelytobepreservedthantheirparentplantsbecauseeachplantproducestheminlargenumbers,theyaremadeofresilientmaterials,andaresmallenoughtobeblownorwashedinto lakes or seaswhere sedimentswill preserve themeven if the parent plant livedonsomeerodinghillslopedoomedtodisappearance.81

The earliest tentative evidence for land plants shows up as microscopic“cryptospores” and enigmatic tubes in the deep recesses of the Grand Canyon andTennessee. 81 These are over 500million years old, about 100million years older thanwhole-plant fossils in the Scottish “Rhynie Chert.” It’s uncertain if those cryptospores

come from a kind of liverwort or from algae, but they do resemble younger liverwortspores 81and theyareonly40millionyearsolder thanconfirmed landplantspores fromArgentina. 118Considering recent evidence for the kind of fungi that are associatedwithplantroots(“mycorrhizalfungi”)in500million-year-oldCambriansoils,538thecasefortheriseofearlylandplantsaround500millionyearsagolooksincreasinglyplausible.

Notlongafterthosecryptosporesappear,westartfinding“Triletespores”–akindthatbearsaY-shapeddent.Theseare linkedtoplants thathadevolvedstems(“vascularplants”),likeCooksonia,hornwortsandmosses.TheyfirstshowupintheearlyDevonian,around450millionyearsago,againwellbeforethewhole-plantfossilsfortheseplantsarefound,placingthearrivalofvascularplantsatatimeofsignificantatmospheric,climateandevolutionarychanges.11912990

Withtheadventofseedplants,pollengrainsbegintoshowupinrocks.Pollenisas diagnostic of its originating plants as spores are for more primitive plants. This istremendously helpful in plotting the spread of conifers, 164 and then (in the Cretaceous)flowering plants, 225 and is invaluable for revealing the traumatic changes at majorextinction events such as at the end of the Triassic, when forests were devastated andreplacedbyferns. 196Pollenisalsousefulinnarrowingdownthetimeperiodrockswereformed in, especially in freshwater sedimentswheremarine fossils normally employedforthisjobaremissing.544

Thesetinyplantfossilsarejustasmallpartofthewidepaletteofmicrofossilsthatscientists use to correlate different rock layers, to establish their dates, and to revealancientenvironments.

Temperature-tellingteeth

Perhaps the most bizarre are tiny, spiky tooth-like fossils called “conodonts,”whichwereusedfordecadestomatchupdifferentstratawithoutanyonereallyknowingwhatkindofanimaltheycamefrom.Thenintheearly1980safossilofabig-eyedeel-likecreaturewasfoundinScottishoilshaleswiththese“teeth”intact,revealingthattheywere indeedakindof feedingapparatus. 545Aside fromdefiningvariousgeological timezones, they can show how hot the rock containing them became during its sojournunderground. The phosphorous-based mineral that conodonts are made from changescolorbasedon the temperature ithasbeenheated to, frompalebrown, tobrown,black,and finallywhite (around 80°, 140°, 480° and 550°C respectively). 546 This informationhelps calibrate how deep rocks have been buried in tectonic upheavals, and tells oilexplorerswhatgradeofoilorgastheycanexpecttofind.ConodontsfirstappearedintheCambrianandtheysuccumbedintheTriassicmassextinction200millionyearsago.544

TheSwissArmyKnifeoffossils

Fortunately there is another group of tinymarine creatures who outlived the conodont animal - the foraminifera, or “forams” asmost people prefer to call them.They are amoeba-like blobswhichhave the advantage(forus)thattheymakeadurableskinwithahugevarietyofshapesrangingfromsimpleballs to elaborate seashell-like structures, spirals, bunches of grapes or even vases,dependingonthespecies.

ForamshavesomanyusesyoucanthinkofthemasthelittleSwissArmyKnivesofmicropaleontology!Being only about half amillimeter bigmeans that even a singleborehole can recover a useful sample of fossils. They are widespread in manyenvironments and they underwent rapid evolutionary bursts, which allows precisematching to other locations across the globe. 544 Oxygen isotopes in their shells give ameasure of thewater temperature they lived in, and different foram species that live indifferenthabitatsrevealancientoceantemperature,salinityanddepthprofiles.544

22AdiversityofforamsfromasinglebeachinBurma.Thephotoisjust5.2mm(2/10in)across.Source:WikimediaCommons/Psammophile

…andmanymore…

Even Swiss Army Knives have their limits. In deep waters where the calciumcarbonateofforamshellsisdissolvedaway,scientistscanturninsteadtothesilica-basedskeletons of “radiolaria.” These perforated balls and y-shapes are only about 1/10 of amillimeterbig,butcanbeusedmuchlikeforams.Theyareespeciallyhandyinrocksthathave undergone intense changes in continental collisions. Their tiny flinty skeletonspreservedinbandsofchertareoftentheonlyfossilsthatsurviveinthoserocks,providingauniqueinsighttoconditionsinlong-departedseas.544

ThereisahostofotherusefulmicrofossilsthatIdon’thavespacetodiscusshere,likeAcritarchs (acatch-allname fora rangeofplanktoniccreatures,cystsandembryosfromtheearlydaysoflifetothepresent),chitinozoa(resemblingsmallvases),ostracods(tiny leggy crustaceans in clam-like shells), diatoms, coccolithophores (whose bodiesmakeupchalk)anddinoflagellates.544

Sobydelvingintotherealmofthingstoosmalltobeseenwithoutamicroscope,scientistshavebeenabletotracetheevolutionandglobalspreadofdifferentplantgroups,learnaboutthedietofextinctanimals,uncoverthethermalhistoryofrockstrata,revealancientoceantemperatures,salinitiesanddepths,andevenmatchupdisparaterocklayers.Butwhatabouttheotherendofthescale-fossilssobigtheycan’tbeappreciatedwihoutabird’s-eyeview?

Realmofthereallybig

Fossilforests

Weoftentalkof“notbeingabletoseethewoodforthetrees”butintheEarthsciencesthiscanbebothafigurativeandaliteralproblem.Muchcanbededucedfromasinglefossil–anisolatedleafimpression,ashelloratoothfor instance–but inferringhowdifferent lifeformscoexistedinacommunity,basedonisolatedspecimens, is fraughtwithdifficulty. Justoccasionallywedouncover fossilsofsuch size and quality, and from a single instant, that they capture an entire ecosystemfrozenintime.TheyareabitliketheruinsofRomanPompeiiwherehumanbodiesandartifactsliepreservedinthemomenttheywereoverrunbyvolcanicashinAD79.

ThePompeiianalogyisparticularlyaptforthefossilPermianforestdiscoveredatWudainInnerMongolia,China,whereover1,000squaremetersofforesthasbeenfoundpreservedinvolcanicash,justliketheunfortunateRomans.Thisexceptionalpreservationhasallowedscientiststoplotthelocationsofthousandsofindividualplantsintheactualspotswhere they lived 300million years ago in their swamp, and so to reconstruct theancientforestoflong-extinctplants–trees,ferns,undergrowthandall.547

Perhaps the best known example is the Joggins fossil forest preserved in thetidelandsofNova-Scotia.Thishasover60separatelevels548offossilforest,whichmadeaprofoundimpressiononSirCharlesLyellandSirWilliamDawson(twofoundingfathersof geology), when they visited the site in 1852. Alongwith fossil tree-trunks standingtodaywheretheygrewintheCarboniferous,theydiscoveredfossilamphibians,reptiles,millipedesand snailspreservedwhere theyhid in thehollowsof those trees. 163 Dawsonreturned in 1877 armedwith explosives to uncovermore, and collected over 100 fossilamphibiansandreptiles!TheJogginsfossilforestinfluencedDarwin’stheoryofevolutionandhelpedresolvethedebateabouttheoriginofcoal.163

There areover60Carboniferous-era fossil forest sites thathavebeenuncoveredacross the globe so far, 548 including a 310 million-year-old rainforest in Ovčín, in theCzechRepublic,preservedinPompeii-likeash,allowingpaleontologiststodistinguishthegroundcoverandunderstoryplantsfromthecreepersandtheforestcanopy.548

Bonebedsandtrackways

OneofthemostimpressivefossilsitesIhaveseenistheDinosaurQuarryat theDinosaurNationalMonumentinUtah,USA.Itisawarehouse-sized,bone-strewnslabofrock upon which the contorted skeletons of dozens of dinosaurs of different species,young and old, lie as theywere buried in theirmass grave 150million years ago. The

quarryisjustonepartoftheMorrisonFormationwhichhasyieldedmanydinosaurfossilsacrossthewesternUSA,includingahugecollectionofdinosaurfootprintsinsoutheasternColorado. This “dinosaur trackway,” with the footprints of more than 100 differentdinosaurs,shedlightontheirgregariousbehaviorandalsoontheratioofpredatorstopreyspecies(whichturnedouttobesimilartotheratioinmodernspecies).549TherehavebeensimilarfindsaroundtheworldincludinginYemen550,France551,China552andArkansas.553

It’snotjustdinosaursandcoalforeststhatarepreservedonabigscale.A15km(9mile)area inCalifornia’sSanJoaquinBasin ishome toa“bonebed”datingback to theMiocene, some 15 million years ago. It is just crammed with shark teeth, bones ofdolphins, seals, whales, turtles, camels, horses, and teeth from the extinct giant sharkcalled Carcarodon megalodon, which was much larger than a modern Great Whiteshark.554

Sopaleontologistsdooccasionallygettheirhandsonfossilsitesthathelpthemgetthe big picture ofwhat an ecosystemwas like at a given point in time and space. Butsometimes they are blessed with discoveries that are special for another reason - theyrevealmuchmorethanbones,shellsandtreetrunks.Theyrevealthedelicatesofttissuesthatnormallyrotorarescavengedawaybeforeafossilissafelylocked-awayinsediment.

SoftStuffIfyouthinkofyourownbody,there’smuchmoretoyouthanyourskeleton,and

yetforthemostpartifafossilispreservedatall,onlythehardparts–bones,teeth,shells,spores and woody trunks – get preserved. Forensic scientists could make a reasonablereconstructionofwhatyoulooklikeifall theyhadwereyourbones,butthinkaboutallthosecreatureswhohavenohardpartstospeakof–thejellyfishandwormsofthisworld.Andeventhosethatdohavebonesorshells–whatdidtheanimalthatownedthebones,orinhabitedtheshells,reallylooklike?Withoutsoft tissuesyouhaveaveryimpoverishedsamplingoftherealdiversityoflifeatthetime.

AndthatwasessentiallyDarwin’sdilemma,sinceinhisdayfossilcollectinghadnotyetencounteredthoserareandpreciousfossilsitesknownas“lagerstätten,”wheresoftparts of creatures aredelicatelypreserved.All theyknewofwere fossils fromafter theevolutionoftheseashell.Lagerstättenhavefleshed-out(punintended)thediversityoflifebeforetheseashellfirstappearedintheCambrian,andsince.

GunflintandBitterSprings

Itwasn’t until the 1950s that the first convincing evidence for Precambrian lifewasdiscoveredinthe2.1billion-year-oldGunflintChert(a‘bandedironformation”)onthe shores of Lake Superior, Canada. A geologist named Stanley Tyler took rockspecimens and ground them into thin, translucentmicroscope slides. The flinty, stripedrockwas found to contain awealth of cells and filaments, even preserving the internalstructureofsome. 555Thesilica in thewaterswhere thesemicrobes livedhadpermeatedtheirbodiesandentombed them in rockhardenough to resist the ravagesof2.1billionyearsofcontinentaldrift,burial,exhumationandiceages.ThesilicapreservedsuchdetailthatthefossilswereunassailableevidenceforearlyPrecambrianlife,ataroundthetimeoftheplanet’s“mid-lifecrisis”thatIdescribedinthefirstpartofthisbook.

Before long, other discoveries of petrified Precambrian lifewere discovered.AtBitterSpringsincentralAustralia,morefilamentsandchainsofcellswerediscoveredin820million-year-old chert.These have been identified as cyanobacteria, algae, possiblefungiandothermicrobes.555

Jellyfishandembryos

In 1947 geologist Reginald Sprigg announced his discovery of fossil “jellyfish”andothersoft-bodiedanimalshehadfoundonamineralprospecting tripatEdiacara inthe Flinders Ranges, South Australia. He assumed they were Cambrian, but they werelater found tobePrecambrian– fromaround560millionyears ago. 555His discovery offronds, disks andother shapes in sandstones borewitness to diverse soft-bodied-animallifeattheendofthe“SlushballEarth”period.

Itsparkedasearchforsimilarfossilsaroundtheworld,andsincethenexquisitelypreservedfossilsofthisperiodhavebeenfoundatDoushantuoinChina,MistakenPointin Canada, in Namibia, the White Sea region of Russia, India, and Shropshire inEngland. 556557Thefinedetailpreservedatsomesiteshasevenallowedscientiststostudyindividualcells.Forexample,theDoushantuofossilsincludetinyanimalembryocells, 558seaweed,spongesandrelativesofjellyfish.49

Reachingup into theCambrian (aged88 inYourLife asPlanetEarth),we findmore examplesof extraordinarypreservation,like a crustaceanwith everydelicate strandof its filter-feedingmouthpartspreserved infools’gold,559orthefossilizedbrainofanancestortoinsects,spidersandcrustaceans!560

The510-million-year-oldCanadian “BurgessShale” is a spectacular location forCambrian soft-bodied fossils. If you have ever discovered a bug flattened between thepagesofanoldbook,youhaveareasonableideaofthefossilsthere.Takingtheplaceofpagesarelayersofshale,andthesquishedbugsareawiderangeofarthropods,mollusks,worms and the earliest precursors to vertebrates. 49 The preservation is so good it evenincludes the guts of some creatures! 559 There are other Cambrian sites with similarlyexcellent preservation in Greenland, Örsten in Sweden, Emu Bay in Australia, andChengjianginChina.

SpidersfromScotland,feathersfromGermany

Movingforwardintimethereareotherlagerstättensitesscatteredacrosstheglobeand the geological timescale, each filling in more detail of the anatomies of ancientcreatures.

TheScottish “RhynieChert” is one. It shines a spotlight on the earliest days ofland plants and the fungi, microbes, spiders, insects, mites and centipedes that livedamongthem.ThisDevoniancommunitylivednearvolcanichotspringsthatperiodicallyengulfed them in silica-rich fluids that petrified them, down to the cellular level 49 410millionyearsago(aged91inYourLifeasPlanetEarth).TheSolnhofenlimestonequarryinGermany is another example. It preserves Jurassic creatures including the early bird“Archaeopteryx” as well as pterosaurs, dinosaurs and fish. The first Solnhofen“Archaeopteryx”specimenhadclearimpressionsoffeathersaswellasareptiliantailandclawedfingers. Itsannouncement just2yearsafterDarwinpublishedhisfirsteditionof“OntheOriginofSpecies”wasgreetedskepticallyatfirst,buteventuallyitwasacceptedasanintermediatebetweendinosaursandbirds.49

IflagerstättenaretheRollsRoycesoffossilsites,ittemptingtothinkofthenextkindoffossilasmorelikeaFordtransitvan–utilitarianbutindispensable.

BurrowsandfootprintsA rafting trip through the Grand Canyon, aside from being one of the most

awesomeexperiencespossible,isabackwardsrecitalfromthepagesoftime.StartingatLee’sFerry surrounded by dinosaur-era strata, you regress throughCarboniferous stratareplete with marine fossils, until towards the end of the second day you reach theCambrian“TapeatsSandstone.”Here thecanyonwalls look like theywereassaultedbyriotingpastrychefs!Millionsoffinger-likeblobsandsmearsbearwitnesstothefeedingofcreatureswhotunneledintheseabedsome525millionyearsago,devouringanythingbefore it had a chance to become a fossil, but leaving their burrows behind instead as“TraceFossils.”

23525million-year-oldwormburrowsintheTapeatsSandtone,GrandCanyon.Watchandshoeforscale,photobytheauthor.

TraceFossilsaretheburrows,tracksandfootprintsleftbehindbylife.TheblobsintheTapeatsSandstoneareakindcalled“Trichophycus”whicharen’tfoundinrocksbeforetheCambrianExplosion,aclearindicationofthenewkindsoflifethatemergedthen.Infact,tracefossilsprovidetheearliestcluestotheemergenceofcomplexcreaturesbecausebackfilled furrows require a creature that has a distinct front and rear end (“bilateran”).SuchtracefossilshavealsobeenfoundinpossiblyolderrocksinUruguay,buttheirageisstilldebated.561

YoucanfindothertracefossilsintheTapeatsSandstone:rope-liketrailssnakingthrough thewormburrows are trilobite tracks (knownas “Cruziana”). In factweknowfromtracefossilslikeCruziana thatarthropods(thegroupthatcontainstrilobites,crabs,spidersand insects)appearedat thedawnof theCambrian,millionsofyearsbefore theoldest trilobite fossil itself, 562 just like the fossil spores that predate their parent plantfossilsasIdescribedin“TheRealmoftheTiny”above.

Careful recording of trace fossils can tell scientists about the environment theyweremadein,itsbiodiversity, 563therelativeoxygenlevel,whethersealevelswererisingor falling at the time, and even changes in salinity. 564 In fact the dinosaur trackway Imentionedin“RealmoftheReallyBig”aboveisalsoatracefossil,revealinggregariousanimal behavior. In the case of the largest dinosaurs, there are relatively few skeletalfossils,buttheirfootprintsaremuchmorecommon.Thisallowsscientiststousestatisticalanalysisonthepatternsofdinosaurfootprintstoteaseoutevolutionarychangesovertime.

Onesuchstudywasable to show thatdinosaursbecamemore specialized intodifferentecologicalnichesastimeprogressedfromtheJurassicintotheCretaceous.565

Just occasionally a dead body liesfossilizedattheendofitstracks,enablingscientiststomakeafirmlinkbetweenthetraceand the creature thatmade it. In one example, fromSolnhofen inGermany (where thatfamous“Archaeopteryx”birdfossilwasfound),ahorseshoecrab liesat theendofa10meter(11yard)death-marchpreservedastracksinthestone.Itsfootprintsareapoignantrecord of the crab’s erratic behavior as it asphyxiated in an oxygen-starved lagoon 150millionyearsago.566

So trace fossils are a valuable part of the fossil record, which can record thecommunities that lived together, their environment, and even their behavior. But thesedayspaleontologistsalsohavehigh-techtoolsto“seethrough”rockandextracteventhemostdelicate,complicatedfossils.

CATscansYou or someone you knowmay have had a CAT scan in hospital. The “CAT”

stands for “Computer Aided Tomography,” a range of techniques widely used outsidemedicine,includinginpaleontology.Oftenfossilsaresoinextricablyboundintheirrockytombs that any attempt to extract themwould destroy them, so scientists use advancedCAT scanning technology to extract those fossils intact, albeit virtually, to reveal thetiniestofdetails.

Inoneexample,everymicroscopichairofadiminutiveflytrappedinamberwasrevealed,andpollengrainsonitsbackshownwithenoughdetailtoidentifythespeciesofplanttheflywaspollinating.The1mm(1/32in)thripswerescannedatfacilityknownasthe “European Synchrotron Radiation Facility” – ESRF – in France, which accelerateselectrons close to the speed of light in a synchrotron the size of a stadium, generatingexceptionallybrightbeamsofx-rays.567Thisisamodernupgradeofthetechniqueusedtorevealthedouble-helixstructureofDNAemployedbyFranklin,WatsonandCrickinthe1950s.Inthosedaystheyusedx-rayfilmsbutinthecaseofourthrips,thescatteredx-raysweremeasuredbyelectronicdetectors.Softwarethencalculatedtheinternalstructureof

thespecimensfromthosemeasurements,revealinga3-dimensionalcomputerimageofthepollinatingflies.Thiswasmorethanavisualjoy-ride,astheresultsshowedthatthethripswerespecializedpollinatorsofcycadplants110millionyearsago.568

Asimilarprojectrevealedhundredsoftinyants,flies,spiders,andevensnailsinopaque lumps of 100 million-year-old French amber 569 and the same technique hasuncovered delicate feeding structures in ammonites that prove they ate plankton, 570

providedclear imagesof the insideofEdiacaranembryos, 571andrevealed thegrowthofteeth in 160,000 year-old humans. 572 It is also ideal for getting inside fossils withoutbreakingthemapart,whichishowweknowthebrainshapeofourpre-humanancestorsSedibaandToumaï.356324

Stadium-sizedsynchrotronfacilitiesaredottedsparselyacrosstheworld,butX-rayscanning is getting smaller, and it is now possible to get comparable images fromequipmentthesizeofalargefridge-freezer.ItwassuchequipmentthatuncoveredsomeoftheoldestmulticellularfossilsfromGabon,71andrevealedatinymitehitchingarideona45million-year-oldspider. 573Neitherdotheimageshavetoremainonscreens.Theycanbe fed into 3D printers tomanufacture enlarged plastic versions, accurate to the tiniestdetail,forpaleontologiststoexamineintheirhands.574

UsingX-raysthiswaycomeswithabonusfeature.Thespectraofthex-raysfromthespecimendiagnose itschemicalmakeup.Thishas, for instance,allowedscientists toinfercolorpatternsonthefeathersoflong-extinctbirds.575

In some cases where the fossils are the faintest of impressions in sandstone, adifferentkindof scanning techniquecanbeused.Anoffshoot fromconstruction surveytechnology,3Dlaser-scanningtrackslasersacrossaspecimen,recordingeverybumpandcrevice. The results are fed into a computer and “rendered” in 3-dimensions. ThistechniquehasrevealedpreviouslyunappreciatedgrowthpatternsinEdiacaranfossils,andhas challenged ideas about feeding and evolutionary relationships between theseenigmaticcreatures.576

SofossilsthatwereunimaginableinDarwin’sday,andtoosmall,fragile,orsubtletostudyeveninthe20thcentury,arenowyieldingtothetechnologiesofthe21stcentury.

Missinglinks–found!Oneoftheearlycriticismsofevolutionwasthelackoffossilsofintermediatelinks

betweenspeciesthatwerethoughttohaveevolvedonefromtheother.Bythebeginningofthe21stcenturymostofthosemissinglinkshaveinfactbeenfoundinthefossilrecord,andit’sworthmentioningsome.

Fish-fromfinstolegsWenowhavefossilsofmostoftheintermediatestagesoftheevolutionfromfish

to tetrapods (4-legged animals) during the Devonian and Carboniferous periods. Theyshow a progression of reducing “fishy” characteristics and increasing land-adaptedcharacteristicsthroughspecimensincludingKenichthys, 142Gogonasus, 144Eusthenopteron,Tinirau, 142Panderichthys,Tiktaalik, 145 146Acanthostega, Ichthyostega, 146andadozenorsootherspecies. 142Thisincludestheprogressivechangefromlobedfinsandafinnedtail,tofeetwithtoesandfinlesstails;butitalsoinvolvesthelossofbonygillcovers,areduction

inthesizeof“postparietalbones”andalengtheningsnout146astheyadoptedair-breathingandchangedfeedinghabits.Theevolutionoftheearisalsorecordedinthisprogressionoffossils.Infishwestartwiththefishspiracle(asmallgillslit)andhyomandibula(abonesupportingthegillcover).InPanderichthysthespiracleiswidenedandthehyomandibulaisshortened.InTiktaalikthespiracleisfurtherwidened,andthesechangescontinueuntilboththespriracleandhyomandibulaformthemiddle-earintetrapods.146

There is a minor wrinkle to resolve in this progression. A recent study ofIchthyostega’shipsshowitcouldn’twalkthewaylatertetrapodsdo577andsoitmusthavebeen another, hitherto undiscovered creature that left its footprints (trace fossils) on thetidalflatsofmiddle-DevonianPoland.147

Forawhileinthe20thCenturyitseemedthattherewasa30millionyeargapinthefossilrecordbetweentheprimitivequasi-fishtetrapodslikeIchthyostega,andclearlyland-adapted,lizard-liketetrapodsintheCarboniferous.Thisbecameknownas“Romer’sGap”afterthescientistwhodescribedit,butsincethenthegaphasprogressivelyshrunk.578 With the discovery of tetrapod fossils in Scotland (including Whatcheeria andPederpes),578thegaphasshrunktoabout10millionyears(about2½monthsinYourLifeasPlanetEarth),anditseemslikelythatfurtherfossilswillbefoundfortheinterveningtimeperiod.

Sothegeneralprogressionfromfishtotetrapods(thencetoreptilesandonwardstomammalsandus)iswelldocumentedandsupportedbythefossilevidence.

DinosaurstobirdsTheevolutionofbirdsfromdinosaursispartlyabouttheevolutionoffeathers,and

partlyaboutthedevelopmentofflight.

As more and more dinosaur fossilshavebeenfoundithasbecomeclearthatfeathers,orthingslikethem,firstappearedveryearlyindinosaurevolution,bytheearlyJurassic.206Infactsomuchhasbeenlearnedinthelast twodecades that the scaly, lizard-likevelociraptors depicted in themovie “JurassicPark”arenowknowntohavebeenfeathered!Eventheleathery-wingedpterosaurswereapparentlycoveredindownyfilaments.207Theextensivefossilrecordofvariouslystubbly,

fuzzy,shaggyorplumeddinosaursshowsageneralprogressionfromfuzzyfilaments,tofilamentsbranchingfromacentralquill,tothetypesoffeathersfoundinmodernbirds.207

Butfeathersalonearenotnecessarilyevidenceforflight.Theyprobablyservedavarietyofpurposesbeforeflight:display 208orinsulationforinstance,anditisonlylateintheirevolutionthattheyshowthestructuresthatenablemodernbirdstofly. 207Scientistshavebeendebatingwhether flightevolvedfromground-dwellingcreatures taking to theair, or from tree-dwelling creaturesgliding to thegroundorother trees.Others say thisisn’tthewaytolookatthisatall–whatmattersishowbirdsdevelopedthe“flightstroke”– the powerful flying wing-beat that keeps a bird in the air. 207 Either way, the debatecontinues,withonesidelinkingthedevelopmentofflighttohowyoungbirds“flap-run”along the ground to climb over obstacles, 210 while the other side refers to climbingadaptationsliketheclawedfingersonthewingsofearlybirds.209

Whichever theory is correct, it seems that flight itself was gained gradually,startingasakindofspeedboost(“flap-running”)210,orasaparachute209tobreakafall.Asflyingdeveloped, earlybirds likeSapeornis,Confuciusornis,Cathayornis, andYanornisused their legsaswell as their armsaswings (likebiplanes) 211but somescientistshavesuggested theseearlybirds’feathersandmuscleswerestill tooweakforflappingflight.210579

Whatmatters here is that a number of transitional fossils have been discoveredshowing the succession fromdinosaur-like animals to creatureswith feathered tails andsmallwingstomorecomprehensivelyfeatheredcreatureswithlegfeathers,andeventuallytounmistakablebirds.Wealsoseethelossoffingersonthewingandthelossofteeth,207212213thedevelopmentofawishbone,abackwards-pointing toe,and theenlargementof thebreastbonetothekeel-likebreastboneofmodernbirds.Thiswasn’tasimplesequenceofreplacement,sinceinChina125millionyearsagothespectrumfromfuzzydinotoactualflying birds (like Jeholorinis)coexisted. 207 They eachmust have had a viable niche forsurvival.

SnakesgetleglessTheevolutionofsnakes–essentiallyleglesslizards–wasapuzzle,butscientists

recentlyuncoveredseveralfossilsofcreaturesintransitionfromlizardtosnake.

Oneexample fromeasternWyoming iscalledConiophisprecedens. 227 This LateCretaceous creature had a snake-like body and a lizard-like head,with a skull thatwasintermediatebetweensnakesandlizards.227Itslizard-likevertebralspinesalsoshowedthatsnakes evolved from burrowing lizards. 227 Another example,Dinilysia patagonica fromArgentinaalsohasaskullwithamixtureoflizardandsnakecharacteristics. 580Oneearlysnake fossil, the Lebanese Eupodophis descouensi, even had vestigial legs that werediscoveredusingsynchrotronX-raytomography.581

Whales–fromlegstofinsEnormousprogresshasbeenmadeover thelastcoupleofdecadesinuncovering

the evolution of whales and dolphins (collectively “cetaceans”). The earliest cetaceanssplit from a common ancestor with hippos around 53 million years ago 582 and fossilexamplesincludeHimalayacetus,andPakicetusfromtheHimalayanslopesofIndiaandPakistan.Himalayacetusstillhadaland-adaptedearandwalkedonlegs,althoughitsteeth

showitatefishandforagedmainlyinfreshwater.582ThenwefindIndohyus,a48million-year-old raccoon-like creature that has the beginnings of the specialized adaptation tounderwaterhearing, anddensebones tohelp itwithneutralbuoyancy. 583 Soon after,wefindAmbulocetus,alarge,furry,web-footedcreature,withthosesameunderwaterhearingadaptations.49

Early whale fossils from the next 10-15 million years are found in marinesedimentsandbegintoshowupoutsidetheIndia-PakistanareaintoNorthAfrica,EuropeandNorthAmerica.Overthistimethefossilsshowthatwhaleancestorsseveredtheirtiesto the landandbecamecompletelyaquaticcreatures. 582For instance, inRodhocetus andGeorgiacetuswe see thepelvis separating from thebackbone, andvertebra therewhichare fused together in humans and most other mammals, becoming separated. Theseadaptationsallowformorepowerfulswimmingbutwouldmakewalkingonlanddifficultto impossible. 582 By 40 million years ago we have completely aquatic cetaceans likeBasilosaurus, 582which resemblesamodernwhale inalmostevery respect, except it stillhasvestigialhindlegs.582

ModernwhalesevolvedattheEocene-Oligoceneboundary(about34millionyearsago) at a time of major changes in ocean circulation and climate. 582 By that time wealreadyseethesplitbetweenbaleenwhales(filterfeeders)andtoothedwhales.582

HumansThere are something like 24 325 359 ancestral human “species” in the fossil record

fromthe7millionyearsbetweenthemostarchaiccreaturesthatfirstseparatedfromthechimpanzeelineage,tomeandyou.Thiswealthofevidenceisnotalwaysstraightforward.It is complicated by often fragmentary fossils and relatively few complete (or nearcomplete)skeletons, 325andit’sclearthatseveral“species”coexistedduringmuchofourevolution. 325 359 The emerging view is that some of those anthropological “species” arereallyjustregionalvariants,orvariantsfromdifferenttimes,orevenjustindividualswholooked different, within far fewer actual biological species (biological species cannotinterbreedwithotherspecies). 584 585Weknow fromgenetics thathumanancestralgroupssometimes interbred fusing separate branches of the evolutionary “tree” back togetheragain, so the pattern of evolution pattern ismore of a braid or a tangled thicket than atree.584

AnythingthatdiesontheAfricanplains,ifnotimmediatelyeatenbythepredatorthatkilledit,isusuallypickedtopiecesbyscavengersfromvulturestohyenastoinsects,beforeitsbonesarebleachedtodustundertheAfricansun.Soweareluckywehavethespecimenswedoforthisrelativelyshortperiodofterrestrialhistory.Theremainswedohave paint a clear picture of the gradual development of human characteristics, likewalking upright on 2 legs, dexterous hands with thumbs, changes in pelvis and skullshape,andincreasesinbrainsize.

Becauseasurprisingnumberofpeoplestillbelievehumanswerecreatedinthelastfewthousandyearslookingaswedonow,517Iwanttowalkyouthroughtheevidencethatthisjustisn’tso,beingopenabouttheuncertaintiesbutalsobeingclearaboutthestrongevidencethatwedohave.

Theape-humansplit

It’s not correct to think of us as descendedfrom chimps, rather we and they had a common ancestor that was neither human norchimpanzee. 326Unfortunately theonly archaic chimp fossils are a handful of teeth fromabout500,000yearsago,326andtheonlyfossilputativegorillaancestorisChororapithecusfrom 10million years ago. 586 That’s a time when we find a diverse collection of apesincludingDryopithecus inEuropeandSivapithecus (theancestoroforangutans) inAsiaand Kenyapithecus, whose teeth, face and elbow begin to resemble our pre-humanancestors,inAfricaandtheMiddleEast.320587

There are fossilsof2 creatures thatmustbe close to that last ancestorwe sharewithchimps.Theyareeachcandidates 325forourancestor,butwedon’tknowforsureiftheyareinadirectlinefromapestous,oronsomeancientsidebranchofourevolution.Interestingly,thedatesofthesefossilsfallsintothetimeperiodshortlyaftertheseparationofchimpsandhumanscalculatedfromgeneticdata,of7-8millionyearsago.588

Sahelanthropusfossilsdatefromaround7to6millionyearsago.Wehaveaskull(nicknamed“Toumaï”),somelowerjawsandsometeeth,butnobonesfromtherestoftheskeleton.325Itsskullandteethdon’tfitwellwithchimps&bonobos,butitistheshapeofits brain (revealed by synchrotron X-ray tomography) and the location of the spinalcolumn at the base of its skull, thatmost clearly places it in the “hominin” line as ourupright-walkingancestor. 324Orrorin is another. There is a fragment of a lower jaw andsomethighbones,13specimensinallfromtheKenyanRiftValley. 325 It lived6millionyearsagoand,giventhefewboneswehave,there’snosurpriseweknowverylittleaboutit.Itappearstohavebeenabletowalkupright,butitsteetharedistinctlyape-like.325

Andthenthere’sArdipithecus,or“Ardi”forshort.

Wehave2skeletonsofArdi,onewhichhaspartoftheskullandgoodpreservationofthehandsandfeet,325andwehavepartsofanestimated35otherindividualslikeArdi.327We also have pieces of skull, teeth and bones from an older and slightly differentArdipithecus.325AllthefossilswerefoundinEthiopiainrocksdatedfromabout6to4.4millionyearsago.Ardi’shandsandfeetareape-like325butherpelvisandskullsuggestshewalkedupright327andherwristsuggestsshecouldn’thaveknuckle-walkedorevenswungfrombranches likechimps. 327Herbrainwasonly slightly larger thanachimp’s,butherfaceandteetharea littlemorehuman.Thetotalityof theevidencehasconvincedmanyscientists thatArdi is our next oldest relative after Toumaï, but this is still amatter of

debate.325327

Archaichumans

Soon after Ardipithecus we find a variety of prehumans in the genusAustralopithecus,includingthefamous“Lucy.”ThereareaskullandbonesfromcavesinSouthAfrica,a lower jawfromTanzania,andanumberofsites inKenya,EthiopiaandChadwhichhaveyielded awell-preserved skull, bits of other skulls,many lower jaws,teeth, and a number of limb bones.Although there are differences between the variousspeciesofAustralopithecus(anamensis,afarensis,africanus,bahrelghazali,prometheus, 589andtheclosely-relatedKenyanthropusplatyops), 325ifwefocuson“Lucy”(aspecimenofafarensis)asbeingrepresentative,wecanseethatinmanywaysshemakessenseasthenextstepontheladderafterArdionthewaytous.

Theactual skeleton thatwasdubbed“Lucy”isofficiallylabeled“AL288,”andisabout25%ofanadultfemaleskeletonfoundin theAfar regionofEthiopia. 325At 3.2millionyearsold she’s from themiddleof theAustralopithecus time range.Her short legs and pelvis shape show shewas capable ofwalkinguprighton2legsbetterthanArdi,327butnotforgreatdistanceslikeus,325andthereareseveralcontemporarytrailsoffossilfootprintsinTanzaniafromaLucy-likeprehumanthatsupportthisidea.325HerbrainsizeissimilartoArdi’s,butherteetharemorehuman.325 Evidence that she still spent significant time climbing in trees comes from a fossiljuvenile found in Dikika, Ethiopia. Specifically, the shoulder blade of that fossil hasimportantape-likeclimbingcharacteristics.335

So Lucy has attributes that are closer to humans than Ardi is. Moreover, it isduring the reignofafricanus thatwebegin to see the first simple stone toolsknownas“Oldowan”tools,353althoughexactlywhomadethemisuncertain.

“Sediba”isthenextsuperstaronthisevolutionarytour.She’smorecloselyrelatedto the southern African Australopithecus africanus than eastern African afarensis thatincludesLucy, 590andshedates toamuchmorerecentageof1.977millionyearsago. 356Thereare2partialSedibaskeletons(possiblymotherandson)thatwerefoundencasedinrock inside caves at Malapa in South Africa 591 along with a leg bone from a third

individual, 590 andmore remainshavebeen found throughX-ray scanningof rocks fromthosesamecaves.592

We have a nearly complete wrist and hand of the adult female, which is partlyAustralopithecus-like(aclimbing-adaptedupperarmandpowerfulforearmsforclimbingtrees 590)andpartlyHomo-like(alongthumbandshortfingersforprecisiongrippingand,possibly,stonetoolproduction).355Sedibahasahuman-likeankle,footarch,andAchillestendon,yetherfoot isalsopartlyape-like,witha thinnerheelandabigger insideanklebonethanhumans. 354Shewalkedawkwardly,withherfootrolledonitsouteredgeratherthanourmodernflat-footedgait, 590 a condition that some runnersandhigh-heelwearerssufferfromtoday.HerbrainisaboutthesamesizeasotherAustralopithecusbrains,butitsshapeshowsitsstructurewasclosertoamodernhumanbrainthanotherAustralopithecusbrains. 356 Interestingly, genetic studies suggest therewas a crucialmutation roughly 2.4millionyears ago thatduplicated thehumanbrain-genecalled “SRGAP2,” 357which fitswell with the emergence of a more human-like brain at this time. Her teeth are alsointermediate betweenAustralopithecus andHomo, 590 rounding out the pretty convincingevidence for Sediba being a transitional species linking Australopithecus to theHomogenusthatincludesus.

ButifSedibaistheancestorofthelaterHomospecies–includingyouandme–then there is aproblemwithher timing.At1.997millionyearsago, she ismore recentthanthefirstputativeHomofossilsataround2.4millionyears–soeithersheisarelicofagroupthatgaverisetoHomomuchearlier,ormaybetheolderHomofossilsaredubious,orabitofboth.ItturnsoutthatthattheoldestreliablespecimensofHomoarearound1.9millionyearsold. 356Theolder assumed-Homo fossils are fragmented or isolated bits ofskullandteeth,whicharetoughtoassigntoanyspecieswithconfidence,orwhosedateissuspect.356SotheupshotisthatthetimingstillmakessenseforSedibatobetransitionalbetweenAustralopithecusandHomo.

Ataround thesame timeasSediba,we find fossilsofweirdly robustprehumanscalled Paranthropus. Fossils of this genus have been recovered from South Africa,Tanzania,KenyaandEthiopia.Thesecreatureshaveawideface,adistinctridgealongthetopoftheirskulls49andmassivejawswithlarge,fast-growingmolarssuggestingtheywereadaptedtoagreatdealofchewing,andanalysisoftheirteethrevealsthattheydidindeedgraze on large quantities of grasses and sedges. 358 Their hip joint is like mostAustralopithecus’suggestingParanthropusdidwalkon2legsbutnotexclusively, 325andcuriouslytheirbrainisslightlylargerthanthatoftheirAustralopithecuscousins.325Somescientists lumpParanthropus in withAustralopithecus, 593 but their anatomical featuresdon’tsurviveintoHomo,suggestingthatParanthropuswasasidebranchofourevolutionthatdiedout.

EcceHomo

Thegroup thatsucceededSedibaandherAustralopithecus relativeswasHomo -whichincludesyouandme(weareHomosapiens,whichisself-congratulatoryLatinfor“wiseman”).

TheHomogenusiscomplicatedandvaried,withupto5differentkindsofHomolivingatthesametimeimmediatelyafterSediba(around2to1.6millionyearsago).They

werehabilis,rudolfensis,ergaster,325andaspecimenlabeled“KNM-ER1802,”whichhasrecently been attributed to a new, unnamed species. 359 They overlapped withParanthropus,soit’sclearthatourancestorsinAfricawereadiverselotatthattime.

These earliest Homospecimenscanbeseenastransitional325betweenAustralopithecusanatomyandthekindofanatomywefindinlaterHomospecies.FossilsforthisgroupcomefromOlduvaiGorgeinTanzania,KoobiForainKenya,aswellasEthiopia,SouthAfricaandMalawi. 325Thefamous“TurkanaBoy”ergasterfossilisanearlycompleteskeletonfromKenya,withoutfeetandfewfingerbones.49Theskullsandskullfragmentstellustheyhadabrainsizeofaround600cubiccentimeters (37-48%ofours),which is a significant stepup from thebrainsofAustralopithecusandParanthropus.49

Recent finds inDmanisi inGeorgia suggest that the variations between habilis,rudolfensis,ergasteranderectusarealljustindividualvariationinearlyHomo,andsocanallbelumpedinwithHomoerectus,butthisremainscontroversial.585325

Homoerectusfossilshavebeenfoundinrocksdatedfromabout1.8millionyearsago to 30,000years ago across the continent ofAfrica, inGeorgia, and as far afield asChinaandIndonesia. 325Thefossilsaremainlyskulls,withsomebonesfromthebodybutfew hands or feet. 325 Inmost respectsHomo erectuswas quite likemodern humans inheight (with much individual variation), proportions, teeth, gait and stature. Thedifferencesareitssubstantial“monobrow,”apointedbackoftheskull,andachin-lessjawthatisthickerthaninmodernhumans. 325Hislimbsalsohadthickerbonesthanours,buthishandswereasdexterousasours.325594Likeushehadaflattishfacewithapointednose,a barrel-shaped ribcage, and his knee and thigh joints show he was just as adapted toupright walking or running as you and me. 49 His smaller teeth than Australopithecusfossilsareprobablytheresultofincreasinguseoftoolstodotheworkformerlydonebyteeth, 49andmicroscopicscratchpatternsonhisteethshowthathehadamorevarieddietthaneitherhabilis,AustralopithecusorParanthropus. 595Toolmarksonbonesandisotopeevidenceshowerectushadageneralizeddietincludingmeat.584

Still,thebrainoferectuswasnotquiteasbigasours(67-95%ofamodernhumanbrain)49whichraisesthequestionofhowintelligenthewas.TurkanaBoy(ergaster)seemsto have a smaller brain 49 than later erectus fossils, so there was some enlargement aserectusevolved. In fact themost recentmutationofourSRGAP2braingenehappenedaround1millionyearsago,inthemiddleoferectus’agerange,beforetheappearanceoflaterHomospeciesliketheNeandertalsandmodernhumans.357

But the fact that by at least 1millionyearsago(probablyearlier)theyhadlearnedtousefire,360suggeststhaterectuswasinfactprettyintelligent.Ifyouhavewatched“Survivorman,”“ManversusWild,”orevenBoyScouts try to make fire without matches, it’s clear that it involves some sophisticatedhigher-level cognitive functions like problem-solving, preparation, learning, andcommunication. The strongest evidence for this comes from theWonderwerk Cave inSouthAfrica, intheformofburnedbonesandplantashfoundinanancientfireplace. 360Fireswere clearly a regular habit rather than chance events, and thequantityofburnedbones strongly suggestserectuswascooking food. 360Other,more tentative evidence forerectus’useoffirecomesfromsitesinEastAfricaupto1.5millionyearsago,andmorerecentdepositsinIsrael.360

Cookingisvitalforbraindevelopment,361becausetherejustaren’tenoughforaginghoursinadayforerectustosustainthenumberofneuronsintheirbrainsonrawfood! 361Cookingsuppliedthecaloriesneededbyerectusandlaterhumans,anditfreed-uptimeforthemtobesocial,maketools,andengageinotheractivitiesratherthangrazingallday. 361There is also tenuous evidence that erectus might possibly have had some form oflanguage,basedontheshapeofthebaseofitsskullandthetimingoflanguage-implyinganatomyfoundinlaterspecies.364

Another sign of erectus intelligence is their distinctive “Acheulean” stone tools.Thesetoolswereclearlymadewithconsiderablecareandskill,andtheybegintoshowupinKenyaandEthiopia1.75millionyearsago, rightat thestartoferectus’daterange. 362Theyareavarietyofspecializedtoolsthatshowrefinementbetweentheearliestandlatestexamples. 362Erectusseemstohavehadtool“factories”forexampleatKonsoinEthiopiaand at Kokiselei andOlorgasailie inKenya, suggesting a degree of social organizationakintomodernhumans.Theanatomyoferectus’waist,elbowandshouldershowsitwas

thefirsthumancapableof throwingweapons 363 -killing fromadistance - so it can’tbecoincidence that at around the same time as the emergence of erectus,we begin to seeextinctionsofanumberoflargecarnivoresinAfrica.365

The fact that erectus spread so quickly across theworld, successfully exploitingenvironments quite different from Africa, shows that erectus was special, and entirelynew.585TheyalsolastedwellintotheeraofHomosapiens,andtheyseemtohaveevolvedthelater“species”ofHomo.

floresiensis(nicknamed“TheHobbit”)isanoddball.Thisdiminutivecreaturewasdiscovered in a cave on the island of Flores in Indonesia andmay have evolved fromerectus. 2 skeletons and up to 100 fossils have been found representing up to 10floresiensisindividuals.325IthasabrainaboutthesizeofanAustralopithecusinanearlyHomo-likeskull,auniquepelvisandlegs,325andinadulthooditisonlythesizeofmodern4-year-oldchild.

Denisovansareanewly-discoveredspeciesfromacaveintheAltaiMountainsinsouthernSiberia.Allwehaveofthemsofarisafingerboneand2teeth,butthebonestillhas good quality DNA in it so scientists were able to sequence the genome of thisDenisovangirlwho livedbetween50,000and30,000yearsago. 370TheDNAconfirmedthatthegirlwasnotNeandertalbutwasinsteadtheproductofadifferentmigrationoutofAfrica,andthatherlineagehadseparatedfromthatofNeandertalsandusabout1millionyearsago,370suggestingerectusasitstentativeancestor.

Fromerectustomodernhumans

Wehaveover80skeletalremainsofantecessor representingsome7 individuals,including bits of skull, teeth, and amandible, fromAtapuerca inSpain.The fossils arefound with stone artefacts and are dated between 1.2 million years and 900,000 yearsold. 596 597 antecessor seems to be an evolved European version of erectus, 598 and someconsiderittheancestorofNeandertalsandmodernhumans,butmostscientistsdonot.325598

heidelbergensis is the earliest of our ancestors to have a brain as large as somemodernhumans,andisperhapsbetterthoughtofas“archaicHomosapiens.”325599Wehavefossils from Europe, the Near East, Africa, China and India dated between 600,000 to100,000yearsago.SomeresearchersclassifyAfricanheidelbergensisasaseparategroupcalled Homo rhodesiensis, and see those as the ancestors of Homo sapiens. 325

heidelbergensis skeletons are more robust than ours, with thick skulls, large eyebrowridges 325 and a forward-projecting face 49. Their leg joints are also large and adapted tolong-distancetravel.325Heidelbergensisevolvedfromerectus,developinganenlargedandmore rounded back of the skull and a broader forehead enclosing a larger brain in theprocess.325Theyprobablyhadspeech,basedonfossilhyoidbonesfromtheirthroats,thenatureofthemiddle-earbones,andthesizeofcertainnervecanals. 364Theyevolvedintomodern humans in Africa, and into Neandertals in Europe, showing a gradation ofcharacteristics over time into those species (for example many fossils at Sima de losHuesos inSpain). 325heidelbergensiswas an accomplished toolmaker andbutcher, usingfire-hardened wooden spears and a variety of stone tools to process animal carcasses.They also hafted stone tips towooden shafts tomake spears, 369 which involves tying asuitably-crafted spear tip to a shaft using some kind of twine and maybe glue. This

suggestshighlevelcognitiveabilityandcommunicationtoteachtheskilltoothers,whichadds more weight to the suggestion they had some form of speech. Evidence for thiscomesfromexcavationsinSouthAfrica,369EnglandandGermany.49

Neandertalswereahighlysuccessfulgroupwhose territory extended acrossEurope and theNearEast and intowesternAsiafrom around 200,000 to around 40,000 years ago, 600 601 overlapping the age range ofmodernhumansinAfricaanderectusinAsia.325ThisagerangeassumestheolderfossilsatSimadelosHuesosareassignedtoheidelbergensisandnotNeandertals,whichisstilldebated. 325598Fossilsofmorethan275individualNeandertalshavebeenfoundfromover70sites,representingastrongfossilrecord.49Theyhaddistinctivelow-browedskullswithlarge eyebrow ridges, big eyes, wide noses and a small-to-non-existent chin, and theirbodies were stocky, robust and muscular, suggesting a very strenuous lifestyle. 325

Neandertal DNA has been recovered from several individuals and their genomessequenced, showing that there was much less genetic diversity in Neandertals than inmodernhumans.325Theydidhavebrainsaslargeasours(actuallyslightlylarger)andhadat least one distinctive culture of tools known as “Mousterian.” 602 They used ochre forpaint, scratched symbols onto bones, 381 and they adorned themselves with feathers andeagle claws. 379 They also constructed 10m (32 feet) wide huts from carefully selectedmammothbones. 381 There is evidence that they had speech based on the shape of theirhyoidboneintheirthroats, 49thepresenceofthespeech-associatedvariantoftheFOXP2geneintheirDNA,theshapeoftheirmiddle-earbones,thesizeofrelevantnervecanals,theircultureandtheirsymbolicbehavior.364

Inotherwords,asidefromappearanceandstature, theywerequitesimilar to theearliest modern humans and not the primitive, brutish creatures so often depicted inpopular culture. In fact they appear to have been about as modern, culturally andbehaviorally,asAnatomaicallyModernHumanswholivedatthesametimeasthem.603

Improvements in carbon dating have now pushed back the dates for the lastNeandertalsfromabout30,000toabout40,000yearsago, 398buttheextentandnatureofmodern-lookinghumans’overlapwithNeandertalsisstilldebated,withthepossibilityofalaterNeandertaloutpostinSouthernSpain.398

Anatomicallymodernhumansemergedaround208,000yearsagoinAfrica,372andthe oldest fossils are fromOmoKinish in Ethiopia. 325 Fossils of ancientHomo sapienshave been discovered on every continent except Antarctica, with the first discovery in1822inaWelshcave, 325andthereareplentyofsapiens fossils ingoodcondition.Whatthey show is a gradation over time from archaicHomo sapiens with robust skulls andmuscularbodies325tothelighter-bonedmodernhumansoftoday.Earlyfossilsalsoshowagradation between heidelbergensis and Anatomically Modern Humans, making theboundaryhardtoset325butsupportingourgradualevolutionfromAfricanheidelbergensis(sometimes called rhodesiensis). It was probably more complicated than that, giveninterbreeding between closely-related but distinct groups, and the high degree ofindividualandregionalvariabilityovertimeinarchaicgroupslikeerectus.

Direct fossil dates formodern humans in Europe are from around 44,000 yearsago399buttechnologyassociatedwithmodernhumansbeginstoshowupinEuropearound45,000 years ago, during a relativelywarm and humid respite in the ice age known asMIS3. 398 That gives a protracted overlapwithNeandertals in Europe of 2,600 to 5,400years.398Interestingly,placeslikeArcy-sur-CureinFranceshowthatNeandertalschangedtheir “Mousterian” culture to a kindof hybrid culture (“Châtelperronian”) oncemodernhumans arrived in the region about 44,000 years ago, and then modern human“Protoaurignacian” and “Aurignacian” artefacts take over completely by about 41,000yearsago,suggesting theextinctionofNeandertalsat that locationaround thatdate. 604 602ThistransitionoccursearlierinsouthernEurope,showingthattheannexingofEuropebymodernhumanswasslowandpiecemeal.604

The traditionally-assumed singleAfrican exodus 60,000 years ago has now been superseded by evidence of olderwanderings, and recognitionofmuchmore complexity to themigrations. 386 584 This is ahotbedofcurrentresearchonallfronts:archeological,fossil,andgenetic;butitsuggestsacomplicatedpatternofdispersal,generallyassociatedwithwarmer,wetterinterglacials. 605Considering that we are talking of a time period in excess of 100,000 years, thiscomplexity is unsurprising, since in just the last 5,000 years we have seen manywholesale populationmigrations throughEurope alone (Indo-Europeans,Hittites, Celts,

Aryans,Greeks,Phoenicians,Persians,Romans,Samartians,Goths,Hunsand Mongolstonameafew).606

So, to sumup, there isawealthof fossil evidence for theemergenceofmodernhumansfromapesthroughaseriesofintermediatespecies.Thisisanexcitingtimeofbigstepsinourknowledgebroughtaboutbyimproveddatingandgenetics,whichIwillcoverin the next section.There are also new fossil finds that haveyet to be spliced into thisstory,likethoserecentlyunearthedintheRisingStarExpeditioninSouthAfrica. 607Butitdoesn’t seem likely that the broad-brush picture of our journey from apes to modernhumanswillchangefundamentally.

24HumanEvolutionFamilyTree.AfterWood,PNAS2010;Pickeringetal,Science2011;Leakeyetal,Nature2012;Krauseetal,Nature2010;Langergraberetal,PNAS2012;Dennisetal,Cell2012;Wilkinsetal,Science2012;Bernaetal,

PNAS2012;Beyeneetal,PNAS2013;Clarke,ThepaleobiologyofAustralopithecus,Springer2013;Herries,etal.Thepaleobiologyof

Australopithecus,Springer2013;Lordkipanidzeetal,Science2013;Parésetal,

J.Arch.Sci2013.

BreadcrumbsofevolutionInthefairytaleofHanselandGretel,thechildrenleaveatrailofbreadcrumbsso

that theymight find theirwayhomeafterbeingabandoned in the forest.This isagoodanalogy for the way clues in our biology today can lead us back, like a trail ofbreadcrumbs,toourorigins.

Thegeneticcodeinallorganismsisbasedona4-letteralphabetrepresentingthe4different bases in DNA, and the patterns of these letters are amenable to computerprocessing,which can find identicalmatches of sequences and even closematches thataren’t identical.Now that the entire genomesofmanyorganismshavebeen sequenced,andwith recent advances in computer processing,we can now parse these vast sets ofgenetic codes formatches between organisms, andwe can rank the closeness of thosematches.

This tool is known as “molecular phylogeny” and provides a new and powerfulway to illuminate the evolutionary tree of life based on how similar creatures’ geneticcodesare.Itisactuallynotrestrictedtogeneticcode–itcanbeappliedtothemakeupandstructureofproteinstoo(the“proteome”-theproteinequivalentofthe“genome”).

LUCAandthedawnoflifeWe can learn a surprising amount about LUCA – the Last Universal Common

AncestorofalllifeonEarth–fromthesestudies.Forinstance,onestudyconcentratedontheproteinsin420organismsspreadacrossthe3domainsoflife:archaea(simplesingle-celledorganisms),bacteria,andeukarya(fungi,plants,animals).Theyrantheiralgorithmstoidentifyproteinssharedacrossall these lifeforms(thereforepresent in theircommonancestor)andfound70familiesofproteins thatallother lifehas inherited insomewayfromLUCA.608

This primordial set of proteins sheds some light on the nature ofLUCA (or the“urancestor” as the authors prefer to call it). It tells us that LUCA had enzymes formetabolismandformakingasimplecellmembraneofglyceroletherandesterlipids,andithadtheabilitytosynthesizeproteins.ItalsohadverysimplegeneticreplicationusingRNAratherthanDNA.608LUCAhadthesimplesttypeofRNAcalled“16SrRNA,”45anditsmetabolismmusthaveusedtheKrebscycleandaprocessknownas“chemiosmosis.”39LUCA did not start life in an especially hot environment. The tolerance for hightemperaturesseemstohaveevolvedafterLUCA,requiringthatwhereverLUCAlived,theenvironmentatthattimewasnotfaroffmodernconditions.609

Ofcoursethesearethecharacteristicsofourlastcommonancestor(asintheonethat could replicate itself andpassongenes to its descendants), andnotnecessarily theearliestprototypeof lifeonEarth.The scenario for theoriginof lifeonourplanetasakindoflivingrockinsubmarinevents39isspeculativebutplausible.41Thissubjectaloneisa book’s-worth, so I can only touch on the highlights here. For a fuller account of theemergenceoflifefromchemistry,seeNickLane’sbook:“LifeAscending.”39

We have to bridge the gap betweentheLUCArevealedbythosemolecularphylogenystudies,andthechemistrythatexistedbefore life in the oxygen-less early Earth. Science has moved on from the famousexperimentsofthe1950swhichsuggestedlightningcouldgeneratethechemicalsoflife.Theywere based on an assumed composition ofEarth’s early atmosphere thatwe nowknow was wrong, and when you repeat the experiments using the more realisticatmosphereyoujustdon’tgetthoseproteinbuildingblockswecallaminoacids. 610Morerecentexperimentssuggestthatyoucangetmoreaminoacidsfromarealisticatmosphereifyouaddotherchemicals,butitjustisn’tnecessarytoinvokelightningnowthatwehavediscoveredmostof life’schemicalbuildingblocksalreadyexist inmeteorites 3441andsowerereadilyavailableon theearlyEarth.Theothersourceofessential lifechemicals isthe natural chemistry of deep ocean vents. Measurements of the chemistry at alkalinesubmarine vents today, such as the “RainbowHydrothermal Field” in theMidAtlanticRidge,havefoundtheyexudehydrogen,methane,andhydrocarbonswithlife-likecarbonchainlengthsofbetween16and29carbonatoms.21

The real problem is getting from those chemical building blocks to complicatedlife-chemicalslikeRNA.RNAisastringofribonucleotides,andeachofthoseismadeupof a sugar, a phosphate group and a base (one of the 4 letters in our genetic code).Scientists have found that if you recreate the chemistry of the earlyEarth oceans, thenribonucleotidesspontaneouslyforminthatsoup, 611buttheyhavenotyetbeenabletogothenextstepandcombinethoseribonucleotidestomakeRNA.OnceyoudohavestrandsofRNA,experimentshaveshowntheyspontaneouslycooperatewitheachotherandgrow.MorecomplexRNAnetworksgrowfasterthansinglemolecules,sothereseemstobeaninherentbiasforRNA-basedchemistrytoevolvethecomplexityoflife.612

Life needs energy and so the earliest life needed some form of metabolism toextractenergyfromitsenvironment.Alllifetodayhasametabolismbasedonacycleofchemicalreactionsknownasthe“KrebsCycle”andaprocessknownas“chemiosmosis.”ThefactthatthesearesharedacrossalllifesuggestsweinheritedthemfromLUCA.39

Archaea todayuse enzymes containing iron, nickel and sulfur to react hydrogenwithCO2toproduceorganicmoleculesand,crucially,energy.Themetalliccoresofthose

enzymes are very similar to naturalminerals in hydrothermal vents. The ventmineralsreacthydrothermalfluidswithCO2 - richwater toproduce“acetyl thioester” (similar tovinegar),whichinturnreactsspontaneouslywithCO2toproduce“pyruvate,”whichkicksoff theKrebsCycle. 39 In otherwords, the natural geochemistry around ancientmineralventsspontaneouslyproducesthechemistrytoarriveattheKrebsCycle.

Chemiosmosis isachainof reactions tocreateachargeacrossamembrane, justliketheelectricchargebetweenthe“+”and“-”terminalsofabattery.Thefactthatalllifeusesprotons(hydrogenions)forchemiosmosis,ratherthananyotherkindofion,seemstoharkback to thenaturalprotongradientathydrothermalvents (like theRainbowvents)wherealkalinefluidsmeetmildlyacidic,carbon-richseawater,producinganaturalchargewhichcanbeexploitedbymetabolism.39613

OfcourselifeonEarthtodayisnottiedtosubmarinevents,soatsomepointourtransitionfromrockchemistrytocellularlifemusthaveinvolvedthedevelopmentofcellmembranes to package the chemicals required for metabolism and replication into anindependentcell.

All cellmembranes are composedofglycerolphosphate phospholipids and this,again,suggestsLUCAhadsuchamembrane. 41Itturnsoutthataniron–nickelphosphidemineral common in meteorites (schreibersite) reacts with sea water in early-Earthconditions(low-oxygen,mildtemperatures)toproducethoseverymembranemolecules.42Other experiments have shown that organicmaterials inmeteorites like theMurchisonmeteoritewillspontaneouslymakelittlebubble-like“cells”inwater,capableofenclosingpackagesofRNAandotherchemicals,andwhichgrowanddividespontaneously.41SothefactthatlifeemergedduringtheheavybombardmentofEarthbyasteroidsisprobablynocoincidence,asschreibersiteprobablyalsosuppliedthephosphorousrequiredtomaketheessentiallifechemicalsRNAandATP.42

Interestingly, archaea and bacteria have their own variants of cell membranes,whichimpliesthatLUCAmayhavehadamixtureofthetwo.41Thisdifferencealsohelpsusunderstandhoweukaryotesincludingusevolved(whichI’llcoverbelow).BythetimeLUCAleftherrockywombshepossessedcomplexlipidmembranesandtheenzymestomakethem.41

Thegreatschism–theseparationofthe3domainsoflifeThe protein study I mentioned above also mapped-out the family tree of life,

supportingthefundamentalsplitinto3domainsearlyon,andsuggestingthatarchaeaareclosertoLUCA(andthereforemoreprimordial)thanbacteria.608Thatstudynotonlygivesus the order of evolution (who begat whom), but it also gives an idea ofwhen thoseorganismsfirstevolved(calibratedusingdatedfossils).This“molecularclock”placesthesplitofeukaryotes(thekindofcellwhichgaverisetomulticellularlifeincludinghumans)frombacteriaatabout2.9billionyearsago.608Thisisaroundthetimewhenabout27%ofallgenefamiliesincreaturesalivetoday,acrossallformsoflife,seemtohaveevolved.609Thesegenefamiliesaremainlyinvolvedinrespiration,andthatmakessensebecausethiswasaroundthebeginningoftheGreatOxidationEvent(themid-lifecrisisofthefirstpartof thisbook)whichwouldhaveallowedorganisms to respireusingoxygen for the firsttime.

Ourgenesinvolvedinreplicationandmakingproteinsareinheritedfromarchaea,yet our genes formetabolism and cellular functions are essentially bacterial. 41 Our cellmembranesarebacterialinorigin,asarethemembranesthatsurroundthecellnucleusandmitochondria (the parts of eukaryote cells that generate energy).Mitochondria, in fact,havetheirownDNAwhichiskeptseparatefromtherestofthecell’sDNAand,unliketherest of our genes, it is only inherited frommothers. This suggests that eukaryote cellsoriginatedasahybridofanarchaealcellandabacterium,orpossiblyfromabacteriumengulfingbothanarchaealcellandanotherbacterium.Eitherway,thebacteriumlivedonasmitochondria, 41 resulting inmoresophisticated,moreenergeticcells (eukaryotes) thatpavedthewayforcomplexlife,andeventuallyus.

BreadcrumbsofmammalevolutionIt’s remarkable that molecular phylogeny, a technique of sleuthing through our

genesandproteins,agreessowellwiththeevolutionaryrelationshipspreviouslyderivedthroughmanydecades of painstakingly detailed fossil comparisons. It complements thefossil record,fillingingapswhere thefossil recordissparse,andthe“molecularclock”canimproveourknowledgeofwhennewgroupsevolved.

Wealsogettwobitesatthegeneticcherry,asthetechniquecanbeusedonbothnuclearDNAandmitochondrialDNA.Forexample,byusingbothkidsofDNA,scientistshavebeenabletoclarifytheevolutionofmammals,showingthatanumberofgroupshadalreadyevolvedbeforethedemiseofthedinosaurs,butthattheyonlydiversifiedinthe20million years after the dinosaur extinction. 614 Another study using both kinds of DNAshowed that a major radiation in whales coincided with the startup of the AntarcticCircumpolarCurrentaround31-34millionyearsago, 615thesameeventthattriggeredtheAntarcticicecapandourspiralintoanice-ageclimate.Thatstudyalsofoundwhaleanddolphindiversificationintheperiodof13-4millionyearsago,whentheTethys,theIndo-Pacific,andPanamaoceanconnectionsclosed.

Scientistsarenowborrowingtechniquesfrommolecularphylogenyandapplyingtheminsteadtophysicalcharacteristics(“phenomes”)oflivingandfossilcreatures.Thistechniquewasusedtotracktheemergenceofthefirstplacentalmammals(ourancestors)toverysoonaftertheend-Cretaceousextinction.252

EvolutioninhumansrevealedbygeneticsSequencing human genomes has led to an explosion of information about our

evolution. Modern human genomes across the globe have shed light on patterns ofancestry,butancientgenomesfromarchaichumanshavealsobeensequenced,includingseveralNeandertals,aDenisovan,anda400,000yearoldheidelbergensis.AsIwritethis,newgeneticdataareemergingeveryfewmonthswithfreshinsightsonourorigins.

Itisveryclearfromourgenomesthatabout2%388ofourgenesareNeandertal(forEuropeans,Asians,616andNorthAfricans 380),andthatsome20%ofNeandertalgenesarescattered throughout different human populations today. 617 Melanesians, AboriginalAustralians,Polynesians,Fijians,andsomeIndonesianshavealsoinheritedabout4%oftheirgenesfromDenisovans,whointurninheritedgenesfromanolder,non-Neandertalgroup. 390 389 618 The genes of aheidelbergensis person from Spain turned out to bemoreclosely related toDenisovans thanNeandertals, despite its quasi-Neandertal skeleton. 619

ThefactthatsomeAsianpopulationshaveDenisovanDNAbutothers(likemainlandEastAsians,westernIndonesiansandsomeothergroups)donot,meansthatanyinterbreedingmusthaveoccurredinSoutheastAsia.ThissuggeststhattheenigmaticDenisovanshadavast range fromtropical southeasternAsiaall theway to frozenSiberia. 390Whatall thistellsus is that themigrationsand interbreedingsofHomopopulationsweremuchmorecomplex–andinteresting-thanwepreviouslyassumed.

Neither were we a “finalproduct” of evolutionwhenwe emerged inAfrica 208,000 years ago.We continued tochangeinresponsetotheuniqueenvironmentalchallengeswefacedaswespreadacrossdifferentregionsoftheglobe.

GenomestudiestellusEastAsianshaveevolvedsubtlydifferentgenesconcernedwithhearingandvision,bloodpressureandmetabolism,andWestAfricanshaveevolvedspecializedgenesfordisease immunity. 620 InEuropeandAfrica, life alongsidedomesticcattleresultedinevolutionofadultlactosetolerance,621somethingthatisswitchedoffafterweaninginotherpopulationsandalllactose-intolerantpeople.AndTibetans,wholiveathigh altitude, have evolved differences from closely-related Han Chinese in genesassociatedwithbloodoxygenlevels.621

Genetics tells us that the San people of southern Africa (sometimes called“Bushmen”)becameadistinctpopulationveryearlyonataround130,000yearsago,longbeforeEurasiansbegantodivergefromAfricanpopulationsbetween64,000and38,000years ago. 622 They also tell us that America was initially populated by several distinctmigrations fromSiberia, and show how those populations spread south along the coastintoSouthAmerica,andeastoftheRockiesfromAlaska.623624

Thesestudiesalsotellusthatthereisfarmoreindividualgeneticvariationwithinour population groups than between them. 622 There is much active research on ourgenomeswhichwillshednewlightonourrecentanddistantpast.Molecularclockswillberefined625andnodoubtrevisionswilloccur,butsofarthegeneticevidencehasfleshed-outthestoryofourevolutionthatwaspreviouslyonlyrevealedbybonesandteeth.

SharedanatomyIt’s not just our genes and proteins that bearwitness to our evolution.Our very

fleshandbonescontainechoesofourancestors.

Takeyourlegsandarms,forinstance,andcomparethemwiththelimbsofall4-limbedcreaturessinceourdivergencefromfish.Weallhavethesamebasicstructureofasingleupperbone inarmsand legs, twobonesforour lower limbs,and thenaseriesofsmaller(wristorfoot)bonesbeforeendingwith5toesorfingers.Thisistrueforhumans,whales,dogs,cats,horses,bats,lizardsandevenfrogs.

Therearemanycaseswhereyoucantracechanginganatomythroughaseriesofdescendentspecies.Wecanseethesophisticatedmammalianearbeginitsevolutionfromthe reptile ear.Smallbonesat thebackof the jaw in reptilesbegan to separate inearlyCretaceous “utriconodont”mammals, before eventually becoming the tiny bones of ourmiddleearinlatermammals. 626Jawsareanotherexample.Aftersomeinitialvariationinvery early fish, their shape settled on a design that stayed through fish and then the 4-leggedcreaturesthatmovedontoland.627

EvolvingembryosOf all the 4-limbed animals, birds would appear to be the exception to the 5-

fingeredrule,becausetheyhavelost2ofthose5digits.Buttheydohavethosefingersasembryoswhentheyarestillintheireggs.Watchingabirdembryodevelopshowsthatitstartswith5digitsbutthenthe“extra”digitsstopdevelopinganddegeneratelongbeforethebirdhatches,628sotheyaren’tanexceptionafterall.

Infact thewayembryosdevelopoffersstillmoreclues to theoriginsofspecies.Vertebrate embryos, including humans, all go through a similar development sequencewhich reveals vestiges of the ancient creatures we evolved from. At an early stage ofdevelopmentweallhavea tail,gillslits,andgillcover.Astheembryodevelopsfurthersomeoftheseremainvestigial(likeourtail)andsomearerepurposed,likethegillswhichbecomeourparathyroidglands.629

BottomLine:Evolutionisafactacceptedbypracticallyalltheworld’sscientists,becauseofthe

overwhelming body of evidence that proves it. The evidence for evolution and for themyriad creatures that existed long before us is writ large and tiny in rocks across 3.7billionyears of our planet’s history.More than that, it iswritten in theverybiologyoflivingthingstoday–theirgenes,theiranatomyandtheirbiochemistry.Evolutionthroughselection has been observed in the lab, outside the lab and in the history of domesticanimals.Butmorethanthat,weareincreasinglyunderstandinghowtheevolutionoflifehasbeenanintegralpartofourplanet’sevolution.

4.TheClocksInRocksSandsoftime

Throughout this book I have referred to events at specific times in the past -thousands, millions and even billions of years ago. Being able to nail events down tospecifictimes,andsoworkouttheorderofevents,ispivotalforworkingoutthehistoryofourplanet.HowweknowthosedatesisthereforeacornerstoneofourunderstandingofEarth’spast.

Fromscripturetonature

Inthe17thcentury,BishopUssherbasedhiscalculationoftheageoftheEarthonaliteralreadingoftheBible.HedecidedthattheEarthwascreatedin4,004BC,630andthisdatehasbecomea fixation forYoungEarthCreationists today.But thebishophadonlyancient texts and family trees to go on, unconstrained by scientific measurements andobservationsfromthenaturalworld.

In the 18th and 19th Centuries, as geologymoved from a collecting hobby to ascience,observationsofnaturewerebroughttobearonthequestion.ItwasJamesHuttonwho first articulated the idea that itmust have taken eons to produce the landscapes oftoday.HuttonwaspartoftheEnlightenmentsocietythatflourishedinEdinburghaftertheJacobiterebellion.Hetravelledandmadeobservationsoftherockseverywherehewent,which convinced him that erosion leads to sediments that form rock.He also observedmarinesedimentshighupinmountains,anddeducedthatthemountainsmusthavebeenraisedfromthebottomofthesea.

Afterthefirsteditionofhis“TheoryoftheEarth”waspublished,hevisitedSiccarPoint, about 50km (30mi) east ofEdinburgh,wherehe saw red sandstone strata drapedover upended layers of gray sandstone. That crystallized his understanding of the deeptimerequiredtoexplainwhathesaw:eonsmusthavepassedfromtheinitialformationofthegreysandstonelayers,tothetimeneededtopetrifyandupendthem,thenlatertodrapetheminfreshredsands,forthosealsotopetrify,andthenthewholeedificetoberaised,erodedandexposedbeforehim.AllthatmusthavetakenfarlongerthanUssher’s6,000years:

“It is only in science that any question concerning the origin and end ofthings is formed; and it is in science only that the resolution of thosequestionsis tobeattained.Thenaturaloperationsofthisglobe,bywhichthesizeandshapeofourlandarechanged,aresoslowastobealtogetherimperceptible to men who are employed in pursuing the variousoccupationsoflifeandliterature.”631

Smith’spuzzleWilliamSmithcamealittleafterHutton.Fromhisworkbuildingcanalsandcoal

mines, Smith deduced that sediments laid down at the same time had the samecharacteristics and, crucially, the same assemblage of fossils. 632 Armed with thisknowledge,Smithwasabletotracestratafromoneminetoanother,andfromonecounty

to another, fitting them together like pieces of a puzzle.He laboredmanyyears single-handedly, mapping strata all across Britain, until he had produced the world’s firstgeological map in 1815. 632 Far more than an impressively-colored piece of paper, thissummarized the entiregeological historyofBritain in a single, graphical document.Bystringingtogethermanyindividualsuccessionsofstrataintheirproperorderofformation,hehadmultipliedHutton’seons.

Geologists follow Smith’s mapping technique to this day, and still use fossilassemblagestocorrelateanddatestrata(althoughthesedaysthosedatesarecalibratedtoradiometric dates which I will come to below). But it was a while before Smith’sachievementwasunderstoodforitstruesignificance,andhewastreatedshamefullyintheinterim–ataledeftlytoldbySimonWinchesterinhisbook“TheMapThatChangedtheWorld.”632

SoSmith had succeeded in defining the relative dates of strata (this layer camebefore that layer, and so on), and Hutton had shown us that the planet’s history wasplayed-outacrosstheeonsofdeeptime.Hutton’sinsightsinfluencedscientistsafterhim,fromSirCharlesLyell(whovisitedtheJogginsfossilforestinNovaScotia,inthe“Proofof Life” chapter earlier) to Darwin. Darwin himself observed the slow rate ofaccumulationanderosionofsediments inBritish landscapes,andmultiplied those timesbythevastthicknessesofsandsandclaysexposedincliffs,toarriveattimescalesinthehundredsofmillionsofyears.515

Yetabsolute dates (exactly howmanymillion years old a particular stratum is)remainedelusive.So,attheendofthe19thandbeginningofthe20thcenturies,scientiststurnedtophysicsforanswers.

Tostartwiththeywerewayoff.Inthe1860sLordKelvin attempted to calculate the age of the Earth by assuming it had cooled from amolten state. He came up with between 20 million and 400 million years. 630 He wasprofoundlywrongbecauseheguessedameltingpointforrockwhichwaswaytoohigh,hisvalueforhowfasttheEarthconductsheatwaswrong,andcruciallyhedidnotknowabout the heat added by natural radioactive decay.He also did not factor in other heatsourceslikeasteroidimpactsandtheheatgeneratedbytheplanet’score.TheheatbalanceofourplanetismuchmorecomplexthanimaginedbyKelvin, 630whichiswhyhisageoftheEarthisfartooyoung.

Fortunately,inthe20thCentury,physicistslikeRutherfordandBoltwoodbegantomeasure radioactivedecay, delivering a new technique formeasuring accurate, absolute

datesbyradiometricdating.

Radiation-freeproofsofanoldEarthSince radiometric dating is dismissed by Young Earth Creationists, it’s worth

touching on a fewof thewayswe know that theEarth is very old, evenwithout usingradiometricdating.

Measuredcontinentaldrift

If we start with modern GPS measurements of the speed of movement ofcontinents today,wecancalculatehowmanyyearsback in timeyouhave togo to joincontinents likeAfrica andSouthAmerica back together.Using theGPS-measured driftrate today of around 26mm (1in) per year, 451 multiplied by the distance between thecontinentsofroughly5,500km(3,400mi),ittakesyoubackroughly200millionyearstojointhecontinentstogetheragain.Loandbehold,thatputsyouattheendoftheTriassicperiod,whichiswhengeologicalandradiometricdatingevidencedoes,indeed,havethebeginning of the opening of the Atlantic. This crude calculation illustrates that we aredealinginhundredsofmillionsofyears,notthousands.

And a similar exercise can be done with Hawaii. The “Big Island” of Hawaiicurrently sits over amantle hot spot that has remained fairly fixed as the pacific platedriftedoverit,likeablow-torch,meltingastringofvolcanoesintotheseafloortocreatetheHawaiian-Emperorseamountchain.GPSdatashowtheplate ismovingatabout70-90mm (2¾ - 3½ in) per year. 633 451 Thatmeans theMidway Islands (around 2400 km(1500mi) away) should be roughly 28 million years old. Potassium-argon radiometricdatesfortheMidwayIslandsarearound27.7Millionyearsold.634

Erosionrates

Measurementshavebeenmadeoferosionratesacrosstheworld’sriverbasins,andas you might imagine, they vary. They range from averages around 0.5 mm per year(1/64in)acrossmostof theworld’srivers toaround5mmperyear(¼in) intectonicallyactivemountainranges,toafewcmperyearinvolcanicareas.488

Thepointof this isyouendupwithaminimum ofmillionsofyears required toerodethemiles-deeprivervalleysthroughtheworld’sgreatmountainchains.Andthat’sifyou suspend geological reality by ignoring tectonics or isostacy (the unloading-a-boateffect), which just prolong that time in reality. We do know for certain that they areeroding (from measurements of sediment loads in rivers, for instance), and we canmeasurewithGPS the rateof their tectonicuplift (MountEveresthasbeen risingabout4mm(about1/8in)ayear486).Onceagain,theseareverysimplisticcalculations,buttheyforceustoaccepttimescalesofmillionsofyearstoexplainsuchtopography.

Treerings

Everybodyknowsthateachringinthewoodofatreerepresents1annualgrowthcycle, socounting thegrowth ringsgivesyou theageof the tree.Unfortunately it’snotquite that simple because some trees havemissingor false rings, so in reality countingringsfromasingletreedoesn’talwaysyieldveryaccuratedates.635Butyoucangetaroundthe idiosyncrasies of individual trees by counting the rings in many trees and cross-matching their thick-thin ring patterns like barcodes. 635 In this way an accurate master

chronologycanbebuiltup,startingwithlivingtreesandgoingbackmanythousandsofyears.

Oak tree rings in Germany can betracedbackover9,000years,andacombinedSwiss-Germanpinetreerecordgoesbackmorethan12,000years. 636TheGermanoakchronologymatchesanIrishoakchronologyasfarbackasthe3rdmillenniumBC.TheNorthAmericanBristleconePinerecordgoesbackover7,000years636andseverallivingtreesofthisspecieshavebeenfoundtobeover2,000yearsold. 637 Thousandsoftree-ringchronologiesfromacrosstheworldandmanytree types have beenmeasured, and they can all be cross-referenced, albeitwithminorerror in some cases (there is apparently a 37 year error comparing the bristlecone pinechronologywiththeGermanpinechronology–orabout0.5%).636

Thisisfurther,unassailableevidencethattheEarthismucholderthanUssher’ssixmillennia.

Coralgrowthbands

Coralshaveannualgrowthbandsmuchlike trees,and theyareused toconstructchronologies in much the same way as tree rings. Cores drilled from coral reefs arephotographed under ultraviolet light which makes the summer (tropical rainy season)growth bands glow. 638 These bands are traced, compiled, and cross-matchedwith othercorestoproduceayear-by-yearcount.Inmanycasesvariationsinstableisotopes(nottheradioactivekind)areloggedalongthecoralrecordtohelpconstrainthatannualcount. 639Corals grow in more complicated shapes than trees, so it’s harder to constructchronologiesasaccurateasthetreeringrecord,butdespitethat,scientistshavebeenableto trace a year-by-year record that extends back around 130,000 years, with just a 2%error.639

Lakebeds

Some lakes have been quietly silting-up for thousands of years. Each year insummertheylaydownalightercoloredlayeroffinesand,andeachwinterathinfilmofblackclaydrapesthelakebed.Drill-coresthroughtheselakebedsrevealblackandwhitestripescalled“varves”–exactlyoneforeachyear,justliketreerings.Countingthestripes

givesyouthenumberofyearsittooktomakethesedimentinthatlakebed.

Lake varves record the passage of up to 30,000 years in a single lake (LakeSuigetsu, Japan) and they occur all across the globe. 640Many records begin today andextendcontinuouslyback,butothersbegininthepastsotheyhavetobematchedbyothermeans to an absolute date. Matching different sequences from many lakes (usingradiometricdatingorvolcanicashdepositsormagneticdata)createsarobustyear-by-yeartimeframegoingbacktensofthousandsofyears,witherrorscalculatedtobejust1-3%.640

IceCores

Icecoresarecylindersoficeboredfromicecapsandglaciersbyspecialdrillingrigs.TheyaremainlyfromthedeepicecapsofGreenlandandAntarctica,andcomprisearecordofsnowsformillenniatogetherwithanytraceimpuritiesthatwereintheairatthetime.The younger part of the record can be annually counted just like tree rings, coralbands and varves, but for the longer timescales we are interested in here, visual layercountingaloneisinsufficient. 639Thedifficultiesaremany;includingicecompressionanddiffusionthatblurs theseasonaldistinctions. 641Nevertheless,detailedchemicalsamplingalongtheicecoresyieldswigglesintheratiosofisotopesofoxygenandhydrogen(again,these are nothing to do with the isotopes used for radiometric dating) which reflecttemperaturevariationsat the time the snowwas falling.These showadistinctup-downwavecorrespondingtosummer-wintercycles–givingatimescaleof1yearperwiggle.Tosupplement the isotopewiggles, scientists also use analyses of dust levels (which varyseasonally)641andashfromvolcaniceruptionstocross-matchdifferentcores.639

Theupshotofallthatisthatwehaveicecorerecordsthatgoback800,000years.642

Cavecrud

We’reallfamiliarwiththespikystalactitesandstalagmitesthatbuildupincaves,ifonlyfrommovies.Manycontainannually-layereddepositswhichgobackevenfurtherintimethanicecores.Theirannualbandingisrelatedtoseasonalvariationsintheamountofwaterpercolatingintothecave,andseveralcanbetracedbackhundredsofthousandsofyears(acaveinBorneogoesback600,000years,andthe“Soreq”and“Peqiin”Cavesin Israel go back 250,000 years). Most of those older dates are based on radiometricdating, but a few younger caves have had stalagmite layers counted going back 6,500years(the“ColdAirCave”inSouthAfrica)andothersabouthalf thatage(egCarlsbadCavern,USA:2800years;northwesternScotland:3600years).643

Someoftheselayer-counted“speleothem”recordshavealsobeenradiometricallydated(egtheCarlsbadCavernsinNewMexico).Theradiometricdatesdoindeedmatchthelayercounteddates,validatingtheradiometrictechniqueatleastoverthetimescaleofafewthousandyears.643

25Annually-layeredrock‘clocks:’coralbands,lakevarvesandastalagmitelayers.Photosby:-corals:RobDunbar,RiceUniversity/NOAAPaleoclimatology;Varves:TakeshiNakagawa,suigetsu.org;Stalactite:Dr.HonglinXiao,ElonCollege/DepartmentofGeology,

UniversityofGeorgia

PhysicsoftheSun

Evidencetaken,quiteliterally,fromoutofthisworldalsocorroboratesanancientoriginfortheSolarSystem.TheageoftheSuncanbecomputedfromtheproportionofhydrogen to helium in its core, because in a “Main Sequence” star like our Sun thishydrogenisconvertedtoheliumataspecificrate.Thetrick,then,istofindoutwhatthoseproportions are. It turns out that the Sun, which is basically a giant hydrogen bomb,resonates with its nuclear reactions. The frequency of that vibration is very stronglyrelatedtotheproportionofhydrogentoheliuminitscore.Scientistsdetectthevibrationsusing Sun-observing satellites like NASA’s “SOHO” and “SDO” 644 and those“heleoseismic”measurementsplace thebirthof theSunbetween4.6billionyears 645and4:57billionyears 646–remarkablyclosetoradiometricdatesofmeteoriteswhichareusedtodatethebirthoftheSolarSystem(includingEarth).

AnoldEarthevenwithoutradiometricdatingThe last fewparagraphs addup to a number of completely independent lines of

evidencefor themillionsofyearsofEarth’sexistence.Whencompared theyagreewitheach other, recording the same climate cycles. So evenwithout recourse to radiometric

dating,thereisamountain(punintended)ofevidenceforthelongtimeframeoftheplanetconventionally understood by science. They also agree with, and therefore validate,radiometricdates.

Radioactiverock-clocksRadiometric dating, using radioactive isotopes, allows us to read the natural

‘clocks’inrocks.Unlikemostofthemethodsabove,radiometricdatingisn’tlimitedtoafew specific situations (like trees or lake bottoms) or to the last few hundred thousandyears. It can be used across the span of geological time and on many different rocks,makingitindispensableforanypracticalunderstandingofourdeeppast.

Itonlybecamepossiblewiththeadvancesinatomicphysicsinthe20thcentury,andthedevelopmentofanexquisitelysensitivetechniqueformeasuringminutequantitiesofchemicalelementsinasample-themassspectrometer.

AtomicracetrackThemass spectrometer sounds complicated but in essence it’s very simple. It’s

basicallyaracetrackforatoms.

Imagine2racecars,onelightweight,theotherheavilyoverloaded,racingaroundabend.Thelightercarmakesthebend,buttheheavycarsmashesintothebarrier.Themassspectrometer works just the same way – it separates atoms that weigh differently, nomatterhowslightthatdifference.Asampleisheatedtothepointthatitsatomslosetheircoveringofelectrons,leavingjustatomicnuclei.Thenakednucleiarethenacceleratedbymagnets along a curved track and, just like our imaginary race cars, the lighter atomsmake it around the bend but the heavier ones slam into the wall. Sensors record thelocations of crashes, and a little calculation gives the weights of the atoms and theirquantities.

There are many different kinds of mass spectrometers, including some that arecoupled to particle accelerators (smaller cousins of the famousLargeHadronCollider),andsomecoupledtomicroscopesandlaserssothattinysamplescanbevaporizedfromasinglecrystalor individualgrowth layers ina tooth.But theyallwork inessentially thesameway,countingtheamountsofatomsofdifferentweights.

Sowhatdoesatomicweighthavetodowithreadingthe‘clocks’inrocks?

26Atomicracetrack:howamassspectrometerdetectsatomsofdifferentmass.

Moldycheese

Radiometricdatingworkslikemoldycheese.

Ifyouleavecheeseoutoftherefrigerator,itgoesmoldy.Thelongeryouleaveit,themoldier itgets.Withtheexceptionofcollegestudents,wetendtothrowthatcheeseawayassoonasthemoldappears,butifyouweretoleaveitoutforafewweeksitwouldeventuallydecaycompletely.

It’s basically the same with radiometric dating. Radioactive elementsspontaneouslydecaytootherelementsovertime.Theatomsofthe“parent”elementandtheatomsofthe“daughter”elementhavedifferentweights,sothemassspectrometertellsustherelativeproportionsofeach.Weusetheratioofparent(freshcheese)atomstothedaughter(moldy)atoms,andtheratetheyconvert(their“half-life”),togetadate.

“Half-life”justmeansthetimeittakesforhalfoftheoriginalparenttodecay.Ifanelement’shalf-life is, say,100years thenafter100yearshalfof theparentwill remain,

after200yearshalfofthathalfwillremain(¼)andafter300years⅛isleft,andsooninanever-decreasingcurve.

Skeptics of radiometric dating claim that there is no experimental proof of thetechnique (because humans haven’t been around long enough) making the dates areunreliable. So it’s probably a good idea to explainwhy such criticism ismisguided bycoveringsomeofthewaysthatradiometricdatinghasbeenprovenvalid.

Timesignal–checkingtheaccuracyoftheradiometricclockLabproof

Firstly, radiometric dating has been proven in the lab. As scientists began tounderstand radioactivityat thedawnof the20th century, they realized that eachkindofradioactiveisotopedecayedatitsownsetrate–its“half-life.”Somedecayfast,andothersdecay over billions of years, but experiments and theory showed they all decay in thesameprecisely-predictableway.Infactitisa“law”ofphysicsthathasbeencheckedandrecheckedmany timesover the last60yearsorso. 437 Still,weshouldprove itsvaliditymuchfurtherintothepastthanthat.

Cross-checkingwithotherkindsofrock‘clocks’

Agoodwaytocheckradiometricdatesistocomparethemwithotherreliablerock‘clocks’ like the tree rings, corals, cavedeposits andvarves Imentionedearlier. Thesehave been used to calibrate radiocarbon and uranium dating to 50,000 years ago. 636 647UraniumdateshavealsobeencheckedagainstannuallayersinstalagmitesfromcavesliketheCarlsbadCavern,USA(2800years)andcavesinnorthernIndia(about700years). 643Carbon-14, lead-210, Cesium-137, lead-206/207 and Plutonium-239+240 datingtechniqueshavebeencross-matchedtovarvesgoingback15,000years.640

So we have independent validation of radiometric dating going back tens ofthousandsofyears,butwewanttoproveitsreliabilitybackintothemillionsandbillionsofyearsinthelifetimeofEarth.

Orbitaltuningandseafloorspreading

Toaddressthis,scientistsmatcheddatesofsea-floorlavas(usingpotassium-argondates) with their expected age based on sea-floor spreading rates, going back severalmillionyears.Theyalsousedthepatternofmagneticpolereversalspreservedinterrestrialbasalt lavas. They successfully compared the radiometric dates of the basalts with thepatternsofmagneticreversalsgoingbackaroundamillionyears.437

Tovalidateradiometricdatesfurtherintothepast,theymadeuseofEarth’sorbitalwobbles.Theseorbitalwobbles,which Idescribedas themajorcauseof fluctuations intheiceages,areknownveryaccuratelyandcanbeprojectedwithprecisionintothepast.Theycanbecorrelatedwithtemperaturevariationsinoceanwaterrevealedtousthroughoxygenisotopesinourmicroscopicfossilfriends, the“forams.”Inthisway,radiometricdatescanbecheckedagainstorbitalcycles–andtheymatchallthewaybackintotheageofthedinosaurs.437648

Ancientnuclearreactor

Beyond thatwemust reach into the deep past, to theweird natural reactor that

occurred inOklo,Gabon, around 2 billion years ago. First off, scientists can tell that anuclearreactorwasoperatingtherebecausetheprofileofelementsinthegroundcloselymatches that in waste from modern nuclear reactors, and the isotopic abundances ofseveral elements can only be explained by nuclear fission reactions (isotopes ofneodymium,Plutonium-239anduranium-235).Thisrelatestoradiometricdatingbecausethe proportions of those isotopes today are exactly as predicted by a constant nucleardecayrateoperatingover the last2billionyears. Inotherwords, the rateof radioactivedecayhasbeenconstant,andthereforereliablefordating,atleastsincethetimeofOklo,around2billionyearsago.437

Immunityfromtemperatureandpressurechanges

Butwhat ifour‘clock’speedsuporslowsinextremeconditionsdeepinsidetheEarth? It turns out that the dates are immune to extreme temperatures and pressuresbecause the radioactive decay happens in the atomic nucleus, shielded from chemicalreactions, which only involve electrons. Lab experiments have put radioactive samplesthroughbombexplosions,frozenthemtonearabsolutezero,takenthemtomountaintopsanddownthedeepestmines,spunthemincentrifuges,friedthemwithelectricityandputtheminstrongmagneticfields,alltonoavail.Theirdecayconstantremainedjustthat–constant.630

Thereisoneexception–akindofdecaycalled“electroncapture.”ExperimentsonBeryllium-7showedthatelectroncapturecanbeaffectedbyextremepressures–thosefoundverydeepinthemantle.Isotopesofrheniumanddysprosiumalsochangetheirhalf-livesiftheirelectronsareremoved,buttheconditionsthatwouldmakethathappenexistonlyintheinteriorsofstars!630InrealityBeryllium-7isnotusedfordating,andtherelated

potassium-Argon (K-Ar) dating that should theoretically be affected, is not used underthoseconditionsbecauserocksaremoltenatthoseextremetemperaturesandpressures(sotheradiometricclockisnever‘set’).Nevertheless,asmallerrorinthedecayconstantforK-Ardateshasrecentlybeencorrected,whichI’llexplainbelow.648

In the final analysis, there are dozens of different radioactive isotopes used fordating,whoeachhavedifferentchemistriesandgetincorporatedintodifferentrocktypes,andyettheyallproduceconsistentdatescompared toeachother. 437Moreover,dating iscarriedout routinely inhundredsof labs around theworldwith consistent results. 437Soradiometric dating has been proven in the lab and validated by cross-checking with arange of other rock ‘clocks,’ and between different radioactive isotopes. Radiometricdatingisconclusivelyvalid.

ErrorsanduncertaintiesThis doesn’tmean that every radiometric age is infallible.As the geologist and

author Brent Dalrymple put it: failures may be due to: “laboratory errors (mistakeshappen),unrecognizedgeologicfactors(naturesometimesfoolsus),ormisapplicationoftechniques (no one is perfect).” 630 To mitigate these risks, scientists analyze multiplesamples, use several independent techniques, and check the geology around the samplelocation.630

As better instruments become available and measurements more precise, somefine-tuning inevitably occurs, and occasionally new techniques supersede old ones. Forinstance,asmallerrorinuraniumdatingwasrecentlyuncoveredthataffectsdatesofthemost ancient rocks andmeteorites, enough to reduce ages at thebeginningof theSolarSystemby700,000years 649–butstillonlyatinyerror(0.015%orabout5daysinYourLifeasPlanetEarth). Theelectron-capturedecayconstant forpotassium-Argon (K-Ar)dates is another example.Usingvalues adopted in the1960s and70s,K-Ar and argon-argon (Ar-Ar) dateswere generally younger 437 by around 2% compared to other datingtechniques. 648TheK-Ardecayconstanthassincebeencorrected,andnewerresults(since2010)areasaccurateasuranium-leaddating.648

Italsodoesn’tmean thedatesareexact to theday.Adateof, say2.024±0.062millionyearsdoesn’tmeanAprilthe24th,intheyear2,021,987BC.Itjustmeansitismostlikely 2,024,000 years old but almost as likely to be any date 62,000 years younger orolder than then. It’s a spread of probability, with the quoted date in themiddle of thespreadthemostlikely.

Settingthestopwatch

Radiometric ‘clocks’ in rocks are reallymore likestopwatches. When rocks are forming there is a point when their chemistry stopschanging, like when some magma cools into basalt. At the point that the rock ischemicallystatic,the‘stopwatch’isset.Andifnothingdisturbsit,thatnaturalstopwatchkeepstickinguntilscientistsmeasureitintheirmassspectrometer,likeatimerinaswimracethatkeepsgoinguntiltheswimmertouchestheplateattheendoftherace.

Sometimestherockcangothroughchangesafteritsformation,likegettingcaughtup in a continental collision. If the temperature and pressure rise sufficiently, they canmakethemineralschemicallymobileagain,resettingthestopwatch.Scientistscanexploitthisbyusingdifferentisotopesthataremoreorlesssusceptibletothosechanges,tolearnwhatchangesarockhasbeenthroughinitslife,andwhentheyoccurred.

Spottingafaultystopwatch

Idon’twanttogetboggeddowninmathandgraphshere,buttheimportantthingtoknowisthatscientistsmeasurenotjusttheradioactiveparentanddaughterisotopes,butalso the quantity of a third element. This removes the need to know howmuch of theoriginal daughter was present when the rock formed. It alsomakes the technique self-checking:iftheradioactiveclockhasbeencompromiseditwillshowupasscatterinthedata points, so scientistswould know not to rely on those dates. Even better, it allowsscientists to calculate the initial quantity of the daughter isotope as one of the results,whichcanbeusefulforotherpurposes.630

Choosingtherightclock–likechoosingtherightrulerBecause there are such a variety of environments that rocks form in (sediments,

reefs,volcanoes,mountainbelts,andsoon),scientistsmustpicktheappropriate‘clocks’thatformnaturallywiththoserocks.Theymustalsochoosedifferentradioactive‘clocks’fordifferenttimeranges.It’sjustlikechoosingtherightkindofrulerformeasuring–youcanchooseanythingfrommicrometerstotapemeasures,butyouwouldneveruseatapemeasure to make precise microscopic measurements, and you would never use amicrometertosurveyalargeplotofland.Injustthesameway,youwouldneverattempttouseashort-half-lifeisotopelikecarbon-14(themicrometeranalogy)tomeasuredatesin rocksbillionsofyearsold,andyouwouldnever try tomeasure thedateof recently-eruptedlavausingthepotassium-argonmethod(thetapemeasureanalogy)whichisusefulbetweenaround10,000yearstobillionsofyears.

Datinghumans

Carbon-datingwith14CIn the most recent past, the time of modern humans, dates are measured using

isotopes with relatively short half-lives, like carbon-14 (14C). Carbon dating is thetechnique most people have heard of and it starts with cosmic rays bombarding ouratmosphere.Everydaysomeatmosphericnitrogenistransformedinto14Cbycosmicraydamage,anditreactswithoxygentomakeCO2,whichgetsincorporatedintoplantsandthenanimalsasfood,orintoseashellsfromcarbondissolvedinseawater.14Cdecaysbackintonitrogenwithahalf-lifeofabout5,700years,soprovidedanuncontaminatedsampleof wood, tooth, bone, etc. can be found, the amount of 14C that has reverted back tonitrogencanbemeasuredtogiveadate.

Carbondatingisusedextensivelyinarcheology,butitsshorthalf-lifelimits it tosamples less than 50,000 years old,which is unfortunately rightwhenmodern humanswere spreading across the globe. 650 Its cosmic origins mean that a correction must beappliedforthevariableratethat14Chaswascreatedinthepast.Todothis,agreatmanysampleshavebeencross-referenced todates fromstalagmites,varves, 651 corals and tree-tings,creatingacalibrationcurvegoingback50,000years.636Abonusthatfallsoutofthiswigglycurveisthatit tellsusaboutspaceweatherinthepast,whichIwillcoverinthenextchapter.CarbondatingofmarinesamplesisalottrickierbecauseittakestimetomixtheCO2fromtheatmosphereintotheoceans,andoceancurrentscreatevariablereservoirsof14C,makingmarinecarbondatesinaccurate.636437Thisdoeshaveabeneficialside-effect–itenables14Ctobeusedasatracerforoceancirculation.437

14Cfalsedates&contamination

Since the1970s,many 14Cdatesweremeasured eitheron contaminated samplesoron samples erroneously assumed tobe thesameageasnearbyhumanremains.652Theproblemwassowidespreadthatsomescientistsestimateupto90%ofmiddletoupperPaleolithic(OldStoneAge)datesarewrong.652Ina30,000year-oldbone,98%oftheoriginal14Chasdecayed,leavingonlyvanishinglysmalllevels that are easily overwhelmed by carbon that seeped into the bone from thesurrounding soil over the ages. The problem stems mainly from collagen, a tissue in

bones,whichhasanannoyingpreponderanceforsoakingupcontaminantsandthrowingoff date analyses. This issue is now being addressed by far more rigorous samplepreparation to remove contaminated collagen, and scientists are busy reanalyzingspecimenstocorrect theirdates.Inoneexample,ahumanjawfromtheUK,whichwaspreviouslydatedat35,000yearsold,hasnowbeenshowntobemorethan41,000yearsold 652. Inanotherexample,SpanishNeandertalremainswerere-datedandfoundtobeatleast 10,000 years older than previously thought, limiting the time Neandertals andmodernhumanscoexisted.601

DatingSedibaWhen it comes to dating the bones of our ancestors like Homo erectus and

Australopithecus,wehavetoreachpasttherangeof14C.Theworkthatwentintodating“Sediba”isagreatillustrationofthewayEarthscientistshavetoworkwiththereal-worldconditionstheyfind,andemployavarietyofmethodstonarrow-downtheageofancestralhumanremains.

The Sediba fossils were found in a dirt-filled cave in South Africa. Scientistsbeganbyidentifyingthesedimentlayersinthecave,thentheycollectedover200fossils(from hyenas, saber-toothed cats, wolves, zebras, etc.) from those layers and identifiedthem.Basedonpublisheddateswhentheseanimalsappearedorwentextinct,theycouldbrackettheSedibaremainsbetween2.36and1.5millionyearsago.NosuitablesamplesforradiometricdatingcouldbefoundinthelayerwiththeSedibafossils,soinsteadtheysampled a layerof “flowstone” (basicallyhorizontal stalagmite) frombelow the fossils.Theysentthosesamplestotwodifferentlabsforuranium-lead(U-Pb)radiometricdating.Thetwodates(2.024±0.062and2.026±0.021millionyears)wereidenticalwithintheirerrorrangesandcomingfromthelayerbelowthehumanremainstheysetamaximumagelimitforSediba.653

So, based on the combination of radiometric dates and the fossils, Sediba wasbetween 2.047 and about 1.5million years old – a big range in the context of humanevolution.Theyhadtonarrowitdownfurther.653

TheyturnedtotheinternationalrecordofEarth’smagneticfieldreversals. 654TheyanalyzedsamplesfromtheflowstoneandthesedimentsaboveitfortheremnantghostofEarth’smagneticfieldpreservedwithinthemfromthetimetheyformed.Theyfoundthatthe sedimentswhere Sedibawas found hadnormal polarity, so given the dates alreadyestablished, they assumed the Sediba layer had to be in the normal-polarity “OlduvaiSubchron,”whichlastedfrom1.95to1.78millionyearsago.SonowSedibawasdatedat1.95to1.78millionyearsold-anuncertaintyof170,000years.653

Buttheywerewrong.

As theycontinued toexcavate, they foundanew layerof flowstonecapping thelayerthatcontainedSediba(soitwasyounger).TheU-Pbdateofthenewflowstonewas2.19to1.91millionyearsold-straddlingtheOlduvaiSubchronandanearliermagneticreversal.Magnetic testing confirmed that thenew flowstonehad reversed polarity, so ithadtobefrombeforetheOlduvaiSubchron(soitwasolderthantheythought).Therewasavery short switch to normalmagnetic polarity at 1.977millionyears ago,whichonlylasted 3,000 years, so Sediba had to be right in thatmagnetic time zone. This tightly-

constrained the age of the Sediba fossils to 1.977 ± 0.002 million years old, 356 anuncertaintyofjust2,000years,or0.1%.

DatinghumanartifactsIn exploring the earliest Homo sapiens culture and survival, scientists found

pigments,stone“bladelets”andaquantityofseashellsinacaveatPinnaclePointinSouthAfrica.Thoseworkersneededtopintheirdiscoveriestoadate,andtheyusedtechniquesthatweredifferent from thoseused todateSediba.Theyused“UraniumSeries”dating,which is good for samples that are too old for carbon-dating and too young for U-Pbdating,onaflowstonejustabovetheartefacts. 437Theresultsgaveayoungestagelimitof92,000 years. They then used a technique called “Optically Stimulated Luminescence”(OSL)onthesedimentscontainingtheartifacts,tocomeupwithanagefortheartifactsof164,000years.OSLrequiressamplestobekeptindarknessfromexcavationtoanalysis,anditexploitsdirt’sabilitytobeanatural‘dosimeter’–thedeviceoneverynuclearpowerplant worker’s coat that measures their radiation exposure. It works because while asampleisburied,itisconstantlyzappedbynaturalradioactivityinthesoil,whichcausesmicroscopicdamagetomineralgrainsinthedirt.Thelongertheburial,themoredamage.Whenthatsampleisrecoveredandstimulatedbylight,itglows,andthestrengthofthat“glowcurve”gives the lengthof time thesamplewasburied!OSLisanother techniquefordatingintherelativelyrecentgeologicalpastfrom1,000to500,000yearsago.650

DatingatyourlocalnuclearreactorForolderremains,likethoseofArdipithecus,workershaveusedatechniquewhich

requires sticking samples inside a nuclear reactor!

You may be surprised how manynuclear reactors are quietly working away on university campuses and other researchfacilities.655Samplesareplacedinthereactortoturnpotassium-39intoargon-39bybeingbombardedbyneutrons.Oncethesampleshavebeenradiated,theyarethenanalyzedinamassspectrometer.InthecaseofArdipithecustheyusedthisargon-argon(Ar-Ar)dating

techniqueonvolcanicashdepositsthatsandwichedArdi’sbones,andcameupwithanageof4.4millionyearsforArdi.656

DatingfossilswithSHRIMPYoumaybewonderingwhytheydidn’tjustdatethefossilbonesthemselves.As

we saw a few paragraphs ago, radiocarbon (14C) dating is sometimes used directly onbones, 652butit’sofnouseonfossilsolderthanabout50,000years.Directdatingofolderfossils is problematic because the process which preserves bones, teeth and shells asfossilsusuallymesseswiththeirisotopic‘clock.’

Living bone and tooth is made up of calcium-phosphorous crystals(hydroxyapatite),embeddedincollagenfibers.Whenabodydecaysindirt, thecollageneventually rots but the bone crystals exchange chemicals with groundwater and aregraduallyconvertedintofluorapatite,whichismorestableandcapableoflastingmillionsofyears.Thenewcrystals are as tiny as theoriginal bone crystals, so they collectivelyhave a large surface area in contact with groundwater. This means they continue toexchange isotopes with groundwater over thousands or millions of years, so theradiometric‘stopwatch’isneverset.Datingthesespecimenstypicallygivesdatesthatarefar too young, sometimes half the true age, and with very large errors. 657 In contrast,molluskshellshavea tendency toabsorburaniumfromsurroundingsediments, 437whichgivesthemanartificiallyoldage.

Thisdifficultyinestablishingmeaningfulradiometricdatesdirectlyonfossils(asopposedtothestrataenclosingthem)hasbeenastickthatYoungEarthCreationistshaveusedtobeatradiometricdatingasawhole.Inreality,meaningfuldatescanbeextractedforfossilswhentheyhavebeenisolatedfromchemicalcontamination.Examplesincludea50-million-year-old shark tooth extracted from clay, a Jurassic dinosaur bone extractedfrom a clay-rich limestone, and conodonts (those mouthparts from ancient eel-likecreatures).Alltheseweredatedusinglutetium-hafnium(Lu-Hf)dating,andevidentlythesedimentsthatencasedthefossilsprotectedthemfromcontamination.657

Highprecisiondateshavealsobeenobtainedfrompristinesamplesofcoralusinguranium-thorium(U-Th)dating,658andmicroscopicshellsofforamshavebeendatedusingstrontiumisotopes.659Uranium-thorium-lead(U-Th-Pb)datinghasalsobeensuccessfulonCarboniferousandSilurianconodonts,andaCretaceousdinosaurtooth. 660Theseanalysesusedamicroscopicprobeknownasa“SHRIMP”(SensitiveHighResolutionIonMicroProbe)tozaptinyspotswithinsamples,tofinduncontaminatedpartsoffossils.

Directdatingoffossilshasalsobeenusedto shed light on early humans.Antelope teeth have been used to date ancestral humanartifacts in South African caves using uranium-lead (U-Pb) dating, 661 uranium-thoriumdirectdatinghasbeensuccessfullyappliedtoaNeandertaljawbonefromSpain,662andanimportantearlyHomosapiensskullfromEthiopiahasbeendateddirectlyusinguranium-seriesdating.663

These examples show that fossilscan be dated directly in special circumstances(so there’s no fundamental flaw in the concept of dating fossils), but inmost cases it’smuchmorepractical todatevolcanicdepositsor flowstones thatbracket thestratawerefossilsarefound,becausetheygiveamuchmorerobust,tightly-constrainedage.

AgesoflavasandgranitesPotassium-Argon (K-Ar) is themostwidely-used dating technique for lavas and

other igneous rocks. It’s good for rocks older than about 10,000 years, which containpotassium-richminerals, like feldspar. 650 A variant of the K-Ar technique is the argon-argon(Ar-Ar)techniquethatwemetwhenweweredatingArdi,above.TheadvantageoftheAr-Armethodis thatbyheating thesample inprecisely-controlledsteps, itcangivethedate of original crystallization (called a “plateau age”), screeningout effects of anylater part-resetting of the ‘stopwatch.’ 630 The Ar-Ar method gives very precise dates(precisionsbetterthan0.1%)onlymatchedbytheuranium-lead(U-Pb)method, 648andsoisbehindmanyofthekeydatesinthestoryoftheplanetbetweenabout10,000yearsand66millionyearsago.648

The otherworkhorse of radiometric dating is uranium-lead (U-Pb) dating, oftenusingzirconcrystals.Becauseofthelonghalf-lifeofuranium,U-Pbdatingisn’tusedonrocksmuchyoungerthanabout40millionyearsold,650anditisthemainmethodofdatingrocksolderthantheageofthedinosaurs.648Zirconisaveryresilientcrystalwhichwemetas the holder of your first memories in Your Life as Planet Earth. When it forms, itstronglyincorporatesuraniumbutnotlead,sooncethe‘clock’issetitisn’teasilyre-set,whichmakesitavitaltoolfordatingoldrocksthathavebeenthroughcomplexhistories.It’s also amineral that forms in a rangeof igneous rocks, so it iswidely applicable. Insomecases,zirconcrystalscansurviveintactevenwhentherockarounditismeltedandlatercrystallizedasecondtime.Thisallowsscientiststomeasuretheoriginalageofthese

“inherited”zircons,andthereforeto“seethrough”morerecenteventstoolderperiods.437

For thosemostancient specksofearlyEarth - theJackHillszircons–scientistsused lead-lead (Pb-Pb) and lutetium-hafnium (Lu-Hf) dates to reveal their origin 4.4billion years ago. 437 There’s a range of other radiometric dating techniques for igneousrocks - like rhenium-osmium (Re-Os), and samarium-neodymium (Sm-Nd) 437 - but I’llspareyouthosedetails.

DatingsedimentsIfyouthinkaboutwhatsedimentsactuallyare,theyaremostlybitsofolderrock

that havebeenbrokenoff andwashed into rivers, lakes andoceans.They are chimerasassembledfrombitsofdiversemountainsandplainsupstream.Sowiththatinmind,whatwoulditmeantodatesucharock,otherthantoattempttoplaceasingledateonmyriadpiecesofolderrocks?437

Except,itturnsoutyoucaninsomecases.

Firstly,somesedimentaryrocks(limestone,saltdepositsandthelike)arechemicalprecipitates rather than grains of older rocks. So, provided they haven’t beencontaminated,it’sreasonabletodatethose.Secondly,theveryprocessthatconvertsloosesand tosandstone, for example, involves the individual grains being cemented together,and this cement can be dated. It’s not the same date as the moment the sand grainsdroppedoutof thewater, rather it is theslightly laterdatewhen theburialpressureandgroundwaterturnedittorock.437

There is an exception. Rhenium-osmium (Re-Os) dating has been used to dateblack shale from theageof “SlushballEarth.”BothReandOsbecomeconcentrated inlow-oxygenmudsontheseabed,andtheygetabsorbedintodecayingorganicmatterthatwill become the black in black shales once the mud has been compressed into rock.BecauseReandOscomefromtheseawatertheydoindeeddatethedepositionofthemud,ratherthanalaterrock-formingeventorindeedthedateoftheparentrockthatthemudswereerodedfrom.664

Uranium-lead (U-Pb) dating can be used onmarine limestones, cave flowstones(aswesawwithdatingSediba),andevenfossilsoils(“paleosolcalcite”). 437Otherdatingtechniques for sediments include uranium series dating (for young rocks with silicacement), 650 thorium-thorium, thorium-230 (with proctactinium-231 as a check), 437 andrubidium-strontium(Rb-Sr-formarinesandstonesthatcontainthemineralglauconite).437

So there are, in fact, techniques for dating the formationof sediments, butwhathappens to those dates when rocks are subjected to the temperatures and pressures ofcontinentalcollisionsandmountainbuilding?

Cookingtime–datingmetamorphicrocksRocksthatstartedoutassedimentsorigneousrocks,butwhichweresubsequently

changedbybeingsqueezedandheatedincontinentalcollisions,arecalled“metamorphic.”Theyarechangedbytheheatandpressure,growingnewmineralstoreplaceoldones,andtheychangetheirappearance.Soindatingmetamorphicrocks,whatwearereallydoingisdatingthemetamorphiccookingeventthattransformedtheoriginalrocks.

Acommonwaytodothisistousetheargon-argon (Ar-Ar) method, to learn about the rock’s “thermal history.” 650 Manymetamorphic crystals are zoned,with the outside of the crystalmore “cooked” than itsinside (like aholiday turkey in theoven, sizzlingon its outsidebut defiantly raw in itsmiddle). Lasers zap tiny spots across these zones, which are analyzed to reveal theequivalentoftheoventemperatureandwhenitwascooking. 437Thisisusuallycombinedwith uranium-lead dating 650 (using zircon if it’s available) to “see through” themetamorphicchanges to theoriginal rock,andsometimeswith rubidium-strontium(Rb-Sr)dating650tolearnaboutitscoolinghistoryafteritwas“cooked.”

Somemetamorphicrocksarestuddedwithdarkredgarnets.Thesesemi-preciousgems formduring thepressure-cookingof rocks, and theyhavehigh levelsof lutetium,makingthemsuitableforLu-Hfdatingofveryhigh-pressure-formedrocks,suchasintheAlpsandsimilarmountainchains.437

So thereareways todate thecookingprocess that rockshavegone through,butwhataboutrocksthatwerecookedinspaceratherthanonEarth?

AntiquityofmeteoritesandimpactsThe time between the formation of the first solids in the Solar System and the

formation of the earliest protoplanet embryos was less than 10 million years. 429 Thiscritical stage inourSolarSystemiscrucial forunderstandinghowourplanet,ourSolarSystem, and others like it evolved, but it’s asking a lot to resolve dates at such highprecisionsfromsolongago. 429Luckily,lead-lead(Pb-Pb)datingdoeshavethisprecisionand, aswe saw inChapter 1, it has been used to date “Calcium-Aluminum-Inclusions”(CAIs)frommeteorites liketheAllendemeteoritewhichfell toEarth inFebruary1969,andbasalticmeteoritescalled“angrites.”TheCAIsandangritemeteoritesyielddatesof4,568.2millionyearsgiveortake400,000years,whichservesasthedatefortheoriginoftheSolarSystem.4296630649

Iodine-xenon (I-Xe)dating isanother techniqueusedonmeteorites. Ituses step-heatingliketheAr-Armethod,to“seethrough”heatingandshocksthathappenedinthelonglivesofmeteoritesbetweentheirformationandtheirdiscoveryonthegroundhereon

Earth. Measurement of the Allende, Bjurbole, Qingzhen and Kota-Kota meteorites allclusteraroundthesameearlySolarSystemdateof4.56billionyearsold.437

Rhenium-osmium (Re-Os) dating has helped tie down the timeframe of coreformationintheearliestproto-planetsinourSolarSystem.Measurementsacrossdifferenttypes of ironmeteorites (shards from the cores of planets thatwere obliterated early intheir lives) reveal an “osmium isotope evolution curve.” The curve plots the earliestevolutionofplanetsinourinfantSolarSystem,fromthemostprimitiveplanetsataround5millionyears after the start of theSolarSystem, to themost evolved (most completecoreformation)around100millionyearslater.437

ThereisevenawaytodatewhenameteoritearrivedonEarth.Asteroidfragmentshurtling through space before they arrive onEarth are constantly fried by cosmic rays,whichconvertanyaluminumintheirmineralstoaluminum-26(26Al).OncetheylandonEarththeyare(comparatively)shieldedfromthosecosmicraysandthe26Aldecaysataknownrate,someasuringthelevelof26Alallowsthecalculationoftheroughdateofthemeteorite’sarrival.

Establishing the date of an asteroid impact on Earth is particularly challengingbecausetheasteroid,andsomuchofthesiteithits,arevaporizedinaninstant.Moreover,the instant melting and cooling hinders the proper setting of the radiometric ‘clock,’resultinginagesthatpre-datetheimpact. 665Incratersthathaveanimpactmeltlayer,Rb-Sr,Sm-Nd,K-ArandAr-Ardatingcanallbeused.Fission trackdating (seebelow)onglassymeltscanalsobeused,and thedatescanbecompared to therecordofmagneticreversals(liketheydidtodateSediba)forimpactsinthelast200millionyearsorso.Fordebris flung from the crater, it’s more complicated, but carefully chosen samples mayyield Ar-Ar or fission-track dates. Young crater walls can be dated using isotopesgeneratedintherocksurfacebycosmicrayssincetheywerefirstexposedtothesky.Andfinally, theglassydropletsof impactmelt that fall far from thecratercanalsobedatedusingAr-ArorevenU-Pbinzircons.665

Theupsanddownsofmountains

TheupsFission-trackdatingisalittledifferentfromtheothertechniquesdescribedsofar

because insteadofrelyingon thedecayofaparent isotope toadaughter, this techniquemakes use of the natural damage radioactive decay (fission) inflicts inside crystals.Radioactiveelementslikeuranium-238ejecthighenergyparticleswhentheydecay,whichare likemicroscopicbullets that leave tiny10-nanometer-wide (0.00001mmor about3-millionths of an inch) tracks of destruction in crystals, which can be seen under anelectronmicroscope. 437Thetrailsofdestructionaresotinythattheycanhealthemselvesgiven sufficient heat – 120°C for apatite, 200°C for zircon, and 300°C for sphenecrystals. 650What thismeans is thatapatitecrystalswillnothavefission tracksuntil theyrise towithinabout3km(2mi)of thegroundsurface (where the temperature reduces tolessthan120°C). 650 Thesloweramountainrises,themorefissiontracksitwillhave,soscientistscancalculateupliftratesoverthelastfewmillionyears.ThistechniqueshowedthattheeasternAlpshaveslowedtheiruplift,andthewesternAlpshavesped-up,overthelastfewmillionyears.437

ThedownsAnd to get a handle on how fast those mountains are eroding away, you can

calculatehowlongrocksatthesurfacehavesatthere,exposedtotheatmosphere.Cosmicrays hitting rocks on the surface slowly generate chlorine-36 (36Cl) from chlorine,potassium and calcium in the rocks’minerals.The cosmic rays don’t penetrate into theground,sothelevelsof36Clareameasureforhowlongarockhasbeenexposedtothesky. This can also be used to date glacially-deposited boulders and even asteroidimpacts!437

Bottomline:Thetake-awayfromthischapter is that therockrecordisfullofnatural‘clocks’

that get set – like a stopwatch –when they are formed or reheated. Scientists have anextensivemenu of dating tools to apply to different situations, and several of these areoftencombinedtobrackettheageofasample.Radiometricdatinghasbeenprovenvalidbycomparingitwithotherdatingmethods,andbyexperimentsinthelab,sowecanbesurethattheEarthisover4billionyearsold,andbeconfidentinthedatesthatformthescaffoldingofourplanet’sstory.

5. Dipping a Toe inAncient Seas, BreathingtheAirofLostWorlds

ThroughoutthestoryofYourLifeasPlanetEarthIdescribedtheclimatesofthosetimes.It’snotasifwecantime-traveltotheJurassicandsetupaweatherstation,orsailEdiacaranoceansto taketheir temperature.Andevenifdinosaurswerealive today,youwouldn’twant to take their temperature in the usualway!But Earth scientists do haveways to reveal the climates of the past, and even the body temperatures of extinctcreatures.

Onceagainwehavethemassspectrometertothankforthis.Thatversatilepieceof lab equipment, which revolutionized the dating of our planet’s history, alsorevolutionizedourabilitytoprobetheclimatesofmillionsandevenbillionsofyearsago.Through incredibly smart forensic science, we can use a variety of “proxies” that arepreservedinrockoricetotellusthetemperature,atmospheric,andoceanicmakeupgoingback to Earth’s earliest days. Many of these proxies are isotopes, and the massspectrometerservestomeasuretheminmuchthesamewayasradioactiveisotopesusedinradiometricdating(describedinthelastchapter).Buttheproxiesforancientclimatesare “stable isotopes” - they are not radioactive. And by learning about past climatechangeswecangetahandleonwhattoexpectfromourmodernclimate.

Thermometersfortimespast

WaterasathermometerWatercanbeusedasakindofthermometerforancient times.Morespecifically,

isotopesofoxygenandhydrogencan.Thesearecalled“stablewaterisotopes.”

Youmayrecallthatisotopesareflavorsofthesameelementdifferingonlyintheirmass.Forinstanceoxygencomesin3isotope‘flavors’:oxygen-16,17and18.Almostalloftheoxygenwebreatheisoxygen-16,butitisalwaysaccompaniedbyatinyfractionofthe slightly heavier isotopes, including about 0.2% oxygen-18. 666 The ratio betweenoxygen16and18iswidelyusedina“thermometer”forancientclimatesandgoesbythemonikerof“delta18-O”ormorecorrectly:“δ18O.”Hydrogeninwateralsocomesintheisotopic ‘flavors’ of hydrogen 1, 2 or 3 (usually referred to as hydrogen, deuterium(0.015%666)andtritium).Theratioofhydrogentodeuteriumisanother“thermometer”forancientclimates-“δD.”

Stable water isotopes work as athermometer because they get more concentrated or depleted through temperature-dependent changes like evaporation or condensation to rain or snow.Watermadewithlighterisotopesevaporatesmoreeasilythanwaterwithheavierisotopes,andheavierwater(i.e. with oxygen-18 and/or deuterium) tends to condense into rain easier than lighterwater.666Soastheoceantemperaturesrise,sodothelevelsofoxygen-18anddeuteriumintheocean,asmoreofthelightwaterevaporatesintotheatmosphere.Stablewaterisotopesinmarinefossilsreflectthelevelsinthewatertheylivedin,sotheycantellustheoceantemperaturewhentheywerealive.Thesameistrueformarinesediments.667

Stablewater isotopesmeasured in corals, tree rings, lake deposits and ice coreshavegeneratedanannual(ornearlyannual)recordoftemperaturegoingbackhundredsofthousandsofyears,fromlocationsdottedacrossmostoftheplanet.Goingfurtherbackintimethanthoserecordsallow,scientistshavemeasuredδ18Oinfossilsandsedimentsallthewaybacktotheearlypartoftheplanet’shistory. 52Fossilmollusks,forams,diatoms,conodonts,668othermarineinvertebratesandevenfishbonescanbeusedtogeneratedate-indexedrecordsofancienttemperatures.53669670671

Butthefurtherbackintime,themoreuncertaintiestherearethattheresultsreflectreal temperatures, rather than some other intervening process like warm groundwaterpercolatingthroughthesampleatalaterdate.SoclimateestimatesgleanedfromEarth’searliestrocks,whichtendtohavebeensquashedandcookedmultipletimesoverthelast3billion years, are particularly challenging. Yet scientists have still been able to makereasonable estimates of Earth’s very early climate, using a combination of painstakingsample collection, comparing results from different techniques, and advances in labequipment.

Verycarefulgeologicalfieldworkandsamplescreeningcanfindsmallhavensofrelativelyunalteredrock,andadvancesinlabequipmentallowincreasinglytinyspotstobe measured from those samples. 53 Comparing the δ18O in one kind of mineral (likephosphates) with those in another (like chert – a flint-like rock) allows scientists totriangulate a reasonable temperature estimate. For example, one study on rocks fromSouthAfrica deduced ocean temperatures 3.5 billion years ago (at the dawn of life on

Earth)tobe26-35°C(79-95°F). 53Adifferentstudyusingchertsfromthesameareaandtimeconcludedocean temperatureswere less than40°C(104°F), 52 anda third studyonolder rocks (3.8 billion years old) from Greenland studied serpentinite – a product ofreactingseawaterwithoceancrust–andconcludedocean temperaturesweresimilar totoday’s. 20 So these 3 separate studies from2 different parts of theworld independentlyconcludedtheoceantemperatureatthedawnoflifewasnotverydifferentfromourowntime.Thiswasasurprisebecausetheoceanswerepreviouslyassumedtohavebeenratherhotbackthen,atabout55to85°C(130-185°F).

But a coupleof interesting things fall out of those temperature soundingsof thedeep past. The first concerns a surprising loss of our oceanwater, and the second is apuzzleknownasthe“FaintYoungSunParadox.”

Waterworld

StablewaterisotopestellusthattheEarthhasbeenlosingoceanwatersincethoseearlydays.Whilethenon-ice-ageaveragevalueofδ18Oseems tohavestayed thesamefor the last3-plusbillionyears, 20 thedeuteriumcomponentofwater(δD)hasnot.Itwentthroughanincrease, 20whichtellsusthatwaterhas been lost – butwhere to? Sincewe know that the amount of continental crust hasgrownsincethen,anditcontainsasignificantproportionofhydratedminerals,scientistscan calculate how much water has been ‘soaked up’ by continental crust. But it’s notenough to explain all of the δD increase.Weknow the planet is leaking hydrogen intospace and has been since its early days. Hydrogen is such a light gas that its atomsbouncingaroundintheupperatmospherecanexceedtheirescapevelocityandflyoffintospace, never to return, and nevermore to bondwith oxygen tomakewater. The planetleakedmorehydrogenwhentheatmospherehadmorehydrogen, 13andagainwhenithadmoremethane,because sunlightbreaksmethanedown in theupper atmosphere, freeinghydrogen.BasedontheδDvalues,scientistscalculatedthatoceans3.8billionyearsagohad26%morewaterthantoday!20

FaintYoungSunParadox

And this is important becauseourSun (a “main sequence” star)was about 30%lessbrightbackthen, 4 11whichmeans theplanetshouldhavebeenfrozensolid.Butweknowfromstablewaterisotopes, theJackHillszircons 2627 29andfrommarinesediments

formed at that time, that it wasn’t frozen. There were oceans of liquid water, andtemperatureswerebalmy.ForawarmclimatetoexistunderafaintSun,theremusthavebeen a thick warm blanket of greenhouse-gasses in the atmosphere. There was, but itdoesn’tseemtohavebeenquitedenseenoughtoexplainthebalmytemperaturesalone. 672SoscientiststhinkthattheextrawarmthcamepartlyfromEarthbeingdarker-tintedthanitistoday(ithada“loweralbedo,”astheysay),673andpartlyfromthepeculiarcombinationofgassesintheatmospherebackthen.Aless-brightEarthabsorbsmoresunlight,thewayanasphaltdrivewaywillmeltsnowcoverlongbeforealawn.Andtheearly,oxygen-freeand low-methane atmosphere seems to have absorbed more heat by the interaction ofhydrogenandnitrogenmolecules,inamannersimilartoSaturn’smoonTitantoday.13

Soeveniftherewasn’tasuper-denseatmosphereofCO2,theextrawarmthtokeeptheEarthtemperatewasprovidedbyacombinationofahydrogen-nitrogenatmosphere, 1326%moreocean(whichislow-albedo)20,andsmallcontinentsdevoidofmountainchains(no reflective snowy peaks) because subduction hadn’t started yet.

55 56 57

Moreover, in avolcanically-highly-active,basalt-covered,oxygen-lessworldwithnoterrestrial life,anydrylandwouldbemuchdarkerthantoday,andsowouldabsorbmoresunlight.Theremayalsohavebeenfewercloudsduetolessdustandbiologicalparticlesintheatmosphere. 673Add-in theoccasionalocean-boilingasteroid impactand throw in tsunami tides (fromacloserMoonandshorterdays22)forgoodmeasure,andyoumaywellhaveenoughheattocompensate for the“faintyoungSunparadox”andexplainwhy theEarthwasn’t frozensolid. It is alsopossible that theSunwas slightlybigger (bymaybe5%) 674 in itsyouth,whichwouldhavedeliveredmoreenergytotheEarthdespiteafaintSun,butitisfairtosaythereissomemileageleftinthisparadox.Asonescientistputit:“itisaverycomplexproblem,withveryfewnon-controversiallimitsthatcanbeplacedonit.”675

Complicatedthermometer

Unfortunately δ18O, although a “common currency” 667 for ancient temperaturemeasurement, is not a simple thermometer. To solve the equations that give youtemperature you have to know the oxygen isotope proportions of the water that yoursample(fossil,sediment,etc.)formedin.676Inoceanwatersthisdependsonthevolumeofwaterlockedupasice,soitisdifferentiniceagesthanwarmphases.Asfarbackasthemost recent ice ages, this value can be measured directly from water trapped insediments, 676butfurtherbackintimeanassumptionmustbemadefor thisvalue,withinthelimitsofwhatisphysicallyreasonable.667

Working out the initial δ18O in land deposits is harder than for oceans, as landvalues are affected by altitude, latitude, distance from the coast and amount ofprecipitation. 666Itisalittlesimplerathighlatitudes,buteventhereitcanstillbeskewedby long-term changes in atmospheric circulation or rainfall. 667 In tropical latitudes it ismorearecordoftherainyseason,whichmeansthatin,say,HongKong,theδ18Orecordmayhavetheoppositerelationshipwithtemperaturethanwefindin,say,Switzerland.677

Scientistsdealwiththeseshortcomingsinthreeways.Firstlytheygetahandleonthe processes affecting stable water isotopes across the globe today, and use thisinformationtocalibrateancienttemperaturereadings.Second,theyrepeatmeasurementsacross a range of different substances and locations, to build a consistent picture. And

third,theytrytousealternativetemperatureproxiesiftheyareavailable.

Wenowhaveadetailedunderstandingofhowstablewaterisotopescorrelatewithtemperature, latitude, rainfall and geographic location today because they have beenmonitoredsince1961atmorethan800locationsacross101countries,andontheworld’soceans, by the International Atomic Energy Authority (IAEA) and the WorldMeteorological Organization (WMO), 677 along with precipitation, pressure, andtemperature.These results canbeused to calibrate temperaturesmeasured fromancientsamples.667

Whentheyusefossilstomeasurethestablewaterisotopetemperatures,scientistsalsohavetocorrectforthecreatures’metaboliceffectsontheisotopelevels.Calibrationsarebasedonlaboratorymeasurementsonarangeofdifferentspeciesthatarebothalivetodayandfoundinthefossilrecord.Careisalsotakentoensureonlypristinefossilsaresampled,andthatenoughsamplesaretakenthattheresultsarestatisticallyvalidandnotjustasamplingartifact.245Inaddition,scientistsscreenoutanytemperaturesthathavebeencompromised by ground fluids, by analyzing tracemetals in the samples,which betraysucheffects.676

In the final analysis,when stablewater isotope temperatures are comparedwithinstrumentalrecords(whichonlygoback130years),theydomatch,evenwhentropicaland land-based stable-water isotopes are included, 678 which suggests they are indeedreliable thermometers.But tobesurea temperature isvalid, it’sagoodidea toalsousealternativesfromthegrowingtoolchestofproxythermometers.

Atoolchestofthermometers…There are many other ancient temperature-measuring tools in the scientific

toolchest,includingmicrobemembranechemicals(TEX86), 679Silicon isotopes(δ30Si), 680microscopic bubbles of brine in ancient salt crystals, 681 magnesium-strontium levels inseashells, 682 683 lipidchemicals inpeat (“MBT-CBT”), 684crystalorientations inmother-of-pearl, 685 and evenworm poop! 686 Atmospheric temperatures going back a few hundredyears ago can even bemeasured by logging the temperature profile in boreholes up to600mdeep.

One of themorewidely usedtemperature proxies is a class of chemicals extracted from dead algae.We try to avoidsaturated fats in our diets, but it turns out that saturation is the important temperatureproxy inalgalchemicalscalled“alkenones.”Coccolithophores, thosealgaewhosechalkskeletons make up the White Cliffs of Dover, change the saturation level in theiralkenonesdependingonthetemperatureofthewatertheylivein-whenit’swarmertheymake more-saturated alkenones. The “alkenone saturation index” is abbreviated to thesymbol“UK’

37,”andtracksthe temperatureof theoceanat itssurfacebecausethealgaelive in near-surface, Sun-illuminated waters. Alkenones happen to be resistant tobreakdowninsediments,sorocksgoingbackmillionsofyearsmaybeanalyzedfortheiralkenone saturation index to derive sea surface temperatures for those times. Even insediments 4 million years old, they remain a very sensitive measure of the oceantemperatureatthattime,havingprecisionsasgoodas0.2°C(0.4°F).683

This range of proxy thermometers in the scientists’ tool chest builds confidencewhendifferentproxiesagreeon the temperatures in the sameenvironment for the sametimeperiod.Buttheyallrequiretheirowncalibrations,andtheirusebecomesproblematicforextinctcreaturesandphysicalconditionsoutsidethelimitsoftheircalibrations.

Capableclumps

One new technique stands out as it seems to give valid temperatures withouthaving toknow thestarting isotopicmakeupof thewaters inwhich thesample formed,and it seems tobeoblivious to thedifferentorganisms it forms in. 676Called “CarbonateClumpedIsotopeThermometry,”whichisamouthfultobesure,itisexploitsthefactthattheisotopes13Cand18Oincarbonatesbondtogetherinamathematicalratiothatdependsontemperature,andapparentlynothingelse.676Itseemstohaveproblemsincavedepositsand insamples formedclose to the freezingpointofwater,butapart fromthat it showsgreatpromiseasanindependentandversatileverifieroftemperaturesintimespast.676

IthasalreadybeenappliedtooceantemperaturesduringOrdovicianiceagesabout

445millionyearsago, 132 themost recent iceages,and inEastAfricaat the timeofourAustralopithecus ancestorsabout4millionyearsago. 676Butperhaps themost intriguinguse of this new technique has been to measure the living body temperature of extinctanimalslikedinosaurs.

TakingthetemperatureofdinobloodCarbonateClumpedIsotopeThermometrycanbeusedonfossilbonesandteethto

workoutthebodytemperatureoflandvertebrates. 687ScientistsusedthistechniqueontheteethoflargedinosaursfromtheJurassicanddiscoveredtheirbodytemperaturewas36°to38°C(97-100°F). 688Humanbodytemperature,bycomparison, isaverysimilar37°C(98.6°F).Thiswasasurprise,becausepreviouscalculationshadestimatedahigherbodytemperaturearound41to44°C(116-111°F)basedonthesheersizeof thecreatures,sodinosaursmusthavehadawaytocoolthemselvesdown.688

The technique has also been used to measure the body temperature of ice-agewoollymammoths,andMiocenerhinosandalligators(fromabout12millionyearsago).687Tocheckthetechniquewasaccurate,scientiststrieditoutonmodernteethfromarhino,elephant, crocodile, alligator and a sand tiger shark, and found the technique gave anaccuratebodytemperatureforthemoderncreatures.TheMiocenerhinobodytemperatureof36.6°C(97.9°F)agreedwellwiththemodernrhinotemperatureof37°C(98.6°F),andthe woolly mammoth samples suggested a temperature of 38.4°C (101.1°F), slightlyhigherthanamodernelephanttemperatureof37°C.687

PlantthermometersLeavesareremarkablygoodthermometerstoo.Youmayalreadybefamiliarwith

thefactthatcold-climateleavesareoftentoothedwhereaswarm-climateleavestendtobesmooth-edged.Itturnsoutthatanumberofotherpropertiesofleaves(size,shapeofthetip,shapeofthebase,shapeandspacingoftheteeth,typeoflobes,andsoon)varywithclimatetoo.Becauseleavesarethefood-enginesofplants,theyareverysensitivelytunedto their environment.For example, the leaves at the topof trees tend tobe smaller andthicker than leaves that are shaded further down the canopy. 689A treewill vary its leafpropertiesinitslifetimeinresponsetochangesaroundit,forexampleifsurroundingtreesare felled. Rigorous sampling of leaves and a statistical process applied to theircharacteristicscalled“CLAMP,”yieldnot justmeanannual temperature,but alsowarmmonthmean temperature, coldmonthmean temperature, length of the growing season,growingseasonrainfallandevenaltitude.689

CLAMPcanbeapplied to fossil leavesso longas therearemanyof them,fromdifferentspecies,andtheyarepreservedwithsufficientclaritytomeetthestrictCLAMPstandards.Whentheseresultsarecomparedwithotherproxythermometerslikeδ18Otheygenerallyagree,689whichincreasesconfidencethattheyreflectrealclimatevalues.

Landtemperatureandrainfallchangesinthepastcanalsobeestimatedfromplantpollenrecords.Pollen isextractedfromlakesedimentsand thespecies itcomesfromisidentified.Thenscientistscount theabundancesofdifferent speciesandconstruct ratios(for example: (birch+spruce) / (oak+Hickory)), 690 calibrated to differentmodern climatezones, toyieldvalues for summer temperatureandannual rainfall.Theyplot thepollenratios for thesediments layerafter layer,coveringmillennia, to reveal theadvancesand

retreatsofdifferentwoodlandtypesinresponsetochangingclimates.

So,aswildastheideafirstsounds,scientistsdoinfacthaveforensictechniquestotellthetemperatureofpastoceansandeventhebodytemperatureofextinctcreatures.Butas we try to get a handle onmodern climate change, one archive of temperatures andatmosphericgassesisparticularlyimportant–icecores.

Icecores

Sticks of ice drilled from icecapsandglaciersareanincrediblydetailedrecordofclimatesofthepast.Beingice,theyare obviously limited to cold locations – ice caps and glaciers.Most come from polarregionslikeAntarcticaorGreenlandandotherArcticislands.Butsomeareextractedfromglaciers across the globe, including places like Tanzania, Kenya, Peru, the AltaiMountains,ChinaandTibet.642

Theycontainanannualrecordofatmosphericgasses,temperature,andimpuritiesgoingback asmuch as 800,000years (EPICADomeC inAntarctica), 642 400,000 years(VostokinAntarctica), 691or123,000years(NorthGreenlandIceCoreProject).641OutsideGreenlandandAntarctica,agerangesaremoremodest,rangingfromafewhundredyearstoroughly5,000years.642CollectivelytheypreserveameticulousrecordofEarth’sclimatefrom today through roughly one third of the Pleistocene ice age, covering 19 glacial-interglacialcycles.

Frozentemperatures

Stablewaterisotopesinpolaricecoresrecordnotjusttheseasonaloscillationoftemperatures,butalsothelong-termdriftsintemperaturethroughiceagesandinterglacialwarmperiods.346Theyshowaveragetemperaturesoverthelast800,000yearsinAntarcticaweremostlyaround8°Ccooler than today, interspersedwithwarmer interglacialsup to3°CwarmerthanmodernAntarctictemperatures.346

Stablewater isotope temperatures are supplemented by isotopes of nitrogen and

argon (δ15N and δ40Ar), which help quantify abrupt temperature changes. 692 384 The δDrecordinGreenlandiceshowsaremarkablyabruptshiftintheyear9,703BC.Atthistimethe δD graph parts companywith the δ18O trace, in as little as 1-3 years. 346 This tellsscientists that the sourcewater for the snowonGreenlandchangedveryabruptlyat theendofthelasticeage.Anditisthisevent–recordedinicecores-thatformallydefinestheendofthelasticeage(technicallytheendofthe“YoungerDryas”)andthestartofthe“Holocene”period. 346 Interestingly, theAntarctic icecoresdonot show the sameabruptshift.693

Bubblesofpastatmospheres

Ice cores also contain tiny bubbles of air that was originally trapped betweensnowflakes.When lumps of this ice are crushed ormelted they release the trapped air,whichcanbeanalyzedforthelevelsofgasseslikeCO2andmethane.346

What they show is a consistent pattern of methane and CO2 varying withtemperature–warmertimeshavemoreofthesegreenhousegasses,andcoldersnapshaveless.Over the last800,000years,CO2 hasvariedbetweenabout 180 and280parts permillion(ppm),comparedwithcurrentvaluesofaround400ppm.346Methanespentmostofthe last 800,000 years between about 350 and 750 parts per billion (ppb) – now it isaround 1800ppb. 346 These past variations in greenhouse gasses are crucial forunderstandingglobalclimatechangetoday.

27Bubblesofpastatmospheres:thetinyspecksinthephotoareairbubblestrappedinice.Theycanbesampledtotellscientiststhelevels

ofgassesinatmospheresupto800,000yearsago.Source:CentreforIceandClimate,UniversityofCopenhagen,Denmark.

DNA,desertdustandseasalt

Afascinatingarrayofimpuritiesgetstrappedintheice,andtheyrevealtheirownsecretsofpastenvironments.

Dusttrappedinicecoresisimprobablyinformative.Dustparticlescanbeanalyzedfortheirisotopes,andtheresultscomparedwithdustfromdesertsallaroundtheplanet,soscientistscantellwhereiceagewindswereblowingfrom.Thisisn’tidlecuriosity–thewindpatternshelpourunderstandingofclimatedriversback then. Italsoshowsus thattherewasupto100timesmoredust intheairduringiceagesthantoday. 346Thismeansthatnotonlywastheclimatecolder,butitwasalsomuchwindier,andfardryer,withlessraintowashoutthedustbeforeitgottoGreenlandorAntarctica.346

Sodiumchloride(seasalt)levelsvaryalotinicecoresbutexactlywhatthismeansisnot certain.Allweknow is that the salt contentof the ice is about5 timeshigher inglacialperiods.Modernpollutionintheformofsulfates,nitratesandleadshowupintheice,asdoesammoniawhichseemstobelinkedtoforestfires.346

Severalicecoreshavedrilledrightthroughthe ice until theymet bedrock, and through the layer of dirty, silty ice that sits on thatbedrock.Thatdirtyicecontainssmearedfrozenremainsoflifethatexistedjustbeforetheice formed. InoneGreenland ice core, they analyzed theDNAandaminoacidsof thisfrozensoupandwereabletoidentifytheremainsoffloweringplantsincludingsaxifrages,dryas, daisies and peas; trees including alder, spruce, pine, birch and yew; and grasses.Theywerealsoabletoidentifybeetles,flies,spiders,butterfliesandmoths.Thispaintsapictureofadiverseopenforest in thehighlandsofGreenlandsome450,000 to800,000yearsago,radicallydifferentfromthebleakicecapthatexiststoday.694

Datingice

Icecoresarestripedwithalternatingclearandcloudylayers.Sometimesthelayersrepresentanindividualsnowstorm,but thekeyis theidentificationofahardsurfaceormelt layer that is at the top of each summer layer. Counting these markers of ancientsummersallowsdatingoficecoresyearbyindividualyear.Therecomesapointwhentheiceissocompressedthatitisnolongerpossibletodothisvisually,whichiswhereother

techniques are employed. Our friends δ18O and δD are analyzed for summer-winter(warmer-colder)wigglestodefinetheannuallayers,asareseasonalpeaksindustlevels,otherimpuritiesandelectricalconductivity.Icecoresarecross-referencedbyvolcanicashlayers, theirmethane record, and even isotopes created by cosmic rays (which are alsointerestingforwhattheytellusaboutspaceweather–coveredbelow).346Volcanicashcanalsobeusedtomatch-upicecoreswithclimateinformationfromsedimentselsewhereontheplanet.

So ice cores give us an unparalleled record of the climate going back nearly amillionyears,buttheyarelimitedtothatcomparativelyshorttimeandtoicylocations.Tofigureoutwhatwashappeningtotheenvironmentfortheother99.98%ofthehistoryoftheplanet,scientistshavehadtofindcluesinrock.

Barometersforoldatmospheres

Seeingred–oxygenRed. It’s the color of blood, rust, and tropical dirt. They’re red because they

contain oxidized iron, and it was that simple sign that gave geologists their first cluesabouttheemergenceofoxygenonourplanet.

Inrocksolderthanabout2.5billionyears,youdon’tseered.InplacesasfarafieldasSouthAfrica 695andAustralia, 696oldersedimentaryrocksarethegrays,greensandblacksofanunrustedworld.Theycontainthingsthatjustaren’tpossibleintoday’soxygen-richplanet,likepebblesofpyrite(fool’sgold),uraninite,andsiderite.696Thesejustrustawayinmodernstreamsandbeaches,butinthoseoldtimestheywerewashedintorivers,rolledaroundandsweptdownstreambeforebeingtrapped,stilluntarnished,inrock.

Butataround2.3billionyearsago,tracesofredbegintoshowuphereandthere,andbyaround1.9billionyearsagowehavepebblesandsandgrainswithrustycoatings,and red-stained sediments. 695 These “red beds” contain iron oxide, only possible inenvironments with significant levels of free oxygen, and their arrival signals thefundamentalchangethatwastheGreatOxidationEvent–Earth’sbigmid-lifecrisis.Andinlaterrocksyoujustdon’tfindpebblesofpyrite,uraniniteorsideriteincorporatedintosediments.

“BandedIronFormations”areanothersign.Theseimportantreservesof ironorearewidelyfoundinrocksyoungerthan3billionyearsold,peakingataround2.5billion

years ago, and then they are absent after 1.85 billion years ago, save some smalloccurrences linked to slushballEarthevents. 24Theyweren’tpossiblewithoutoxygen totake thedissolved ironout of the seawater andprecipitate it onto theocean floor.TheGreat Oxidation Event also triggered a huge diversification of mineral types in rocksformedafter2.5billionyearsagoasoxygenreactedwiththegeosphere,24soit’sclearthatafundamentalchangeinvolvingoxygenhappenedtotheplanetatthistime.

That cyanobacteriawere responsible, rather than somethingpurelygeological, isindicated by genetic ‘molecular clock’ studies, fossil stromatolites, chemical fossils(steranes),andalsomicrofossilsofcyanobacteriadatingbacktonearthistime. 697Anotherclue is amajor uptick of a tracer for organic carbon burial (δ13C) 24 at this time,whichwould be expected as cyanobacteria took over the world and their dead bodies rainedorganiccarbondownontotheseafloor.Butothercauseshavebeensuggested,includingthe lossofhydrogen from theupperatmosphere (leavingbehindorphanedoxygen fromwatervapor-H2O), 20achangein thechemistryofvolcaniceruptions, 440or theevolvingchemistryofrocksascontinentsgrew.698Theymayallhaveplayedarole,butbasedonthelevels of carbon-burial and other chemical budgets, scientists have concluded thatcyanobacteriacontributedthelion’sshare.698

To better understand the link between life, Earth and oxygen,wewant to knowquantitatively what the levels of oxygen were. While ice cores contain bubbles ofatmosphericairthatwecansampleforoxygenlevels,theydon’ttakeusfarintothepast.Sowemustturn,again,toproxies.68Unfortunatelymostoxygenproxiesareonlygoodfortelling us that oxygen was above or below a certain level, rather than providing acontinuous scale. 68 For this reason scientists workwithmultiple proxies to bracket theestimatedlevels.6887159699

Forinstance,inrocksfrombeforetheGreatOxidationeventweseesedimentsthathadtheirironandmanganeseleachedoutofthembywater.Thatcanonlyhappenatlevelsof oxygen less than1/1,000th of present levels. 68After 1.9 billion years ago,when redbeds begin,we see very little solution of iron, sowe can tell that oxygenwasat least1/10thofpresentlevels.68Theoxidationstateofironinancientsoilslookslikeitmayyieldmorequantitativeoxygenlevels,especiallyfortheearlyyearsofatmosphericoxygen,butmoreworkisneededonthattechnique.700

Sulfur isotopes (“δ34S”) can tell us when there was enough oxygen to dissolvesulfate – which needs more than 0.4% of present oxygen levels. 68 They are the mainingredientforcalculatingtheoxygenlevelsIreferredtomostinYourLifeasPlanetEarth.But that studyused the difference in δ34Sbetween sulfate and fool’s gold, combined itwithcarbonisotopedata,andappliedarangeofcorrectionsforrockweathering,erosion,landareaandmountainbuildingchangesover time, inmillion-year increments,usingasophisticatedcomputermodel. 159Soit’farfromasimpleproxyforoxygen,yetitslevelsdomakesenseinthecontextofalltheotherenvironmentalchangesatthetime.

Molybdenum(andvanadiumanduranium)aremetalsfoundcombinedwithsulfurinblack shales,which start life asmuds in thebottomof lakesoroceans, and they areblackbecausetheycontainalotoforganiccarbonthathasbeenpreservedbytheabsenceofoxygen.Eventhoughthemudsaredevoidofoxygen,thetinymudparticlesreflectthe

oxygen levels on land where they first weathered from rock, and the chemistry of thewaterstheysettledthrough,intheirmolybdenumisotopicratio.Molybdenumvaluesalsoshow the rise of oxygen in stages through the Great Oxidation Event, and also theincreasedoxygenlevels intheDevonian,around400millionyearsago. 701 90 InterestinglyuraniumdatasuggestthatoxygenlevelsdroppedbetweentheGreatOxidationEventandtheEdiacaran,about1.8billionyearslater.70272

Onequasi-quantitativeoxygenmeasureusesourfossilSwissArmyKnives– theforams.Theabundanceofvariousforamspeciesthatliveinhigh-oxygenwaterisdividedby the abundance of other forams that live in low-oxygen water, to derive an OxygenIndex. 703But although this allows scientists to graph relative changesofmarineoxygenlevelsovertime,itstilldoesn’tgiveabsoluteoxygenlevels.703

Nofirewithoutoxygen

Land plants have been plagued bywildfiressincetheyfirstappeared,andfossilcharcoalcanbefoundinrocksasoldas419millionyears.141Thecharredremainsofplantscanretainsomeoftheircharacteristicanatomyandsurviveinsoilswheremoistunburnedplantswouldjustrot.Andsinceoxygenlevelsneedtobeabove13%inairtosupportfire,charcoalinrockstellsusthatoxygenlevelsmusthavebeenatleast13%atthattime.141Buttheabundanceofcharcoalinsedimentscanalsobeused todeduce the levelsofoxygen in theatmosphere– themorecharcoal themoreoxygen, broadly speaking.By extracting fossil charcoal from rocks formed in swamps,scientistshaveconstructedagraphoftheestimatedoxygenlevelssincetheearliest landplants.165Sothiswouldappear,finally,tobeaproxythatcangiveustheabsolutelevelsofoxygen in a scale, but it is much more “spiky” than the computer-modeled, δ34S andcarbonisotope-basedstudyIreferredtoearlier.159

Butitdoeshighlightsomeinterestingquirksoftheoxygenrecord.ItconfirmsthehighlevelsofCarboniferousoxygenthatledtothegiantinsectsofthatage,buttherearealso someswingsduring theageof thedinosaurs, anda coupleof “charcoalgaps” thatsuggestverylowoxygenlevels.Oneoftheseisintheearlydaysoflandlife,395millionyearsago(aged91 inYourLifeasPlanetEarth).

141

Theother is rightafter thePermianmassextinction(theGreatDying).

141

Oceandeathandglobalwarming

Even when the atmosphere had plenty of oxygen, there were times when largeportions of theworld’s oceans became stale and oxygen-less, from the shallows all theway to the ocean depths. These “Ocean Anoxic Events” are particularly important tounderstand,sincetheyseemtobeassociatedwithtimesofhighatmosphericCO2,globalwarming and mass extinctions. In fact warmer oceans do hold less dissolved oxygen(Henry’sLaw),butanoxiceventsseemalsotoberelatedtonutrientimbalancesspiralingoutofcontrol.233OnesuchtimewasattheGreatDyingeventattheendofthePermian.Onthreeseparateoccasionsthedeepoceanslosttheiroxygen,afactthatisrevealedinlevelsof cerium, uranium and thorium present in the fossil mouth parts of eel-like creaturescalledconodonts. 184Othermajor anoxic eventsoccurred93.5,120,200and183millionyearsago,allassociatedwithglobalwarmingeventsandlargevolcaniceruptions.233201202

Carbondioxide(CO2)

TheEarthbreathes.

Asassuredlyasyouandme,ourplanetinhalesandexhalesfromthethinbubbleofairthatsurroundsthisplanetinitstravelsthroughthevacuumofspace.Weseethisannualcycle of inspiration and expiration in the air at the top of the Mauna Loa volcano atHawaii704andothermonitoringstationsaroundtheworld.TheyshowthateverywintertheplanetexhalesCO2fromdecayingleaflitterandothersoilprocesses,anditinhalesCO2withthere-greeningofeverynorthernhemispheresummer.Thiscycleisdominatedbythenorthernhemispherebecausethereislessvegetatedlandareainthesouthernhemisphere.

Theannualzigzag(inhale/exhale)of theMaunaLoaCO2 record ispegged toanincessant background rise, fromaround310parts permillion (ppm) in 1958, to around400ppmtoday.704Thistrendisthesameinairsamplingprogramsallovertheworld,frompoletopole. 704Backin1750thepre-industriallevelswereonlyabout280ppm.Anditisthis, combined with the fact that CO2 is a “greenhouse gas,” that makes it the mostcontroversialgasofourage.WeneedtounderstandhowtheplanetwillrespondtorisingCO2,soit’scrucialforustobeabletogageCO2levelsthroughtheclimatechangesofthepast.

Aswithoxygen,wecan sample icecores togetusaCO2record thatgoesbackhundredsof thousandsofyears, butno further.Once againwemust turn toproxies forCO2levelsinolderatmospheres.

Carbondioxideintheoceans

When marine proxies are used to deduce atmospheric CO2, it’s important tounderstand that CO2 in the atmosphere doesn’t automatically equal CO2 in the oceans.Thereissomechemistry,whichIwon’tgointoindetail,thatlinksatmosphericCO2with6 separate chemical parameters: dissolved CO2, acidity, salinity, temperature, howsaturatedtheseawateriswithcalciumcarbonate,anddepthintheocean.Thepointhereisthat to establish what ancient atmospheric CO2 levels were from ocean-based proxies,scientistsneedtoknowatleast2292ofthosechemicalfactorswhentheyuseocean-basedproxiesforCO2.

Fortwowidely-usedCO2proxies,scientistssampletheremainsofplanktonfromsedimentsforalkenones(thosemembranechemicalswesawusedtodeducetemperatures)andthecalciumcarbonateintheirshells.Withthealkenones,insteadofusingtheamountof“saturation”astheydotomeasuretemperatures,theyinsteadmeasuretheproportionsofcarbonisotopes13Cand12C.Theydothesamefortheshells.Theyapplyaformulathattakes account of factors like growth rate, cell size and other physiological factors,temperature, phosphate and nitrate nutrient levels, and use that to calculate theconcentrationofCO2inthewateratthetimethesedimentswerelaiddown.Theformulahas been derived from lab experiments with live plankton, and by comparing analysesfrompre-industrialsampleswiththevaluesofdissolvedCO2forthosetimes.705706

The number of factors that have to be accounted for in the formula can causeproblems.For instance,different species react indifferentways to short termchanges 707and long termchanges 708 inCO2 levels, andalsophosphateandoxygen levels. 309 709 Thisagain underscores the need to have data from several different proxies agree beforescientistscanbeconfidentoftheirconclusions.

So, to complement other proxy measurements, scientists often analyze boronisotopes (δ11B) found in the shells of plankton. Boron isotopes are actually a way tomeasure ocean acidity (pH). 292 Scientists apply a formula which calculates pH afteradjustingforthebiochemistryoftheplankton,oceantemperature,andsalinity.Theratioofborontocalciuminplanktonshellsisthenusedtocalculatethelevelsofcarbonateionsinthewaterwhentheplanktonwerealive,andthusarmedwith2chemicallevels(pHandcarbonate),theycancalculateCO2levels.710292

Land-basedCO2proxies

Plantsbreathetoo.

At least, they need to get air insidetheirleavestomakefoodbyphotosynthesis,andtheyhavetinyporesonleafundersides,called“stomata,”thattheyusetodothis.Itturnsoutthatplantstunethenumberofthesestomata inconcertwithCO2 levels.The lower theCO2 in theair, themore stomata the

leavesgrowtocompensate.151Scientistscountthenumberofstomatapersquaremillimeteronfossil leavesusingcomputer-aidedcountingsystems toderiveCO2 levels. 711But leafstomata as a proxy suffer their own complications. For the stomata density to giveaccurateCO2levels,thetemperatureandhumidityoftheairtheplantlivedin,andalsothemicroscopicshapeofthestomatathemselves,mustbefactoredin, 151andthismaybeonecause of the large scatter in those data. There is also evidence that some plant speciesresponddifferentlytolevelsofCO2.Forinstance,araucarianconifersarelessaffectedbyhigh levels of CO2,making them useful for estimatingCO2 at timeswhen levelsweremuchhigherthantoday.712

Itmay be a stretch to say that soils breathe, but they do exchange air with theatmosphere.Theyareanamazingmixtureofbrokenrock,life,air,rainandgroundwater.Somesoilsbecomefossilized,andthesefossilsoils(“paleosols”)areanotherland-basedproxyusedforassessinglevelsofCO2throughthelast400plusmillionyears.Thisproxyinvolvesanalyzingcarbonisotopes(δ13C)incarbonatesfoundinfossilsoils.ThesereflectabalancebetweentheCO2oftheatmospherewhenthesoilformed,andtheCO2givenoffbythetotalityoflife(microbes,plants,andfungi)thatlivedinthesamesoilatthetime.The value used for the atmospheric δ13C ismeasured from either marine limestone orfossilized terrestrial organic material for the same time period. Again, this proxy iscomplicatedbyuncertainties in thevalueof the life-producedCO2 in the soil, andCO2valuescanvarygreatlybetweensoilsorevenwithinthesamesoil.713Nevertheless,thesoilcarbonrecordofCO2doesbroadlyagreewithotherproxies(likeleafstomata,plankton,boron isotopes), 289 and with computer modeling of the expected CO2 levels (based onbroad-brush calculations of expected CO2 levels based on factors like land area,weatheringratesandsoon).159

ThereareotherproxiesforCO2 includingsoilgoethite (an ironmineral),carbonisotopes in corals and tree rings, and even a new one that uses 3 different isotopes ofoxygeninfossilteethandbones!714Takentogether,thedatafromleaves,algaealkenones,fossil soils and boron isotopes are converging, but a twofold variation between sometechniquesremains. 289RecentworkhasfocusedonimprovingthecalibrationofsoilCO2proxiesindifferentsoiltypesandimprovingthecalculationfortheamountofCO2givenoffbythelivingcomponentofsoil,715sohopefullythescatterinthesedatawilldiminishinthefuture.

ProxieslinkCO2toclimatechange

Combined, CO2 and temperature proxymeasurements confirm the link betweenCO2andclimateformostofthegeologicalrecord.

Forexample,evenasfarbackastheCarboniferous-Permianiceage(300-260millionyearsago,aged93-94inYourLifeasPlanetEarth),CO2 levelssank tovaluesaboutas lowas theydid in themost recent iceages(around220ppm).166Muchlater,inthetimeofthedinosaurswhentherewereice-freepolesandamostly“greenhouse”climate, theproxyrecordshowsCO2 levelswereover1,000ppm.Therewerebriefcoldperiodseventhen,andthosetoocorrelatewithdropsinCO2 levels. 216 Present dayCO2 levels (around 400ppm)were last seen in the Pliocene,about3millionyearsago, 336butarehigher thanthegeneral trendfor thelast30millionyears.289

Thereisoneiceagethatdoes,atfirstblush,seemtobreakthelinkbetweenCO2andclimate.Thelast2millionyearsorsooftheOrdovicianperiod,443millionyearsago,sawanicecapcovermuchofwhatisnowAfricaandSouthAmerica,whichwereattheSouthPoleatthetime. 132716Thiswasaperiodofclimateupheavalandsea-levelchange,withatemperaturesdropping5°Ceveninthetropics,andmassextinction.ThepuzzleisthatwehadanicecapwhenCO2levelswereapparently8to16timeshigherthanrecentlevels.132ButthosehighCO2valueswereextrapolatedfromasingledatapointwhichmaywellpre-datetheglaciation, 216and there isgood isotopicevidenceforhighratesof rockweatheringdrawingdownCO2levelsatexactlythistime.716MoreoverwiththeSunbeing4%lessbrightbackthen,theCO2thresholdforglaciationwasatamuchhighervalueofCO2thantoday, 216andtheicecapprobablystartedabovethesnowlineathighelevationsattheSouthPole,thewayitdidagaininthelaterCarboniferous-Permianiceages. 166 Inotherwords,despiteappearances,theOrdovicianiceagedoesnotactuallybreaktheCO2-climatelink.

Unsurprisingly, given the complexity of our planet, there are other significantfactorsthataffecttheclimate,notjustCO2.DuringthelateMiocene(around12-5millionyears ago), oceans remained warm despite low CO2 values, apparently due to oceancurrentsandthethermalstructureofthePacificOceanatthetime. 717ThePliocene(5.3to

2.6Million years ago) saw summerArctic temperatures some 8°C (14°F)warmer thantoday(eventhroughcyclesoforbitalwobbles),butwithCO2valuesaroundtoday’slevels.

336 ThissuggestspresentCO2levelsareoutofbalancewith themodernclimate,andwehavealreadyemittedenoughCO2 todrive theplanettoPlioceneconditions.

Some people have claimed that CO2 doesn’t rise ahead of (or cause) globalwarming,butratheritfollowswarming.WewouldindeedexpectCO2toriseinresponsetorisingoceantemperaturesbecausewarmerwatercanholdlessCO2dissolvedinitthancoldwater (Henry’s Law again). Until recently the dating and precision of the proxieswasn’t fine-grained enough to delineate what lead and what followed, suggesting CO2mayhavelaggedwarmingbymaybehundredsofyears,basedespeciallyonAntarcticicecores. Recent studies withmuchmore precise correlation of ice cores and temperatureproxies have shown that temperature and CO2 changed synchronously in Antarcticaduring the end of the last ice age, 718 719 and slightly before global temperatures. 720

CalculationsshowthatincreasedwarmthfromtheSunduetoEarth’sorbit,especiallyinthe northern hemisphere, is linked to the southern hemisphere through deep AtlanticOceancurrents, 721whichresults innorthernandsouthernhemisphere temperaturesrisingout of sync with each other. 720 719 This orbit-induced, ocean-current-enhanced warmingreleasedmoreCO2intotheatmospherewhichcausedfurtherwarming,andsoon,toendtheiceage.720719

Soknowing thatglobalclimateandCO2 levelshavebeencorrelated in thepast,allows us to test our understanding of current climate change, providing signposts towhereourmodernplanetmaybeheaded.

28400millionyearsofCO2levelsfromproxies.Source:DanBreecker,UniversityofTexas.

Fingerprintingfossilfuelsinancientextinctions

Carbonisotoperatios(δ13C)domorethantelluspastCO2levels.Theδ13Cvalueisalsoa tracerfor timeswhenlargereservesofdeadvegetablematter(coal,oil,naturalgas) get turned into atmospheric CO2. Since plants prefer carbon-12 when theyphotosynthesize, the carbon in their cells and woody tissue is mainly carbon-12. Thismainly-carbon-12biasstayswiththeplantmatterasitisfossilizedintocoaloroil.Iflargequantities of these fossil fuels are burned intoCO2, the proportion of carbon-12 in theatmospheregoesup,andtheproportionofcarbon-13goesdownasitgetsdilutedbyallthatcarbon-12.

Thiskick inδ13Cvaluesover timecanbe logged in rock successionsassociatedwith mass extinction events. We see a big negative swing in δ13C values during thePermianGreatDyingextinctionevent–aresultofalltheSiberianmethaneemissionsandcoalburning,and the releaseofBrazilianmethaneafter thesmallasteroid impact there.180177Weseesimilarnegativeδ13CswingsintheTriassicextinctioneventwhenoilymudswerebakedintomethane,195andagaininthemodernerafromourburningoffossilfuels.666Interestingly, we see a big swing in the opposite direction during the Great OxidationEvent, probably as a result of aboom inphotosynthesizing cyanobacteria locking awaylotsof12Cinsediments.24

AbriefhistoryofspaceweatherIt’seasytothinkoftheplanetasaballspinningininertspace(apartfromasteroid

encounters),bathed in theconstant,benign lightof theSun.But theSun isnotconstantandspacenotinert.Earthis,infact,subjectto“spaceweather.”

InconstantsunlightImentionedtheslowincreaseinbrightnessoverthelifeoftheSunandthe“faint

youngSunparadox”earlier.ButtheSunalsogoesthroughmorerapidchanges:smallbutmeasurablefluctuationsinbrightness,whichaffecttheEarth’senergybudget.722

ThemostobviousfluctuationoftheSunisitssunspotcycle-the11-yearcycleinthenumberofsunspots.Thishasbeenobservedeversincetelescopeshavebeenpointedatit, and archived photographs of the Sun taken through sunspot-revealing “calcium K”filters extend back to the 1890s. 722 But accurate measurements of the quantity of solarenergyarrivingattheplanet(“TotalSolarIrradiance”)havebeenaroundonlysince1978.These showa small increase anddecrease in irradiancewith the sunspot cycle,but thisvariation amounts to only 0.1% of the total energy (more in the ultraviolet part of thespectrum)andchangesEarth’sclimatedirectlybyperhapsafewhundredthsofadegree.722

29Sunspotcyclesobservedbacktothe1600sandcanbeusedtocalculatevariationsintheSun’sbrightness(TotalSolarIrradiance).Basedon(clockwise)Arlt&Abdolvand,IAU2010;NASA2013;Foukal,SolarPhys2012;MeudenSpectroheliogramfromL’ObservatoiredeParis.

The Sun may have a largerindirecteffectonEarth’senergybudget.WhentheSunismostactive,ozoneinthelowerstratospherewarms as it absorbs the extra ultraviolet light. This seems to expand largescaleatmosphericcirculations(“HadleyCells”)shiftingthejetstreamnorthwardsalittle.WhentheSunisleastactivethere’sacorrespondingreductioninstratosphericwarming.722

Butit’simportanttonotethatboththedirectTotalSolarIrradianceandtheindirecteffectsareonacycle,sotheycancel-outover11years,sotheycan’texplainlongerlastingclimatechangebythemselves.722

TheSunalsogoesthroughlongerfluctuationswhichcanvarytheSun’slightwereceivebyupto0.3%overcenturies.Therehavebeenprolongedperiodswithfewifanysunspots, like the“MaunderMinimum” in the17th century,when theSunwasvirtuallywithoutsunspotsfor70years.ObservationsfromotherSun-likestarssuggestthattheyallundergoquiescentperiodslikethisforlessthan3%ofthetime.722TheMaunderMinimumandsimilarsolarvariationsdoshowupintemperaturesrecordedbytreeringsinEasternEurope,fromtheoldestrecordin1040ADuntil1850.Afterthat,thetreeringsshowtheclimate warmed markedly in line with greenhouse gasses, and not in line with solarchanges.723

Cosmicrays,cloudsandshiftedweatherOurplanetisincessantlyzappedbyhigh-energyparticlesfromspace.Somecome

from theSun, like“CoronalMassEjections” (massive solar eruptions),whicharemorefrequentwhen the Sun ismost active (when it hasmost sunspots).Others are “cosmicrays,” which mostly comprise extremely energetic protons from supernova explosionsvery far from Earth. 724 Paradoxically, when sunspot activity is high and the Earth isbatteredbyincreasedsolarwind,thisshieldstheplanetsomewhatfromcosmicrays.Butwhentherearefewsunspotsandlesssolarwind,moreofthosecosmicraysmakeitintoouratmosphere,withsubtlebutinterestingeffects.722Cosmicraysaremoreenergeticthansolar particles, so they penetrate deeper into the atmosphere where they trigger theformationofwaterdroplets,andthereforemayincreasecloudformationalittle.Theymay

alsoaffecttheelectricchargeoftheatmospherearoundthunderstorms,whichinturnmayaffectprecipitationalittle.722

Cosmicraysandenergeticsolarparticlesionizeatmosphericmoleculeswhichtheneatawaytheozonelayer, 722 725 triggeringacascadeofknock-oneffects.Reductionof theozone layer cools the stratosphere,whichmoves the jet stream and expands the “polarvortex” (the persistent low-pressure areas at the poles), which in turn changes clouddistribution and storm paths. This cosmic ray effect lasts through solar quiet periods,whereastheeffectfromsolarparticleeventsisshort-lived.722

The equatorial Pacific climate has a bigger correlation with solar activity thanelsewhere, which may be due to cloudiness. 722 It’s possible that this is a climate“amplifier,”takingarelativelyminorsolareffectandenlargingitseffectsonclimate,butobservationsshowthatanyamplification isnomore thanabout20%on topof theverysmallTotalSolarIrradiancechange.722726

MeasuringspaceweatherbeforemodernmanCosmicraysareenergeticenoughtomesswiththeatomicnucleiofgassesinour

atmosphere.Theyconvertnitrogenintocarbon-14(14C)andtheyshatteroxygen,nitrogenandargonnucleitocreateberyllium-10(10Be)andchlorine-36(36Cl). 437Theenergylevelneededtomakethese“cosmogenicisotopes”issohighthattheycanonlybeformedbygalactic cosmic rays (and rarely by “solar proton events”) so they record the level ofcosmic rays (and therefore periods of low solar activity) in times before we hadinstrumentstomeasuresolarvariationsdirectly.

14Cisthesameisotopeweuseforcarbon-dating,andwhilefordatingwehavetocorrect for its variable production by cosmic rays, for a space weather proxy we areinterested in theopposite– thevariation itself.10Be in icecores from thepolesgivesastrongersignalthaninlowerlatitudesbecausetheEarthislessshieldedfromcosmicraysat the poles. The 10Be proxy record shows that even in periods of no sunspots (theMaunderMinimum)there isstilladetectable11-yearcycle incosmicraysshowingthattheSun’smagneticfieldstillfluctuatedregularlyinthattime.722Inadditiontothe11-yearcycle,proxydatareveala22-yearcyclecausedbytheSun’smagneticfieldreversals,andtherearealso88-yearand207-yearcycles.Theseallshowupinthe10Beand14Cproxyrecords, but there is no evidence in those records for solar cyclesof theorderof 1,500yearsthatwouldexplain(ascontendedbysome)theclimatefluctuationsknownas“BondEvents” (since the last ice age) or “Dansgaard–Oeschger Events” (during the last iceage).722

Aswithotherproxies, spaceweatherproxieshave their challenges. Inparticulartheyareaffectedbyvariations in theEarth’sownmagnetic fieldover timescales longerthan500years,soit’shardtoseparatetheEarth’smagneticsignalfromthespacesignal. 722AnatomofcarboncanremainintheatmosphereasCO2for7or8years,whichsmearsout the 14C signal from a single cosmic event,whereas 10Be only hangs around in theatmosphereforafewweeksto2years.Thismakes14Cabetterindicatorofspaceweatherfluctuationson11-yearorlongercyclesaveragedacrosstheglobe,where10Beisbetteratpinpointingthedateofasinglecosmicevent.437

Some ice cores show a correlationbetween local temperature and Sun cycles (using 10Be for the Sun cycles and 18O fortemperature), but other locations fail to show this correlation, so it seems not to be aglobalphenomenon.722Otherdatadoshowacorrelationbetween14Candrainfall(recordedincavestalagmites)goingbackaround9,000years.Ingeneral,proxyrecordsshowthatduringtimesofhighcosmicrayflux(timesoflowersolarirradiance)monsoonrainsareslightlyweakerinOman,IndiaandSouthernChina,highlatitudesarealittlecolderwithmoreseaiceintheAtlantic,andEuropeiswetterandcooler.RainfallpatternsalsoshiftinAfrica,Central-andSouthAmerica.722

Thereissolidevidencethatsolarcycleshavebeenstableatleastoverthelast1.2billionyears.GeologistsstudiedlakedepositsfromDevonianandProterozoicperiodsinScotland.Theyare,infact,well-preserved“varves”withalayerperyearforthousandsofyears. Geologists measured the thickness of every tiny layer and analyzed them forrepeatedcycles.Theyfoundcyclesofapproximately11-years,22years,and,curiously,60years,727showingthesamerhythmweseetoday.

ViolentgalacticeventsSupernovaeandgammarayburstsnearusareveryrare,andpotentiallyextremely

destructive.Alargeandverysharpincreaseinboth14Cand10Beshowsup inJapanese,AmericanandEuropeantree-rings,andAntarcticicecores,correspondingtotheyear774-775AD. 728Themagnitudeoftheincreaseissome20timesthenormalsolarcycle,and10times larger than thenormalgalacticcosmic ray signal. Itmayhavebeencausedbyanexceptionalsolar flare, 729 the debris froma supernovawashingpast us, or a gamma-rayburst from two black holes or neutron stars colliding. The fact that roughly 300 timesmore14Cwasproducedthan10Be,tellsscientists that thateventwasmost likelyashortgamma ray burst. 728 A direct hit from a nearby gamma ray burst could fry the planetcausingmassextinction,butscientistsestimatethe774ADeventoccurredroughly3,000to 12,000 light years away, so was mercifully weak enough to not extinguish life onEarth!IntriguinglythiscelestialcalamitymayhavebeenseenbyAnglo-Saxonsasa“redcrucifix”intheheavens!730

Fossilraindropsandpackraturine

Idon’thavespace togo into themallhere,but thereareproxies formanyotherenvironmentalparametersused tounderstand thehistoryof theEarth.Thesecoversuchdiversefactorsasrock-weatheringrates,731monsoonrainfall,732oceanacidification,181oceanmixingandconnections, 437 ocean salinity, hurricane activity, 733 drought, stream flowandmanyothers.734Theyareminedfromdiversesourceslikelakesediments,treerings,corals,cave deposits, deep-seabed drill cores, maize harvest records, ice cores and even peatbogs.

It sounds far-fetched, but scientists even used fossil rain drops to calculate thedensity of the early atmosphere! 2.7 billion-year-old slabs of South African rock werefound covered with small circular pits that bore all the hallmarks of rain pits from apassing shower over freshly-fallen volcanic ash. Scientists performed experiments withwaterdropletsonvariousashdeposits,andusedtheresultstocalculatethedensityoftheair through which those ancient raindrops had fallen to make the observed pits. Theirresultsshowedthattheatmospherecouldnothavebeenmorethantwicethedensityofourmodernair, andmost likelywas just slightlydenser. 672These results poured fuel on the“faintyoungSunparadox”debatethatImentionedearlier.

Perhapsthemostbizarreproxiesarefrom“packrat middens,” ancient nests of packrats containing “plant and animal remainsencasedincrystallizedpackraturine!”735Thevegetationentombedthereisatreasuretroveof information about shifting vegetationwith climate change, and the leaves have beenused for stomata-counting for CO2 levels. The crystallized urine itself is a record ofchlorine-36(36Cl),whichasImentionedintheprevioussection,isatracerforcosmicraylevels, forming a valuable supplement to the cosmic ray record from polar ice cores.Evidentlyscientistswillgotoextraordinarylengthstogettheirdata!

Yettherearesomethingswehavenoproxiesfor,suchasmethane.ThisisashamebecausemethaneisimplicatedinseveralclimateshiftsinEarth’shistory.

Rock-solidevidenceofancientclimatesThe most fundamental, rock-solid evidence for different environments in the

Earth’spastisthescienceofgeologyitself.Theverymakeupandstructureofsedimentsbetraytheenvironmenttheyformedin.Thesamerippledsandsfoundonseabedstodaycanbeseenpreservedinancientrocks;ancientriverfloodplainshavethesamesiltylayers

that we find beside today’s rivers; and the same can be said for desert dunes, reefs,estuaries,glaciersandalltheothermodernenvironments.

For instance,at the lowCO2 timeof theCarboniferousandPermianglaciations,we find “diamictites” (also called “till” - a sediment diagnostic of glacier deposits) inArgentina,Antarctica, SouthAfrica andTasmania.These are associatedwith sedimentscontaining isolated stones that are identical to modern “dropstones” that fall into softsediments from melting icebergs. 166 Glacial sediments like those tell us about the“SlushballEarth”eventsoftheearlyEarth, 24but“hummockycross-stratification,”whichbetraysstormyinfluenceonseabedsediments,showstheworld’soceanswereexposedtowindat those times. 85Wealso seedustdeposits called“loess” inAustraliaat that time,indicativeofglacial timeswhen ice-ground rock flour isuncoveredby summermeltingand blown away, to be deposited elsewhere. Both the loess and the hummocky cross-stratification tellus that theplanetwasnotcompletely frozenover (somorea slushballthan a snowball). Large, thick loess deposits are also found across Europe, the USMidwestandChinaresultingfromthemostrecenticeages.342

Desertsandsareanotherexample.Sandsfrommoderndesertshavegrainswhich,ifyoulookatthemunderthemicroscope,arenicelyrounded,frosted(sand-blasted)andallaconsistentsize.ThesameistrueforancientdesertsandsliketheCoconinosandstoneintheGrandCanyonarea,evendowntotheslantingoutlinesof270million-year-oldsanddunes.736

30JurassicdesertdunesinZionCanyon,Utah.Theirdriftingoutlinescapturedlikeframesinastop-motionfilm,slicedthroughbyrecent

erosion.Photobytheauthor.

Other rocks have tell-tale signs of an estuarine environment (like “flaserbedding”),othershavedesiccationcracks thatshowmudshavedriedin theair,andstillothershavesaltsindicativeofaridsoils.Therearemany,manymorebutIwon’tsubject

youtothemhere!ThepointisthatthesamephysicsthatresultsinsedimentstodaycanbeinferredforidenticalsedimentsthroughEarth’shistory.

Thepresent is indeed the key to the past, asSirCharlesLyell observedback in1832.737

Bottomline:Even thoughwecan’tdirectly sticka thermometer inancientoceans,ordirectly

measuretheconcentrationofCO2inJurassicair,therearearangeofindirectmeasures–proxies-forancientenvironments.Proxiesarecomplicatedandarenotinfallible,buttheirprecision has been improving, themenu of them keeps expanding, and the differencesbetweenthemhasbeennarrowinginrecentyears. 705289Fundamentally,whenwecomparetheproxyrecordwiththephysicalrockrecordwefindtheymakesensetogether.Andbyrevealingwhathashappenedtoourenvironmentinthepast,wecanputthisknowledgetoworktoassesswhatisinstoreforourmodernplanet.

6.TheFutureAccordingtothePast“Thefartherbackwardyoucanlook,thefartherforwardyouarelikelytosee”Churchill

Sowhatcanwelearnfromourplanet’spast?

First,thisisreal.

This is not fiction – there is solid scientific evidence behind this terrestrialchronicle.Yes,thereareuncertainties,whichIhighlightedinthepreviouschapters,butthegreatmajorityofourunderstandingoftheplanet’spaststandsonfirmscientificground.SciencenowhasagoodhandleonmanyofthechangesofEarth’spast,theircausesandtheireffects.

Second,theEarthhasalwaysbeendynamic,sothere’snoreasontoexpectittobestaticnow.

Change has always been a part ofEarth’s story.Orbital changes have imprintedtheirrhythmoniceagesandclimateoscillationsthroughoutgeologicaltime.295149169269Buttherehavealsobeenmuchbiggerchanges,whethercausedbymassivevolcaniceruptions,lifeinnovations,oceancirculationchanges,orasteroidcollisions.Everysignificantchangetriggered a proportional reaction, like the appearance of cyanobacteria,which filled theatmospherewithoxygenandcausedtheplanet tofreeze,or theevolutionofwormsthatchurned the sea bed and caused ocean chemistry to change, or the greenhouse gasemissionsfromLargeIgneousProvinceeruptions,whichtriggeredglobalwarming,oceananoxicevents,andmassextinctions.

We should therefore expect similar causes now to enact similar results. It justdoesn’tmakesensefortheEarthtoworkanyotherway.

Third,humanstodayarechangingtheplanetthewayLargeIgneousProvincesdidinthepast.

Abruptclimatechangeswerehighlydestructive inEarth’spast.Thecreaturesofthose times didn’t get off lightly – many went extinct. Several mass extinctions werecausedbyhugegreenhousegasemissionsfromLargeIgneousProvinces,andsome(likethe Permian) came perilously close to extinguishing all complex life on Earth. Theparallelsbetweenpastgreenhousegas-drivenextinctionsandouractivities todayshouldgiveuscauseforconcern.

And as we saw in Your Life as Planet Earth, modern-looking humans evolvedsome200,000yearsago,butitwasn’tuntiltheclimatestabilizedafterthehelter-skelteroftheiceages,thatagriculture,738thencivilizationbegan.414

ThepowerofhumansThe 2004 tsunami-generating quake in the Indian Ocean released 23,000 times

more energy than theHiroshima atomic bomb. 739 Even the largest hydrogen bomb ever

tested (The “TsarBomba,” a 50megaton device)was just a tenth of the energy in thatsinglequake.740Bythosemeasureshumanpowermayseempuny.

Butourability tochangetheEarthlies less inmomentsofexplosivepowerthanintherelentlessaccumulationofsmallunwittingactsonapersonalscale.

In just the same way that the oxygen released by a single cyanobacterium isinconsequential, but oxygen from trillions of them transformed the planet in the GreatOxidation Event, there are so many of us now on this planet that the sum of all ourindividualcontributionscanfundamentallychangeourEarth.

The smoke stacks from individual power stations and factories, the exhausts ofindividual cars, airplanes and somany other discrete sources have combined to nearlydouble the heat energy trapped in our atmosphere compared to pre-industrial values. 741This extra energy in our atmosphere has built to the point where in 2013 it was theequivalent of 561million Hiroshima atomic bombs, or 224 thousand “Tsar Bombas,”beingdetonatedinouratmosphereeachyear![B]

But less than half of Americans believe modern global warming is caused byhumans.742Forsure,thescienceofclimatechangeisimmenselycomplex,anduncertaintiesremain,butlet’stouchonwhatwedoknow.

31Buildingenergy:radiativeforcingbygreenhousegassesintheatmosphere,relativeto1750levels.Simplifiedfrom:NOAAAGGI,2014

Theglobalclimateiswarming–beyondreasonabledoubtThe planet is warming. Study after study has established this beyond doubt.

Instrument drift, heat-island effects, poor quality data, selectionbias andother spuriouscauseshavebeenruledout.463743744258Theeffectisstrongerinthenorthernhemispherethanthesouthernhemisphere,and it ispronouncedathighnorthern latitudes. 463 743 744 258 Some90%ofthisexcessheathasbeenabsorbedbytheoceans.745Landtemperatureshaverisenfaster thanoceantemperatures,onaverageby1.5°C(2.7°F)in thepast250years,withabout60%ofthatoccurringinjustthepast50years. 463Ifyouincludetheoceans,globalaveragetemperatureshaverisenbyabout0.8°Cinthelastcentury 258andarerisingabout0.17°C per decade (faster in the northern hemisphere), despite a recent flattening ofsurfacetemperaturessincethemillennium.744743744746

Sideeffects

Arctic sea ice ismeltingat13%adecade, 747 and ice-loss inGreenland andwestAntarctica is accelerating, 748 although curiously, Antarctic sea ice has actually beenincreasingby1.4%perdecade.258Sealevelsarerising749748andoceansalinityischanging.750Oceanacidityhas increasedby30%(pHdropped8.2 to8.1) in response toCO2 levels,moreacidicthananytimeinthelast2millionyearsandtheyaregettingevenmoreso.292751 260 Ocean acidification is already affecting shell-forming creatures, reducing marineorganism egg production, delaying larval development, causing abnormal fish behaviorandproblemsinoysterhatcheries.752Someshelledplanktonspeciesareactuallydissolvingalive.753

These effects are déjà-vu of theLarge Igneous Province eruption-driven climatechangesinthepast.

InAlaska the thawing ground has rendered tundra roads unusable for twice thenumberofdaysperyearcomparedto30yearsago,whiletreesrootedinno-longer-solidgroundhavebecome“drunkenforests.”754Warmerwintershaveledtoanepidemicofpinebark beetles causing destruction of forests in Canada and the western United States 755while theArctichasexperiencedvegetationchangesequivalent toa6degreechange inlatitude(400milesor700km)injustthelast30years.756

Acrosstheworldextremeheatwavesaremorefrequent,hightemperaturerecordsarebeingbroken, andweather agencieshave addednewcolors to their charts todepicttemperatures previously considered “off the scale.” 757 Many areas have experienced agreater frequency of heavy downpours, 755 with more frequent flooding an inevitableconsequence.TheThamesBarrier–agatewaytoprotectLondonfromstormsurge-hasbeendeployedwithincreasingfrequencyinthelast2decades.758

Weatherisnotthesameasclimate

One of the most frequentdismissalsofclimatechangegoeslike:“theycan’tpredicttheweatherinafewdays,howcan theypossiblypredict climate change?”Butweather isnot the sameas climate, anymorethanthestockpriceforanindividualcompanyistheeconomy.Likethatstockprice,weather has all kinds of short-term variability whereas the climate is more like theeconomy – the underlying long-term trend. Weather has always been variable, so it’simpossibletopinonehotday,oronemassivedownpouronclimatechange.Butovertime,climatechangemakesheatwavesanddownpoursmorelikely,injustthesamewaythatanimprovingeconomytendstoseerisingstockprices,butnotallstocksriseandshort-termvariabilitycontinues.

Apauseinwarming?

It’struethataverageglobalsurfacetemperatures,whichhadrisenrapidlyfromthe

1970stothemillennium,havenotrisenmuchsincethen,despiteever-risingCO2 levels.Sothequestionis:isthisashort-termwobblewithinthenaturalvariabilityoftheclimate,orisitanactualpauseinclimatechange?Infact,lookingatthelandtemperaturerecord(whichisnotasaffectedasglobaltemperaturesarebyvariationcausedbyoceancurrents)the ‘pause’ is statistically insignificant over the natural ups and downs of the climateduringthelast50years. 759Also,oceanprobesshowthatextraheat isbeingabsorbedbytheoceans,and therewasalsoasmall reduction in thesolarenergyreceivedduring the‘pause’ timeframe. 760 761 So the ‘pause’ seems to be just a short-term wobble in thecontinuing globalwarming trend. 761 It reduces projectedwarming for themiddle of thecenturybyonlyabout10%,anddoesnotmateriallyaffectprojectionsfortheendofthecentury.761

Despitetheso-called‘pause,’theyearsfrom2001to2013stillrankamongthe14warmestin133yearsofinstrumentrecords,7622013wastiedforthefourthhottestyearonrecordglobally, 763andhotextremesof temperaturearestillbecomingmorefrequent. 755 764And despite recent headlines gleefully declaring “global cooling” based on a return ofsomeArcticice, 765therealityisthatevenin2013,theextentofArcticicewasstillmuchless than theaverageforeachof the last3decades. 766The2013sea ice ‘recovery’ isanexample of “weather,”whereas the decades-long decline inArctic ice is a reflection ofclimate.

SowhytheobsessionwithCO2?

AplanetatourdistancefromtheSunshouldbeaballofice.Deepfrozen.Lifeless.

AsimplecalculationoftheenergyreceivedfromtheSunbytheEarthtellsusitsaveragetemperatureshouldbe-19°C(–2°F).295767Clearlyrealityisdifferent–why?

It’sdue to theblanketeffectof“greenhousegasses” inouratmosphere, trappingtheEarth’sheat.Sunlightshinesmainly in thevisiblepartof thespectrum.Youcanseeyourneighbor’shouse,aviewonaclearday,andtheroadaheadwhenyoudrive,becausetheatmosphereistransparenttovisiblelight.ThewarmthofsunlightonyourfaceisthesensationoftheSun’sraysgeneratingheatinyourskin,whichglowsininvisiblethermalinfraredlight.Roads, trees,rocks–everythingdoesthesamething,theyallwarmupinsunlightandthatwarmthglowswiththermalinfraredlight.Buttheatmosphereisopaqueto thermal infrared, so thatwarmthcan’tescape. Itgetsabsorbedandre-radiatedwithintheatmosphere,keepingtheplanetwarmerthanitwouldbewithoutgreenhousegasses.

It’s like a 2-way mirror in a policeinterrogation room. The detectives can see into the room (like sunlight penetrating theatmosphere),butthesuspectlookingbackjustseeshisownimagereflectedbacktohim(likeEarth’strappedheat).

99%oftheatmosphereisnitrogenandoxygen,whicharenotgreenhousegasses.CO2 is in the remaining 1% alongwith argon, nitrous oxide,methane,CFC gases (theozone-hole-makingrefrigerants),andwatervapor.

CO2, nitrous oxide,methane andwater vapor are all greenhouse gasses becausetheir atomicbonds resonatewith thermal infrared radiation, andCO2happens to absorbinfraredatthepeakwavelengthofEarth’sheatglow.Itdominatesthegreenhouseeffect,and80%oftheincreaseingreenhouseeffect(“radiativeforcing”)from1990to2013hasbeenduetoCO2.741CO2andnitrousoxidelevelscontinuetoriseunabated,whilemethanelevelshavebeenvariable.741

A simple combination of CO2 and known volcanic eruptions is sufficient tocalculatepracticallyallthetemperaturevariationinthelast250years. 463Sincevolcanoesproduceshort-livedcoolingfromtheirsulfuremissions,practicallyallthewarminginthattrendisexplainedbyCO2levels.That’swhywe’reobsessedwithCO2.

Watervapor

Watervaporisapowerfulbuthighlyvariablegreenhousegasthatsomebelieveisbehindclimatechange.Butasinglemoleculeofwatervaporlastsonlyabout9daysintheaironaverage, 768soitdoesn’thangaroundlongenoughtobeaprimarydriverofglobalwarming, because the Earth takes much longer to change (“thermal inertia”), and theamountofwatervaporin theatmosphereis limitedbytemperature. 769Thismeans it isa“slave” or “feedback” that only rises in response to warming caused directly by othergreenhouse gasses, 767 but in this way it amplifies the warming caused by those othergasses, doubling their warming effect! 770 767 Scientists have proven that water vapor iscontrolledbytemperature,ratherthantheotherwayaround,bywatchingitsresponsetotheshort-livedglobalcoolingtriggeredbytheeruptionofMountPinatuboin1991.771

ThetwofacesofCO2

AsImentionedinthelastchapter,weknowthatCO2andtemperaturehavemoved

inlock-stepthroughoutmostofEarth’shistory.CO2isbothadriverofwarming,andalsorises inresponse towarming.Whenwarming iscausedbyorbitalchanges,weseeCO2levelsrisesimultaneouslybecausewarmeroceanscandissolvelessCO2thancoldones.719

We also know that when massive volumes of CO2 have been added rapidly into theatmosphereinthepast,thishasdrivenglobaltemperaturerises,likeintheTriassic193andthePermian181extinctionevents.

Basic physics (Planck’s equation for blackbody radiation, absorption spectra forgasses,Stefan–Boltzmannlaw,Boyle’sLaw)tellsusthatextraCO2intheatmospherehasto have a greenhouse effect. If CO2 was not operating as a greenhouse gas, the Earthwould be that ball of ice, 767 so rising levels ofCO2must have a globalwarming effectunlesssomethingelsenegatesit.And,asI’llexplainbelow,eventhoughtherearesomeclimateprocessesthatworktoreducetheeffectofCO2, theydonotnegate it.So,asidefromorbitalwobbles,CO2istheprincipal“controlknob”onclimatechange.767

WearethecauseHowdoweknowhumansaretoblameforrisingCO2levelsnow?

Forastartthequantitiesmatch.TheincreaseinCO2intheatmospherematchesthemeasuredindustrialCO2release,772773774722andglobaloxygenlevelshavebeengoingdownasyouwouldexpectfromburningfossilfuels.775772

Second,theisotopicfingerprintoftheCO2showsitcamefromfossilfuels,ratherthanvolcanoes.772258

Third, the timing of the changes matches the timing of the industrial era. ForinstancetemperatureandCO2levelsarelargelyexplainedbyorbitalwobblesandvolcaniceruptions (withminor solar cycle influence) up to the industrial era, and thenCO2 andtemperatures increase regardlessof theorbitalor solarparameters. 463 723Warming in theoceans is still mostly confined to the near-surface showing it’s a relatively recentphenomenon.776

The magnitude of warming we have seen greatly exceeds natural climatevariability, 772 so it’s virtually certain thatwe are to blame for theCO2which is drivingglobalwarming.772258

It’snottheSunSome scientists have claimed that globalwarming is caused by solar activity or

cosmicrays.Althoughtheamountofsunlight(totalsolarirradiance)hasfluctuatedsincesatellitemeasurements started, overall there has been no net increasewhile the Earth’sclimate warmed. Solar activity rose and then fell in the 20th century, but globaltemperatureskept rising, so there isn’t adirect causal relationship.Thecosmic ray fluxdecreasedintheearly20thcentury,andfromthesecondhalfofthe20thcenturytorecentlyithasbeenhigh.Ifcosmicrayswereasignificantfactorinclimatechange,thenthelatterhalfofthe20thcenturyshouldhavebeencloudierandcoolerthanitwas,socosmicrays

areclearlynotamajorfactor.722

10Beisotopedataledsomescientiststosuggestthat theSunhasmuchmore influenceonourpast climate thanmainstreamsciencehasit, 777 but recent research has refuted those conclusions, 722 so the most up-to-date andwidely-acceptedunderstandingisthatsolarvariationisonlyaminorcontributortoclimatefluctuations. 722Therewasalsoasuggestionthatthecoldclimateinthesecondhalfofthe17th century was caused by low solar activity during the sunspot-free “MaunderMinimum,” but temperature records show that the climate beganwarming 20-30 yearsbefore the end of theMaunderMinimum, so solar activitywas also unlikely themainclimatedriverinthosetimes.722

Mostscientistsstudyingthelinkbetweensolaractivityandclimateagreethatthesolareffectonclimateaccountsforonly0.1%oftheobservedtemperaturechanges.722258

AcceptedscienceYou may not get this impression from some sections of the media or the

blogosphere,buttherealityisthatscientistsarenotdebatingtheevidenceforman-madeglobalwarming. 778That isacceptedscience-scientistswithexpertise inclimate sciencearevirtuallycertainthatrecentglobal-warmingisduetohumanactivities.779258Thatisalsotheofficialpositionofamanyveneratedscientificbodies,governmentsandcorporations,including:

TheGeologicalSocietyofLondon 780,TheAmericanGeophysicalUnion 781, theGeologicalSocietyofAmerica 782, theNationalAcademiesofScience783,TheRoyalSociety772,TheAustralianCSIRO784,theAAAS785,TheAmericanChemicalSociety786,TheAmericanPhysicalSociety787,AmericanMeteorological Society 788, NASA 789, EPA 790, National Center forAtmospheric Research 791, Canadian Meteorological and OceanographicSociety 792, Science Council of Japan 793, Russian Academy of Science 793,Brazilian Academy of Sciences 793, Royal Society of Canada 793, ChineseAcademy of Sciences 793, French Academy of Sciences 793, DeutscheAkademie der Naturforscher Leopoldina, Germany 793, Indian NationalScienceAcademy 793, AccademiaNazionale dei Lincei, Italy 793, AcademiaMexicanadeCiencias,Mexico 793,AcademyofScienceofSouthAfrica 793,AustralianAcademyofSciences 794,TheChinesegovernment 795, the Indian

Government796,andevenmajorcorporationssuchasDuPont.797

Thesepositionstatementsincludephraseslike:“Theevidenceisincontrovertible:Global warming is occurring,” 787 and “Global warming is unequivocal and primarilyhuman-induced.” 791 What scientists are debating is the extent, timing and regionaldistributionofchanges,andthemostaccuratemethodsforpredictingthese.778

Hoax,climategate,andlobbying

Eventhoughhuman-causedglobalwarmingisacceptedbymostscientists,itisn’tbymore thanhalfofAmericans, 742because theyhavebeenmisledbya smallbutvocalcommunity of climate change deniers that attract disproportionate attention from themedia.Thedeniersseizeonshorttermweathereventsandmisusescientificuncertaintyinakindofsmoke-and-mirrorsexercisetocastdoubtontherealityofclimatechange.765778

This phenomenonwas illuminated byNaomiOreskes andErikConway in theirbook: “Merchants ofDoubt:How a handful of ScientistsObscured theTruth on IssuesfromTobaccoSmoketoGlobalWarming.” 798Theydetailedtheexploitsofawell-fundedlobbyingindustrywhoseaimisto“keepthecontroversyalive”anddeliberatelysowdoubtin the minds of the public and government.

Oreskes and Conwayconcluded:“increatingtheappearanceofscience,themerchantsofdoubtsoldaplausiblestory about scientificdebate.TheyerectedaPotemkinvillage [a fakevillage for show]populated, in only a few cases,with actual scientists.” They found that journalists feelobligedto“balance”mainstreamsciencewiththeviewsoftheextrememinority,inflatingthelatter’simportance,effectivelyresultinginamediabiasagainstmainstreamscience.

The Guardian newspaper of the UK has also uncovered evidence that “Secretfunding helped build a vast network of climate denial think tanks,” and: “anonymous

billionaires donated $120m to more than 100 anti-climate groups working to discreditclimatechangescience.”799

Thismanufacturedappearanceofdoubt(asdistinctfromgenuinescientificdebate)taints theopinionsofpoliticians, thedecisionsofgovernment 798 and even the courts. In2011,theUSSupremeCourtruledonacaseofAmericanElectricPowerv.Connecticut,concerningCO2regulations.TheSupremeCourtJusticesapparentlyplacedequalweighton the unfounded opinions of climate change deniers as on the evidence-basedconclusionsof theoverwhelmingmajorityofscientists.AsonecommentaryinNature–one of themost prestigious scientific journals in the world - put it: “That the nation’shighestcourtwouldrepeatthismisleadingrefrain,andseeminglyendorseDyson’sviewsasequaltothoseoftheIPCCandtheEPA,simplytakesthebreathaway.”800

So it’s not surprising that some scientistsmay feel defensive in the face of thismanifest conspiracy. 798 The so-called “Climategate” affair involved emails from theClimaticResearchUnit(CRU)oftheUK’sUniversityofEastAnglia,whichwerehackedand uploaded to theweb in 2009 and 2011. Theywere seized upon by climate changedeniers who claimed that they showed CRU scientists had manipulated and falsifiedclimatedata.SomeoftheemailsgavetheimpressionthatCRUscientistsmayhavetriedtoabusethepeer-reviewprocesstopreventpublicationofpapersthatconflictedwiththeirviews,andmaynothavecompliedwithdatarequests.

We rightly expect the highest standards of intellectual honesty and transparencyfrom scientists, 801 so claims of misconduct and fraud were investigated by severalindependent inquiries. They concluded that the CRU scientists’ “rigour and honesty asscientistsarenotindoubt,”declaring:“exoneration,” 802andfinding:“noevidenceofanydeliberate scientificmalpractice.” 803 Across the pond, theUSEnvironmental ProtectionAgencydeclared that: “EPA reviewedevery e-mail and found thiswas simply a candiddiscussionofscientistsworkingthroughissuesthatariseincompilingandpresentinglargecomplexdatasets.” 804Finally,UKlegislatorsconcluded:“it is time tomake thechangesand improvements recommendedandwithgreateropennessand transparencyandmoveon.”805

ThecriminalwhohackedtheemailsandreleasedthemsuspiciouslyshortlybeforetheUNclimatesummitinCopenhagenwasneverfound.806

The point is that the only hoax here is the unrelenting stream of manufactureddoubtanddenialthatisneithersupportedbytheevidencenorbytheconsensusofthevastmajorityofscientists.AstheAGUPresident,MichaelMcPhaden,putit:“Thatconsensusisrootedinafoundationofscientificknowledgegainedthroughcareful, thoughtful,andthoroughresearch,notpoliticalorideologicalrhetoric.”807

Or as US Senator Daniel PatrickMoynihan once said, “Everyone is entitled totheirownopinion,butnottotheirownfacts.”807

ThelasttimewewerethiswarmTrue,theEarthhasbeenthiswarmbefore.

Wewereonacoolingtrendoverthelast6,000years,scheduledforthenexticeagebyaroundtheyear3,500AD, 808duetochangesinourorbitaroundtheSun.Butthatallchangedwhenwereachedtheindustrialera,whichhas reversed6,000yearsof cooling in a century. 419 The planet is hotter nowthan theMedievalWarm Period (AD 800-1300) 258 809 419 810 or the Roman Warm Period(500BCto400AD), 811812oranytimeinthelast6,000years.Even6,000yearsago,duringthe “Mid HoloceneWarm Period,” when the warmer climate was caused by the sameorbitalchangethatendedtheiceage,295813globaltemperatureswereaboutthesameorjustslightlywarmerthanthe2000s.419295

Beyondthen,youhavetogobacktointerglacials–warmphasesbetweeniceages- to see temperatures like today. During the last interglacial (known as “MIS5”or the“Eemian”–around130,000yearsago),eventhoughArctictemperatureswerearound5°C295 to8°C 384warmer than today,globally the temperaturewasonly about1°Cwarmer. 295TheEarthwaswarmthenbecauseourorbitwasdeliveringabout11%moresolarenergytotheArctic.295Sealevelswere9meters(30feet)higherthantoday,385asaresultofdrasticreductions in Antarctic and Greenland ice sheets – all in a world barely warmer thantoday.To find thenext timeaswarmas the20th century, youhave togoback400,000years ago to “MIS11.” That anomalously warm interglacial, again caused by orbitalchanges,wasinthesametemperatureballparkasMIS5,295814withtheArcticseaiceatleastpartiallymelted,815justaswehaveseeninthelastfewyears.

But in thoseorbitally-driven interglacials,with temperatures justslightlywarmerthantoday,CO2levelsrarelyroseabove280ppm,comparedwithmodernlevelsofabout400ppm. 346 So they were fundamentally different from today – driven by slow orbitalvariations in solar energy, rather than rapid CO2 rise. To find sustained globaltemperatures tomatch those projected for the end of this century, with CO2 at today’slevels,wehavetogobackbeforetheiceage,allthewaytothe“Pliocene”epocharound2.5to3millionyearsago.295333258336

That’satimebeforetheemergenceofHomo.

SpeedofchangeHowdowereconcileCO2’scontrolonclimatewiththeknowledgethatlifethrived

atmuchhigherlevelsofCO2inthedeeppast?

It’sallaboutthespeedandvolumeofthoseCO2emissions.

OurCO2isrisingfasterthaninpreviousmassextinctions

YoumayrecallfrommydescriptionofCO2proxiesthatatmosphericCO2islinkedto six chemical parameters in the oceans: dissolved CO2, acidity, salinity, temperature,carbonatesaturation,anddepth. 292SlowchangestoCO2(longerthanabout10,000years)canbeabsorbedintheoceanswithoutcausinganenvironmentalcrisisbecausetheexcessCO2 is buffered by those other parameters throughout the vast reservoir of the deepoceans.292

32HowchangesinseawaterchemistryprocessCO2emissions.SlowchangesallowoceanchemistryandatmosphericCO2toremainin

equilibrium.Fastchanges,suchasthosecausedbyLIPsandhumans,aretoofastfortheoceanchemistrytoprocess,turningsurfacewatermoreacidicandallowingabigbuildupofCO2intheatmosphere,causingmorewarming.CH4=methane,CO3

2-=carbonate,CO2inseawaterismainlybicarbonate,CCD=CalciteCompensationDepth,

belowwhichcalcite(shells,limestone)dissolvesfasterthanitaccumulates.

BasedmainlyonZeebe,Annu.Rev.EarthPlanet.Sci.2012,withadditionalinputfromA.Ridgwell(personalcommunication).

Human CO2 emissions, in contrast,havehappenedmostlyinthelastfewdecades.Inthistimeframe,theatmosphereinteractswith the biosphere and the surface layer of the oceans. This reservoir is only about 4trillionmetric tons (4.4 trillionUS tons) of carbon – about the same as our fossil fuelreserves - so we have the capacity to completely overwhelm this short-term naturalcompensation system. 292The surface layer of theoceanshas so far been able to absorbaboutathirdofourCO2emissions,butthishasknockedoceanchemistryoutofkilterbymakingitmoreacidic.Infactoceanacidityisrisingaboutahundredtimesfasterthananytimeinthelast56millionyears. 108Worryingly,therearesignsthattheoceansurfacemayalreadybebecominglesseffectiveatabsorbingourexcessCO2.816

Onathousand-yeartimeframe,extraCO2getsabsorbedintothevastreservoirofdeepoceanwaters.At thesetimescales theoceanisstillmoreacidicthanbeforeall thatextraCO2was introduced,and thewarmerocean is inbalancewithhigheratmosphericCO2levelsandtemperatures,becausewarmwaterdissolveslessgasthancoldwater.292Inotherwords,climatechangecontinuesonthistimescale,eveniftheCO2emissionsstop.

Overseveral thousandyears those extraCO2 emissionsbegin to be removedbychemicalreactionwithdeep-seacarbonatesedimentsandcarbonatesonland.Overevenlongertimescales(100,000yearsormore)carbonburialinsedimentsasaresultofsilicaterock weathering is the major long-term remover of CO2, balancing volcanic CO2production, so preventing a runaway greenhouse like onVenus. It was these long-termadjustmentsthatkepttheworldhabitableinthewarm,high-CO2worldofthedinosaurs,andbeforetheiceages.292

33Somefeedbacksareslowerthanothers:Thetimeframesofinputs(forcings)comparedtoclimatefeedbacks.PartlybasedonSchmidt,PAGESnews2012&Zeebe,Annu.Rev.EarthPlanet.Sci.2012.

It’swhenCO2 is produced too fast for the oceans to absorb it, that excessCO2ramps up in the atmosphere, producing the rapid global warming, ocean acidification,seal-levelrise,oxygenstarvationevents,andthemassextinctionswesawseveraltimesinYourLifeasPlanetEarth.292

It’struethatrapidtemperaturechangeshaveoccurredbeforehumans,forexample“Dansgaard-Oeschger events” during the last ice ages.These large, sudden temperaturefluctuationsoccurredinaslittleas20years(changesofabout0.5°Cperyear),295buttheyare not a goodguide for ourmodern times because theyweremainly in the north, andwerenotdrivenbyCO2 but insteadwereprobablycausedby ice shelf collapsesdue tooceancurrents,392258andpossiblytheopeningandclosingoftheBeringStrait.345

Glacial-Interglacial cycles took much longer – over thousands of years. ThesurfaceoceanwasabletoprocesstherelativelysmallriseinCO2thataccompaniedthesecycles(itrarelyroseabove280ppm346)byadjustingitschemistryinanon-destructiveway,292 817withcold timeshavinga slightlymorealkalioceanwithhigher levelsof carbonateions, causing some marine creatures to grow thicker shells than in warm times.Throughout thecold-warm,glacial-interglacialcyclesof the iceages,calciumcarbonate“saturationstate”(andthereforethedepthintheoceanwherecalciumcarbonatedissolves)variedbyroughly20%tobuffertheCO2changes.292

Inotherwords, theoceans and atmosphere remained inbalance (in equilibrium)throughtheiceages.

It’sjusttoomuch,toofast,forouroceanstocatchup

Theannualreleaseof35billiontonnesofhuman-producedCO2 818 is3to7times

larger,and70 timesfaster, thanat theendof the last iceage,overwhelming thesurfaceocean’sabilitytoprocessitharmlessly,anditwillbemanythousandsofyearsbeforetheoceansandatmospherewillreturntoequilibrium.292

VolumeofchangeOur CO2 dwarfs recent volcanic emissions and rivals those of past mass

extinctions

It’s a surprise tomost people that humanCO2 emissions have dwarfed volcanicemissions over the last couple of centuries. Globally every year volcanoes (includingunderseavolcanoes)onlyemitaboutasmuchCO2asastatelikeOhioorMichigandoes.428

The combined volcanic output of CO2 averages at around 260million tonnes per yearcomparedtoabout35billiontonnesofhuman-producedCO2annually. 818EventhemajorvolcaniceruptionofMountPinatubo in1991onlyproducedabout50million tonnesofCO2. 819 HumanCO2 emissions are equivalent to an extra 11,200 Kīlauea volcanoes orabout360moremid-oceanridges!820

OfcourseaLarge IgneousProvinceeruptionwould rivalor exceed theCO2weemit,butwehaven’thadaLIPeruptioninthelast16millionyears.272

Uncertaintyinclimatechange

“Uncertainty” is a much-misunderstoodscientificword.Ineverydaylanguage“uncertainty”meanstentativeandunreliable,withnegativeconnotations.Thatisnotthewaythewordisusedinscience,andthisdifferenceconfusesthemediaandthepublic,somethingthemanufactureddoubtindustryexploitstothefullestextent.

Uncertaintyinscienceisjustanotherwayorexpressinghowconfidentwecanbeinaconclusion. 778Ascientist expressing“uncertainty” isnot saying theyknownothing,rathertheyarestatinginaprecisewayhowconfidenttheyareinwhattheydoknow.Wewronglyassumescientistsareeither100%certain,orknownothingatall.But“certain”is

like“perfect”–anetherealidealrarelyattainable. 778Ifyouthinkaboutit,therearemanythingswetreatascertainineverydaylifebutwhicharen’tso.Youcan’tbe100%certainyouwon’tbestruckbylightninginthenextminute,butyoucanbeveryconfident(unlessyouarereadingthisontopofamountaininathunderstorm,inwhichcaseallbetsareoff).Wecan’tbecertainwewon’tbestruckbyacaraswecrosstheroad,butwecrossanyway.Whendoctorsgiveanexpectantmotherher“duedate,” it’s aguide rather thana100%certainprediction.

There are almost always uncertainties in science. No measurement has perfectprecision,andwhenyoustartcombiningdata,theseerrorsadduptoo.TheEarthsciencesare particularly challengedby less-than-perfect and sparse data sets – scientists have toworkwiththerealworldastheyfindit.Thedataimperfectioncreatesuncertainty,whichismeasuredandincludedwiththeresults,oftenexpressedasanumber±(plusorminus)another. The first figure is the average value, and the ± value (confidence interval) isusually set so that 95% of the data fall between those numbers. The bigger the spread(uncertainty),thebiggerthe±valuewillbe.

TheIPCC(IntergovernmentalPanelonClimateChange)representstheconsensusview of an international panel of scientists. They call conclusions “virtually certain” ifthey are 99% to 100% likely (a very high bar), “very likely” if the likelihood is above90%,andjust“likely”ifitisabove66%.258

Uncertaintiescompoundintothefuturewhenweattempttopredictclimatesinthecomingdecades.Startingwithprojectedgreenhousegasemissionsandclimatesensitivity,we already have a significant spread of possibilities. Adding-in feedbacks like watervapor, methane, additional CO2, precipitation, clouds, and so on, each with their ownuncertainties,widensthespreadofpossibleoutcomes.Thenfactoring-inuncertaintiesforpopulationgrowth,development,adoptionratesofcleantechnologies,andsoon,andyoucanseethattheconeofuncertaintywillgetquitelargeintothefuture.

34Compoundinguncertaintieswhenpredictingclimatechange(left)isabitlikecompoundinguncertaintypredictingahurricanepath(right).

Sources:IPCCAR5,NOAA

It’ssimilartotheprojectedpathofahurricanethatweseeonTV.Acurvedconeofuncertaintystartswithitsapexatthepresentstormcenter,wideningastheeyeisprojected

intothefuture.Thecurveiscontinuallyrevised,butanyoneinthefootprintofthecurvetakesnoticeandactsaccordingly.

Computermodelsandprojections

Climate science, in common with other areas of science that deal with largeamountsofdata,oftenencapsulates itsunderstandingof theway theclimateworks inacomputer model to handle the many complex calculations. Models are inevitably asimplificationof therealworld,a likeness thatcan’tpossiblycaptureeveryaspectof it.But if the computer model can reproduce actual, measured climate changes, then themodelisatleastagoodapproximationforhowtheclimatereallyworks.Themoretimesthatmodelistestedwithnewdataandsuccessfullyreproducesreality,themoreconsensusbuildsthat thecalculationsinthemodelarea truereflectionof thewaytherealclimateworks.

Climate models have in fact been found to do a pretty good job matchinginstrumental observations so far, 821 even if they have generally predicted more surfacewarming over the last decade than actually occurred. 761 Their results alsomatch valuescalculated independently frompast climate changes, 822 so theydo seem to represent theinnerworkingsoftherealclimatefairlywell.

Uncertaintiesremain

Theintricaciesofallthefeedbackloops,likethelossoforganiccarbonfromsoils,theeffectson theozone layer 772 823and thestratosphere, 824 and the futureuptake ratesofCO2byoceans(becauseofuncertaintiesinhowmarineorganismswilladapt),arenotallwell-quantified.Incombinationtheyaregenerallyagreedtogenerateanetwarming,buttheuncertainty lies in byhowmuch. 772Rainfall changes and regional specifics are alsohard topin-down 823 asare theexpectedchanges inoceancurrents in response toall theotherchanges.772

Theroleofcloudsinclimatechangeisstillpoorlyunderstood,772258andthisisabigblindspotsincesomehavesuggestedcloudsmay be nature’s way of combating global warming (“Thermostat Hypothesis”). 825 But

recentstudieshaveinsteadfoundtheopposite-asmallnetwarming(amplifying)effectascloudschangewithtemperature.826Hopefullyourunderstandingofcloudswillimproveasnewsatellitedatacomeon-streaminthenextfewyears.772

Ofcourse, it’s possible that somenewdiscoverywill changeourunderstanding,butoveralltheuncertaintysurroundingclimatechangehasdiminished.In1990scientistsconcludedglobalwarmingwasaboutaslikelynatural-causedashuman-caused.By1995theyagreedtherewas“discerniblehumaninfluence,”by2007itwas“verylikely”duetohuman-produced greenhouse gas emissions, and by 2013 it was “virtually certain,” agreaterthan99%probability.772258That’soddsbetterthan100-to-1,notwisetobetagainst!

AsensitivequestionIn order to predict how much the Earth will warm in response to greenhouse

gasses,wemust account for all those intricate feedback loops in the climate that standbetweentheinput–greenhousegasemissions–andtheoutput–futuretemperatures.Thenumberthatpredictstheoutputfromtheinputiscalled“ClimateSensitivity.”Sensitivityislikeacargearlever–foragivenenginespeed,highergearsturnthewheelsfaster.Iftheclimateismoresensitive(likeahighergear),thetemperaturerisewillbelarger,ifitisless sensitive (like a low gear) then it will be smaller. Real sensitivity is much morecomplexthanthat,ofcourse,becausetheeffectisnotinstant,differentfeedbacksworkondifferent timescales,andcaneitheramplifyordiminishtheeffect,andawarmingworldgeneratesfurtherfeedbacks-asifthewheelmotionisfedbackintothegearbox.

35Agearleverasasimplifiedanalogyforclimatesensitivity.Theenginerepresentstheenergyinputfromgreenhousegasses,thegearbox

representsallthedifferentfeedbacks,andthespeedofthewheelsrepresentstheamountofclimatechange.Climatesensitivityislikethegear

leverchangingthespeedofthewheels.

Thetimescaleofthatoutputmatters.Weneedtoknowhowtheplanetwillrespondin the lifetime of our grandkids (which I’ll call the short-term sensitivity, but climatescientists call it “Transient Climate Sensitivity”). Andwewould also like to know thelegacyweare leavingforourgrandkids’grandkids todealwith in thecomingcenturies(whichI’llcallthelong-termsensitivitybutismorecorrectlycalled“EquilibriumClimateSensitivity”andsometimes“EarthSystemSensitivity”).

Calculationsofclimatesensitivitythatarebasedonthemoderninstrumentrecordhavetheadvantageofwell-constrainednumbers,buttheytendtodealonlyinthefastestfeedbacks,sotheymaybeundulyinfluencedbyshorttermfluctuations–liketheflattishsurfacetemperaturetrendsincethemillennium.746827Theyalsoneedtheircomputermodelstoaccuratelyrepresentallthefeedbacks–soarehostagestotheuncertaintiessurroundingthose.

An alternative approach is to calculate sensitivity from how the Earth actuallyrespondedinthepast.ThishastheadvantagethatitautomaticallyincludesalltheEarth’sfeedbacks, but it is hostage to proxy uncertainties for the exact volumes, timing anddurations of past climate changes. 822 There’s probably no single magic number for thelong-termsensitivityoftheEarth,becauseeachpastclimatechangewasalittledifferent,begunfromadifferenttemperature,CO2levelandicecapstatus.828822SosensitivityvaluesfromEarth’spastmaybeanimperfectanalogueforourfuturebecauseofthepeculiaritiesof human-generated climate change, 265 822 but they probably provide the best guide forwhat’sinstoreforourchildren,theirchildrenandsucceedinggenerations.

Sensitivitybasedonrecentobservations

The IPCC calculated long-term sensitivity to a doubling of CO2 since pre-industrial times as 1.5-4.5°C (2.7-8.1°F). 258 This was based mainly on modeling frominstrument data, but did include some observations from Earth’s past.

They also calculated the short-term,transient sensitivity as 1-2.5°C (1.8-4.5°F). 258 This short-term number predicts a globalwarmingbetween2.6and4.8°C(4.7-8.6°F)bytheyear2100,assumingbusiness-as-usual

CO2emissionrates.258

SensitivitybasedonEarth’spast

Incontrast,studiesgroundedinhowmuchtheEarthactuallyrespondedinthepastsuggestourplanet’slonger-term(equilibrium)sensitivitymaywellbemuchhigher.

Onerecentstudyofthelast65millionyearsofnaturalclimatechangesarrivedatalong-termsensitivityof2.2–4.8°C(4.0-8.6°F),marginallyhigherthantheIPCC’srange.822But that studydrewheavily from theglacial cyclesof the last800,000years,when thecausesofchange(orbitalwobbles)wereoperatingslowlyenoughforallthecompensatingfeedbacks to keep up. As I explained earlier, current climate change is quite different,operatingmuchfasterthannaturalcompensatingprocessescancope.

Otherstudieshave found that theclimateof thepastmayhavebeenmuchmoresensitivethantheIPCC’svalue. 333

829

Thesecalculatelong-termsensitivitytobearound6-8°C 830 831 for a doubling of CO2, valuesmore in linewith past LIP eruptions andmassextinctions. Even the IPCC acknowledges (with “medium confidence”) that actual longtermsensitivitymayinfactbedoubleitsquotedEquilibriumClimateSensitivity.258

Inotherwords,calculationsofEarth’sresponsesinthepastsuggesttheeffectsofmodernglobalwarmingmayendupbeingfarlargerthanwecancalculatejustfromrecentmeasurements.260

NotjustCO2Even though most of the recent warming has been due to CO2, 741 humans are

responsibleforazooofotheremissionsthataffecttheclimateinotherwaysaswell.

Soot (sometimes called “black carbon”) has recently been found to have awarmingeffectsecondonlytoCO2. 832 Itcomesfromtransport tailpipes,coalandwoodburning, burning trash, wildfires, grass fires, and slash-and-burn forest clearing. 832 Incontrast,fineparticles(aerosols)likesmoganddustoffertemporarycooling.

Ourtransformationofthelandscapealsohasaneffect.Developinggreenlandintocitiesandroads,overgrazing,irrigationanddesertification,erosionandclearingforestsallhaveaneffect.Theychangehowthelandabsorbssunlight(albedo)andtheychangethetotalamountofCO2thatgetsabsorbedintoplantsandsediments,andtheygenerateotherclimate-changing pollutants. Agricultural activity generates methane and nitrous oxide(both greenhouse gasses), and this is likely to increase strongly as global populationincreasesandmore3rdworldcitizensareabletoaffordmoremeat.258Fertilizersinrun-offanderodedsoilaffectoceanproductivity,deadzonesandcoralreefs,eachwithknock-oneffectsoftheirown.258

Methane is generated from sources as diverse as cow digestion, natural gasproduction, coalmining, landfill sites,wetlands, permafrost,mud volcanoes, submarineearthquakes833andeventermites.834Abouthalfofcurrentatmosphericmethanecomesfromhumanactivities,258andeventhoughitonlypersistsintheatmosphereforabout12years,in relatively small concentrations compared to CO2, it is roughly 25-32 times morepowerfulasagreenhousegas.258259

Thepointhereisthatwhilethemainfocusiscorrectlyonthemainclimatedriver-CO2 -evenifwemanagetoreturngreenhousegasses topre-industrial levels,wewouldnothaveremovedallthehumaninfluencesontheclimatesystem.801WehavemanyfingersinthemachineryofEarth’sclimate.

SignpostsfromthepastAswesawinYourLifeasPlanetEarth, therehavebeenmanyepisodesof rapid

greenhouse-gas-drivenglobalwarminginEarth’shistory.Theseweregenerallycausedbyenormous Large Igneous Province eruptions like those in the Permian and Triassicperiods,andeventheDeccanTrapseruptionsattheendoftheCretaceous.LargeIgneousProvinces (LIPs) emit farmoreCO2 thanmoderneruptionsbecause theyareabsolutelyhuge,andtheyreleaseprodigiousquantitiesofgreenhousegassesastheyinjecttheirwaythroughthecrust.17888

InordertobeconclusiveaboutthecomparisonofLIP-causedmassextinctionsandour own time,we need to be able todate the carbon emissions and the eruptions to adecade-orcentury-levelofprecision.Unfortunatelythemostprecisedatesusingthelatesttechniquesarestillatthemulti-thousandyearlevelatbest.179Butaseachnewdatecomesout,thetrendhasbeentoshortenthetimeframeforthoseancientemissions,andthereforetopushthemcloserasanalogsformodernclimatechange.Andtheirsymptomsareeerilysimilartowhat’shappeningonourplanettoday.

ThePermianmassextinctionsawatleast100trilliontonnesofCO2andmethane88181178emittedintotheatmosphere,andtheinitialphaseofemissionsmayhavelastedaslittleas 2,100 to 18,000 years. 179 That’s at least 5 times the volume of our entire fossil fuelreserves, 835 emittedoveramuch longer timeframe thanhumanemissions, so therateofemissionsmighthavebeencomparable.AswesawinYourLifeasPlanetEarth,theresultwas catastrophic climate change and the extinction of 90% of species. 184 The Permianextinction was probably made worse because key calcifying plankton groups(coccolithophoresandforams)hadnotyetproliferated–theyimprovedtheocean’sabilitytobufferCO2changesfromaroundtheCretaceousonwards.178181191EvenifthevolumeofPermian carbon emissions and its ocean chemistry are very different from today, if ourratesofemissionarecomparable,thenthePermianGreatDyingservesasawarningforwhat’spossibleattheextremeendofglobalwarming.260

The end-Triassicextinctionsawaround40trilliontonnesofCO2andmethane 195releasedinperhapsafewthousand years or less. That’s around double our fossil fuel inventory 835 emitted overmaybe10timeslonger. 194Subsequenteruptionsspreadover600,000years 194exacerbatedthesituationandit’shardtopindowntheexacttimingandvolumesofgassesinvolved,but thismass extinction event, one of the 5 biggest inEarth’s history, is considered bysomescientiststobeacloseranalogytothenextfewcenturies.195

The183million-year-agoToarcianextinctioneventseemstohavebeencausedbythe releaseof about 15billion tonnesofCO2peryear over about a 650 year period. 202

That’s less thanhalfmodernemission rates,but spreadovera longer timeframe. 258Thiswas a timewhen sea surface temperatures increasedby around10°C (around18°F) foraround300,000years,andtheenvironmentaleffectsincludedextinction,dissolvingreefs,oceanacidification,oceandeadzonesdevoidoftheusualsea-bottomfauna,andincreasedchemicalweatheringon landdue to theheat andacid rain. 201 202The similarAptian andCenomanian/Turonian extinction events also seem to have involved slower releases ofCO2thantoday.233292

The56-million-years-agoPETMeventmaybeabetteranalogformodern times,but it’s still an area of active research. It released the equivalent of all our fossil fuelreserves over millennia (perhaps as little as 13 years 836 – but that’s not generallyaccepted). 837292260Thevolcaniccauseof thePETMisnotuniversallyaccepted,andsomescientistsbelievelargequantitiesofmethane“clathrates”(akindofmethane-iceindeepsediments)weredestabilized.822260Butthatbegsthequestion:whatdestabilizedthem?

Still more recently, the Ethiopian-Yemeni eruptions between 31 and 29 millionyears ago occurred just after the Antarctic Circumpolar Current (ACC) started up. Assuch, this was the first large CO2 emission event in a world with ocean currents andcarbon“sinks”similartoourmodernconfiguration. 307Theseeruptionsmayhavereleased

alittlelessCO2thanourfossilfuelinventory 307andtheyareassociatedwithashort-livedspikeinCO2valuesandacorrespondingblipinoceantemperaturesbymaybe2°C, 309butagaintherateofCO2emissionswasfarslowerthantoday’s.

EventhewarmMioceneclimate16millionyearsago,triggeredbytheeruptionofthe Columbia River Basalts in the USA, had slower emissions. Hundreds of eruptionswerespreadoverafewhundredthousandyears,causingatmosphericCO2toincreaseandmeltingAntarcticicetoaround10to25%ofitscurrentsize.SomewhatlessCO2thanourfossil fuel inventory was released, 307 but CO2 peaked at levels similar to today’s, withglobaltemperaturesrisingby2to4°C310andsealevelsrisingroughly50m(160ft).308

SomescientistsbelievethatwearelesslikelytocreatethekindoflethalhothouseclimatesthatoccurredinthePermianand the Triassic, because unlike then we are today starting from a lower CO2concentration,withicecapstomelt,andwiththeAntarcticCircumpolarCurrentwhichisefficient at drawing down CO2. 307 Instead, those scientists see us generating a climatesimilar to the Miocene with hot tropics and oceans warm even into the deep sea,widespreadoceandeadzones,shiftedweatherpatterns,andmeltingofmostnortherniceandperhapshalfofAntarcticice(causingsealevelriseofmaybe50m(160ft))308withinafewhundredyears.307

Butotherscientistspointoutthat,becauseweseemtobereleasingcarbonatratesmuch faster thanLIPs in the past, 292 andwe are already seeing large changes in oceanchemistry,wecan’tbecomplacentaboutthepossibilityofarrivingatanend-Triassic-likeorevenanend-Permian-likeenvironmentinthatsametimeframe.260

Whycare?-PrognosisSo the diagnosis is clear: we are already suffering from amajor case of global

warmingduetohumanactivity,ofwhichthemaindriverisCO2.Buttheprognosisisless

clear,witharangeofoutcomespossible,frommoderatedisruptiontosevereimpactsonsocietyand theeconomy.Scientists’ability topredict the future is imperfect,butpolicymakersroutinelyrespondtomilitarythreatsoreconomichazardswithsimilarorevenlesscertainty thanwehave forglobalwarming. 801Thequestion shouldnotbe “doweknoweverything?”butshouldinsteadbe“doweknowenough?”778

I don’t want to be “alarmist,” but I do think alarm is an entirely appropriateresponse to scientists’ projections of Miocene-like or maybe even end-Triassic-likeconditionsforourgrandkids’worldandbeyond,albeitwithuncertainty.Itrulyhopetheyareprovenwrong,andthatourclimateprovestobeless“sensitive”thanmore.Iferventlywish for someunforeseen cooling feedback to swoop in like a fairytalehero, savingusfrom destructive climate change. But Earth’s past shows no sign of that. We mustrememberthatwe’redealingwithnature,redintoothandclaw,soit’simportantwefacerealitywithaclear-eyed,rationalassessmentoftherisksitholdsforfuturegenerations.

Even using conservative climate sensitivities, we are talking of averagetemperaturerisesfortheendofthecenturyof2.6to4.8°C(4.7-8.6°F),andfortheyear2300ofroughlydoublethatifCO2emissionsarenotcurtaileduntilthelatterhalfofthiscentury.258260Thatmaysoundtinycomparedtotheweathervariationsweexperiencedaybyday,but that is about the same jump inglobal temperatures that resulted in theTriassicMassExtinction,albeit fromaclimatealreadymuchwarmer in theTriassic thanwearenow.Globalwarming is like inflation,building littleby littleeveryday. Itaddsupoveryears and decades, relentlessly accumulating a whole lot of extra energy to drive theclimatetonew,inhospitableextremes.

TheIPCCissuedafullandauthoritativeaccountoftheirprognosis,whichisfreelyavailableontheirwebsite,andwhichIdon’twanttoduplicatehere. 258Instead,I’llbrieflytouchonsomekeypoints,basedmainlyonevidencefromEarth’spastclimatechanges.

Sealevelrise

Currentsealevelrisewillaccelerateasmoreheat is incorporated into theoceans,whichwill risedue to thermalexpansionandmeltingice.ThelatestIPCCestimateisariseof0.53to0.97meters(1ft8in–3ft2in)bythe end of this century, 258 based on continuing the current trend of emissions.But past

climatechangeshaveresultedinfarmorerapidsealevelrises:ratesof1.2meters(4ft)percentury838toasmuchas5m(16ft)percentury.308258Otherrecentprojectionsforthiscenturythatclusteraroundthe1to1.5meterrange(3’to5’)aremoreconsistentwiththeexpectedPliocene-likeclimateat theendof this century. 839Those levelswere at least 10m (33ft)higherthantoday,butitcouldtakecenturiestomeltthatmuchice–soPliocenelevelsareprobablywhatourgrandkids’grandchildrenwillhavetodealwith.822258

Becauselandbytheseashoreisoftenfairlyflat, 8401meter(3ft)ofsealevelrisecan easily mean 100 meters (330ft) or more of inland encroachment by the sea. Thepractical implications of this vary by location, but this would result in millions morepeople experiencing coastal floodingwith associateddamage to near-coastal roads, rail,electricitydistributionandsoon.Althoughcurrentsealevelriseismuchslower,ifitdoessoon accelerate to 1meter per century, then about 400,000 homes in the states ofNewYork, New Jersey and Connecticut alonemay be vulnerable to occasional storm surgefloodingbythemid-century. 841Bycomparison,HurricaneSandy’sstormsurgeinundatedabout170,000homesinthatsamearea.842

Thatrateofsealevelrisewouldalsodisplace3.8millionpeopleinthemostfertilepartoftheNileDeltainEgypt, 843withsimilareffects incoastalcommunitiesaroundtheworld.Someestimateasealevelriseof2meters(6½feet)ispossiblebytheyear2100,whichcoulddisplace187millionpeopleglobally.843Eventhose“extreme”projectionsarestill just half the fastest rate of sea level rise observed inEarth’s past, at 5m (16ft) percentury.Unfortunatelythespreadoftheseprojectionsissolargethatitmakesplanningforthemdifficult,843butthepointisthatEarth’spastsuggeststhatsealevelriseismorelikelytobeatthefasterendofprojections.

Heat,droughtandrain

With temperatures rising and heatwaves becoming 10 timesmore frequent, 844 258crop-stresscouldreduceyieldsbyasmuchas15% 845andextremedroughtswilloccurinsomeplaces, especially themid-latitudes, but other areaswill experiencemore frequenttorrential downpours. 755 We could well see a doubling or quadrupling of wildfirefrequency. 845Vegetationzoneswillchange,withanorthernUSstatelikeNewHampshireresembling the growing conditions of theCarolinas by the end of the century. 754 Plantscurrentlyabsorbabout25%offossilfuelemissions,butasdroughtandheatwavestaketheirtoll,plantswillbecomealesseffectivecarbon“sink,”844makingmattersworse.Someanimals, birds and insects (including pests) will migrate in response, whereas lessadaptablespecieswilldieout.755

Some major rivers may see reduction in flow, 845 and groundwater reserves inalready water-stressed regions like the Middle East look likely to become severelydepleted, placing additional strain on relations between upstream and downstreamcountries.846

Oceans

Thetrendsalreadyseenforacidificationandtemperaturerisewillworsen.Acidityisexpectedtoincreasebyupto150%(pHdropto7.7)bytheendofthecentury,108turningsurface waters of the Southern Ocean, the Arctic and the Baltic corrosive for calciumcarbonate(shells,corals,limestone). 108Sinceacidity(pH)isafactorinmanybiochemical

reactions,entireecosystemsmaybedecimated.108Coralswillbebleachedmorefrequentlybywarmerseasurfacetemperatures,whichwilleventuallykill themoff.Thisprocessisexpected to spread tomost of theworld’s reefs bymid-century,with a quarter of reefshangingon20yearslonger.847Ifwe’reluckythatextra20yearsmayenablereefstoadaptbyencroachingintopreviouslycoolwatersaswesawthemdointheJurassic.847215

Fish in warmer oceans have a higher metabolic rate so they will need moreoxygen,butwarmwaterholdslessoxygen.Warmeroceansarealsomorestratified,withawarmsurfacelayerstubbornlyrefusingtomixwithdeeperlayers.Thisrestrictscirculationofnutrientswhich,addedtotheoxygenlimitations,couldstuntfishgrowthtoabout75%ofcurrentsizesby2050 848andseverelyreducefishstocksrelieduponbyabillionpeopleinmanypartsoftheplanet.849

Storms

StudiesoftheAtlanticOceanhavefoundasignificantincreaseinthefrequencyoftropical-storm-strength storms since 1923, and the frequency of the most intensehurricanes has been twice as frequent in warm years. 850 These observations supportprojectionsofanincreaseinhurricaneintensityandanincreaseintheannualnumberofintensehurricanesby50%foreach1°Cincreaseinsurfacetemperatures. 851Somesuggestthat Europe can expect formerly rare destructive hurricanes (like Hurricane Lothar in1999)tobe25timesmorefrequentbytheendofthecentury. 844Incontrast,theIPCChaslow confidence in any increase in the frequency or intensity of tropical cyclones orhurricanesoutsidetheAtlantic.258

Methane–a“carbonbomb”?Asglobaltemperatureswarm,methanereleasefromlandfills,soilandwetlandsis

expectedtoincreaseasafeedback.It’suncertainexactlyhowthismethanewillreactwithotherchemicals in theatmosphere,but it looks likemethane levelscoulddoubleby theendofthecentury,259amplifyingglobalwarmingconsiderably.

Butthat’snotthebiggestworryaboutmethane.

InthepermafrostthatcoversaboutaquarterofnorthernlandandextendsbeneathArcticseas,thereisareservoirofcarbonabout4timeslargerthanallthecarbonemittedbyhumanssofar. 852Theremainsofplantsandanimalshavebeenaccumulatinginfrozensoilthereforthousandsofyears.Thawingpermafrostencouragesmicrobestodigestthoseremains,and theyrelease thecarbonpartlyasmethane. 853Therate isuncertain,but it isalready happening. Since 1900, 4.5 million square km (1.7 million square mi) ofpermafrost area has begun to thaw, 854 and the southern limit of permafrost has alreadyretreatednorthwardby80km(50mi)since1975. 258Ignitable 855concentrationsofmethaneare now bubbling from some tundra lakes and from parts of the Arctic seabed. 852 856

Widespread thawing of the Siberian permafrost is expected if Siberian temperaturesincreaseanother0.7°C fromnow, 814which is likely this century. 258 Calculations suggestthispermafrostmethaneandCO2couldadd theequivalentof another5-8%ofour totalinventory of fossil fuels by the end of the century, even further accelerating globalwarming.852

On top of that, scientists areconcerned that vast reserves of a kind of methane-ice, called “methane clathrate” or“methanehydrate,”trappedinseabedswillbecomeunstableasoceanswarm,addingtheirreservesofmanybillionsoftonnesofmethanetotheatmosphere,ontopofeverythingIhavementionedsofar.AccordingtotheIPCC,therateofclathratereleaseintotheoceanis expected to be slow, and then the rate it would be released from the ocean into theatmospherewillprobablyalsobe slow. 258Butdestabilizedmethaneclathrateshavebeenimplicated in largeclimateperturbations in thepast, like thePETM, 260 anddisturbingly,therearesignsintheNorthAtlanticthatabout2.5billiontonnesofmethanehydratearealreadydestabilizing. 857Theprospectof thesevastmethane reservesbeingunleashedontopofhumanemissions, likea slowbutunstoppable“carbonbomb,”withcatastrophic,irreversibleconsequencesfortheEarth,isanightmarescenarioforscientists.259258

The“carbon-bomb” isdefinitelyaworst-casescenario,but if itcomes topass itsuggestsparallelswiththemoreextremeclimaticeventsliketheend-Triassicoreventheend-Permiancatastrophes.182260

So yes, alarm is an entirely appropriate reaction to this climate “sword ofDamocles” hanging over us, especially considering that if you have kids in elementaryschoolnow,yourgrandkidswillexperiencestrongeffectsofthisbytheendofthecentury,and their grandchildrenmayhave to contendwith extreme environmental catastrophe acenturylater,unlesssomethingisdoneaboutitnow.

WhattodoaboutitFacedwith all that “doom and gloom,” youwould be forgiven for throwing up

yourhandsandabandoningthesituationashopeless.Butitisnotyethopeless,andfailureto actwill onlymake itworse.We can all take steps to reduce the chanceof inflictingthoseworst-casescenariosonourgrandkids.

Some steps don’t have to be painful or very costly. Since heating and coolingconsumethe lion’sshareofenergy(andsoemitmostCO2)athome, itmakessensefor

individualstoinvestmosteffortandmoneythere 858byimprovingatticinsulation,sealingdrafts, and installing energy-efficient appliances, light bulbs, thermostats andwindows.Sourcingfoodandotherproductslocallyhelpstoo,asdoesreducingtheconsumptionofmeat.859Weallcould,andshoulddomore,likeinstallingsolarpanels,switchingofflightsand ‘vampire’ electronics when they’re not in use, carpooling, switching to electric orhybridvehicles,andsoon.

Butthesemeasures,asimportantastheyare,arestill“smallchange”inreducingglobalCO2emissions.801Themillionsofpeopleacrosstheglobewhoarecomingonto“thegrid” in the coming decades will have a far larger quantitative effect. 801 China, forexample,nowemitsaroundtwicetheCO2oftheUSAandthattrendisrisingsharply. 860

ThepopulationsofdevelopingnationshaveasmuchrighttoacomfortablelifestyleasyouorI,buteven if theyallcomeonstreamwithexemplaryenergyefficiencyathomeandtheir commute, just their comingonto the gridwill explode global energy consumptionandCO2emissionsinthecomingdecades.801

36Risingfast:worldenergyconsumption.Source:U.S.EnergyInformationAdministrationJul2013.

DecarbonizationanditslimitsIt’sveryclearwemustdecarbonizetheworldeconomy,quickly.Todecarbonize,

wemusteitherreduceeconomicgrowth,reducepopulation,increaseefficiency,orswitchto low-carbon energy, or some combination of those - a formula known as the “KayaIdentity.”801

BillMcKibben in his book: “Eaarth” shows howwe can live “lightly, carefully,gracefully”inourfoodandenergyuse,tosignificantlyreduceourCO2emissions. 859Thisrequires our whole economy to “back off” a focus on growth and instead focus on

resilience.Buttheproblemwiththisisthateachtimewearefacedwithachoicebetweeneconomic growth and decarbonizing, we pick growth every single time. This is whatRogerPielkehasdescribedasan“IronLaw”ofeconomics,andanyattempttobreakithassofarprovedfutile.801

Ifwe can’t break the IronLaw (reduce economic growth) and population keepsrising,thatjustleavesefficiencyandswitchingtolowcarbonenergyforustoconcentrateon.

AccordingtotheIPCC, 861renewableenergyintheformofbioenergy,directsolarenergy, geothermal energy, hydropower, ocean energy andwind energy, canmeet up to77% of the world‘s total energy demand by 2050, a big ramp up from 13% in 2008,requiringahugeinvestmentofbetween3and12trilliondollarsbetweennowand2030. 861But although renewable energy costs have been gettingmore competitive, they are stillgenerallymoreexpensive than traditionalsources, 861 sowe’regoing tobe slow toadoptthemunlesstheygetmuchcheaper,orthemarketisskewedintheirfavorbyregulation.Currentlythemarketisskewedintheotherdirection,throughgeneroussubsidiesandbyallowingCO2emissionswithimpunity.

Pielke calculates that in orderto reduceCO2 emissions to half of 1990 levels (required to stabilize the climate) usingnuclearpoweralone,wewouldneedtobuildmorethan12,000newnuclearpowerplantsacross theglobe.That’sabout1perdaycomingonstreamfromnow to2050,which isclearly unrealistic.And that’swithout allowing for the 1.5billionpeoplewho currentlydon’thaveaccesstoelectricitycomingontothegrid(thousandsmorenuclearplants).Inbroadtermsittakes7solarthermalplants,or200windturbinestogeneratetheelectricityof 1 nuclear power plant,[C] so the numbers get even crazier if you substitute thoserenewablesourcesforthenuclearpowerplants.

Intheinterim,convertingpowerstationsacrosstheglobefromcoaltonaturalgaswouldhalveCO2andcutnitrousoxideemissionstoonethirdfromthatsector,whichis

the biggest emitter. 862 863 Butmoving from coal, a cheap and plentiful source of energyemployingmany,ispoliticalpoison,andmuchofthegaswouldcomefromcontroversialfracking.

37Nolet-upinfossilfueluseforelectricitygeneration.Source:U.S.EnergyInformationAdministrationJul2013

Energy efficiency also needs to improve sharply because it allows growth anddecarbonization simultaneously. Edison’s incandescent light bulb technology lasted 130years,butnowwehaveLEDsthatuseafractionoftheelectricity.Weneedmoreofthosekinds of innovation across every energy-consuming sector, from fridges to cars, fromcomputerstoovenstogreenbuildings.

Evenso, theworldwillstillneed“vastlymoreenergy”(Pielke’swords) thanweareabletogenerateevenwithpedal-to-the-metalburningoffossilfuels,ignoringclimatechange.801So,asPielkeobserves,thisisanenergycrisisaswellasaclimatecrisis,whichdemands radical, out-of-the-box thinking in energy generation and conservation. 801 Theproblem with his undeniably true deduction is that it is “pie in the sky” – we’re stillemittingtoomuchCO2whilewewaitforsalvationbythoseelusiverevolutionaryenergysources.

Sowewon’tbeabletomitigateclimatechangebydecarbonizingalonebecausewe can’t afford to wait for new energy sources to come on stream, the scale of thechallengeistoobig,globaldemandforenergyisrisingtoofast,801andbecauseexcessCO2alreadyintheatmospheretodaywillremainthere,warmingtheplanetformanycenturies,unlessitisremovedthroughgeoengineering.

Geoengineeringanditslimits

Geoengineering seeks to reduce climate change by tinkering directly with theclimate. There are two very different kinds: one seeks tomanage the climate by directintervention, and the other seeks to remove CO2 (remediation). Intervention is fraughtwithissues,butremediationlooksessential.801

Climateintervention–theThalidomideofclimatechange

Climate intervention includes things like building reflectors in space, spreadingaerosolsinthestratosphere,enhancingrockweathering,changingthereflectivityoflandorclouds,andevenfertilizingtheocean.864Thesearebestleftintherealmsofspeculation,andhere’swhy.

Thepharmaceutical industryspendsbetween4and11billiondollarsperdrug tobringittomarket.865Itcostssomuchbecausedrugsgothroughrigoroussafetytesting(onanimals,onhealthyvolunteers,dosingstudies,double-blindrandomizedtrials,etc.)beforetheycanbeprescribedtopatients.Thiscostlysystemofpresumingadrugharmfuluntilproven safe was introduced after a relatively untested drug called “Thalidomide” waswidely prescribed to pregnant women in the early 1960s to combat nausea. 866

Unfortunatelyitcausedbirthdefectsinmanybabies,anditwaswithdrawnin1962.Butthatwastoolatefortheaffectedfamilies-thecostofnottestingthoroughlywasinfinitelygreater.

Climate intervention is like prescribing an untested experimental drug, becausethereisgenerallyalimitedunderstandingofthecausallinkbetweentheinterventionanditsassumedeffect.801Theeffectsaresointertwinedwithnaturalprocessesthattheyarenotseparately measurable (scientifically, the experiment has too many dependent variablesandnocontrol).Thismeansthereisnowaytoassesstheirtrueeffectiveness.801

Forexample,oneexperimenttowhitencloudsshowedthatithadbothdesiredandundesired consequences, sometimes generating a net warming, not cooling. 867 Anotherexperiment which injected iron into the ocean successfully fertilized a diatom bloom,whichthendiedandhalfthecreaturessanktothedeepocean,takingcapturedCO2withthem, but there are “important uncertainties” in that study, even without consideringknock-oneffectsonthemarineecosystem.868

Wedon’thavealab-rat-planettotrythese

things out on, 801 and like Thalidomide, the unforeseen consequencesmay far outweightheirgood.Evennewsoftwareisrigorouslytestedbeforeitisreleasedtousersanditcanoften be “rolled back” should something unforeseen occur.Climate intervention has no“undo,”no“rollback,”andnotestplanet.

Remediation

But remediation, which involves removing greenhouse gasses from theenvironment,isamorepromisingenterprise.

Amaincontenderis“CarbonCaptureandStorage,”capturingCO2inpowerplantsmokestacks and injecting it underground.This sounds attractive until you consider thescalesinvolved.TheInternationalEnergyAgencyproposedanambitiousplanin2009tomitigatealargebutunclearquantityofCO2emissionsfromsmokestacksby2050. 869Thenumberstheyenvisionedhavebeencriticizedforbeingwildlyimpractical–spending$5.8trillion, completing 85 projects per year (1 every 4 days), and developing 3400 carboncapturereservoirsusingenoughsteelpipetomake9tripsaroundtheplanet!801SinceCO2scrubbing projects reduce power plant efficiency, add cost, and lower energy output, 869thereneedstobeaneconomicandregulatoryincentivetoimplementthem,butsofartheincentiveshavenotbeensufficienttooutweighthecosts.869Becauseofthis,thetechnologyhasbeenlimitedtodemonstrationprojects.869

CapturingCO2fromtheambientairismorepromising, 801andseveralstartupsareworkingonit. 870Butagain,theseareinprototypemodeandseemalongwayfromlarge-scaledeployment.

CapturingCO2fromambientairiswhatplantshavedonethroughgeologicaltime,withdemonstratedeffectsontheclimateaswesawinYourLifeasPlanetEarth.Thegoodnewsis thatforestscanbeasubstantialsinkforCO2 871but thebadnews is therequiredareaofplantingishuge,andtheeffectisslow.872Withrisingpopulationthechallengewillbetofindlandforforeststhatisn’tneededforagriculture.SomeprogresshasbeenmadeplantingtreesinaridregionsinAfricawitheconomicgainforthelocalpopulation, 873butthiswouldneedtobereplicatedonahugescale.Inshort,wecan’tjustplantourwayoutofthisclimatecrisis.872

AdaptationanditslimitsAdapting to the effects of globalwarming can takemany forms, like protecting

cities from rising sea levels, relocation away from vulnerable shores, and shiftingagriculturelocationsandpractices.

It could cost $32 billion to build coastal defenses to protect just theNewYorkMetroAreafromrisingsealevels. 874Applyingthatfiguretojust30oftheworld’slargestcoastalmetropolisescomes to$1 trillion,withoutprotectinghundredsofsmallercoastalcities and towns.A simple seawall costs about $35million permile,with perhaps $3millionperyearinmaintenance.874TheDutchgovernmentalonespendsabout$1.3billionperyearonfloodcontrol,875afigurelikelytobereplicatedbymoreandmorecitiesastheyseek to adapt to rising sea levels and more frequent storm surges. In Alaska today, 6coastal villages need relocation at a cost of $30 to $50million each, and a further 160

villagesarethreatenedbycoastalerosion.876Tomakemattersworse,withrisingpopulationandprosperitywekeepbuildingincoastalareas,whichincreasesthefinancialandsafetyrisk, even without factoring-in climate change. By 2050 many of the world’s coastalmetropolises will experience around a 20% increase in annual financial losses due tofloodingover2005figures.That’saround$1billionto$3billiondependingonthecity,everyyear.877

Butwehavetoadapttomorethanjustrisingsealevels.

According to “The Economist,” plausible scenarios for the end of the centurycouldseeagricultureandmanufacturingshiftnorthwardsbyabout10degreesoflatitude,equivalenttomovingDallastoChicago,orFrankfurttoOslo. 878Somepartsoftheworldmay become unpleasant or uninhabitable, whereas other areas may remain oases ofmoderatechangeorevenbenignimprovement.878

38Shiftingclimate:thestateofNewHampshirecouldexperienceasummerclimateformerlyassociatedwithSouthCarolinabyend-century,ifbusiness-as-usualemissionscontinue.Source:EPA/GlobalClimateChangeImpactsintheUnitedStates,Karl,etal,Cambridge

UniversityPress,2009.

Iftherateofchangeisslow(ifclimatesensitivityisatthelowend),itispossiblethatasmoothgeographicadjustment-mainlythroughhumanmigration–maybeabletopreserve economic stability. But moves still cost money. A firm leaving say, lowerManhattan for an upstate city,will incur relocation costs, and a general pull-outwouldleave investors in theabandonedareawith large losses.Asmorepeoplemigrate tonew

areas, a significant investment in new infrastructure will be required, even as oldinfrastructure invulnerable areas is abandoned.Given sufficient investment, innovation,anddeterminationwecanprotectcoastalcitiesforawhile,buteventuallythesedefenseswillbeoutflanked,overtoppedorbreached.Thismeansthatsuchcitiesneedtoincentivizepeople to leave vulnerable areas in the medium term, even as they invest in coastaldefenses.878

Thecostofglobalwarmingadaptationwilldependlargelyonhoweasilypeople,andevenwholecities,canrelocate.Economistsprojectthatnortherncountries’agricultureandmanufacturingwillgrow,tradingwithneedysouthernnationswhoseeconomieswilldoworse.Pressuresofmigrationfromsouthernnationsnorthwardsarelikelytoprovokegeopoliticalinstability.878

In themost extremeglobalwarmingscenarios, adaptation will be impotent. Economists project that those worst-casetemperatureswouldreduceagriculturalproductivityinmanyareastozero–acatastropheforhumankindandacosttoohightocontemplate.878

Soeveninthiscursoryglanceatadaptationtoglobalwarming,evenformoderatewarming scenarios, the projected costs are into trillions of dollars, without countingmigration costs or losses from extreme events. Clearly adaptation alone, withoutaddressingremediationanddecarbonization,isnotgoingtobepractical.

TheneedtodothemallAsthesayinggoes:howdoyoueatanelephant?Onebiteatatime!

Wecan’t justcarbon-captureourwayoutofglobalwarmingbecauseof thecostand scale of the projects involved.We can’t rely on climate intervention because of itsrisks.Wecan’tonlyrelyonadaptationbecausetherearelimitstoourabilitytoadapt,anditscostsarehigh.Andwhilewewaitforrevolutionarynewformsofenergytoappear,westilllackthepoliticalwilltodecarbonizeifitslowsgrowth.801

Wealsoknowwedon’thavethecapacity,eitherinfossilfuelsorcleanenergy,tomeetthedemandoftheworld’sgrowingpopulation,andtheimprovedlivingstandardsof

formerly off-grid populations. 801 As Pielke observes: for thosemillions andmillions ofpeople,thisisn’taboutclimatechangebutratherit’saboutaccessingtheenergytheyneed,butthatthecurrentenergysectorisincapableofdelivering,irrespectiveofclimatechange.

Sowehavetodothemall–smartdecarbonization,remediationandadaptation.801

Theyeachhavecosts,butthecostofinactionisfargreater.Theeconomicburdenof inundated coastal cities, diminished agriculture, restricted fresh water, and largesectionsoftheglobalpopulationbecomingnonproductiveenvironmentalrefugees,wouldbeunsupportable.Anddenyingenergytothepeopleofdevelopingcountriesisuntenable.

We need regulatory and financial incentives to innovate clean energy, andefficiency. 801We needmore nuclear power (with newer, safer designs and safe nuclearwaste disposal) and we need much more renewable energy. 801 We need to grasp thepoliticalnettlebyreplacingcoal-firedpowerstationswithnaturalgasstations,withaneyetoreplacingthosetoowithrenewablesinthemediumterm.Weurgentlyneedtoreversedeforestation,andweneedtoplantforestsineveryavailableareaofland,especiallyinthetropics.We need smart power grids to allow a decentralizedmultitude of small energysources, including home and workplace energy generation and storage (through solar,windmills,flowbatteries,etc.).

Populationgrowthurgentlyneedstoslow,sinceitispeoplethatdrivedemandforland,resources,energy,goods,foodandthegeneralcarbon-emittingeconomy.Voluntaryfamilyplanning,andalso improvedeconomicstandingfor traditionalsocieties 879 isvitalforbringingthepopulationdown,andthereforeslowingenergydemandinthelongrun.Oncurrent trendspopulationgrowth isexpected to slowbymid-century,butnotbeforereaching10billion,879comparedto7billionnowandjust2billionasrecentlyas1930.

We need to deploy carbon capture in every feasible location where it can bepowered by clean energy.We should be as concerned with reducing our nitrous oxidefootprint,ourmethanefootprint,oursootfootprintandourfluorocarbonfootprintasourCO2 footprint. Just reducing those four pollutants would help reduce the rate ofwarming. 880 The Montreal Protocol succeeded in controlling ozone-eating gasses – asimilartreatycouldtacklethose.801

Eachsteprepresentsa“bite”outofourclimatechange“elephant.”

But even if we do, miraculously,stabilizeCO2emissions, thewarmingeffectofgreenhousegassesalready in theairwillcontinueformanyyears.AsMcKibbenobserves,wearealreadylivingonastrangenewplanet. 859Sowemustalsoadapt,withstrategiestoenablemigrationawayfromadverselyaffectedregionstomorebenignareas.Wedoindeedneedtoinvestinamoreresilient,lesscentralizedeconomy.Delayingactiononlymultiplies thecosts, and thecostof inactionmaywellexceedourabilitytoadapt.

Wecannotaffordtowaitforeveryuncertaintytoberesolved.Mobilizingnowisaninsurancepolicyagainstanuncertainfuture.876

Bottomline:

“Thegreatestdangertoourfutureisapathy.”―JaneGoodall

Surviving climate change requires more than our own individual incrementalenergy efficiencies. It requires strategic thinking and actual action by governments. Itrequireswholesectorsoftheeconomytoapproachenergyusedifferently.Itrequiresthekindoftransformationintheenergygeneratingsectorthattheassemblylineorcomputersdeliveredtobusinessesinthe20thcentury.Butnoneof thatwillhappenunlessyouasavoter,consumerandshareholderinsistonit.

And climate change is just one of the Earth hazards that humanity faces.Earthquakes, volcanic eruptions, landslides, sinkholes and asteroid impacts can only beunderstood,predictedandultimatelymitigatedthroughscience.Werelyonsciencetofindandrecoverthemineralresourcesthataretherawmaterialsformanufacturingeverythingfrombuildingstoelectronics,manyofwhichareprojectedtobeexhaustedbytheendofthiscentury.881

Ittakespoliticalleadershiptoactonscientificinformation,andscienceneedsoursupporttoassessthesehazardsforthesakeofourownprosperityandourgrandchildren’sfuture.

Buttoomanyofourcurrentcropofpoliticalleadershaveacompletedisregardforscience.Inthelastfewyearsalone,wehaveseenelectedofficialsinAmericagoonrecordwithbizarrepronouncementsthathavenobasisinfact,882evenclaimingprovenscienceis

“lies straight from thepitofHell”! 883 It’s not just inAmerica – judges in Italy recentlyjailedscientistsbecausegovernmentofficialsfailedtograspthemeaningoftheiradvice.884

Forthesakeofourgrandchildren–andtheirs-weneedtoreleasethebrakesonour progress caused by pseudoscience and political propaganda created by themanufactured doubt industry and fundamentalists for ignorant politicians to regurgitate.Makingallowanceforreligioustoleranceorpoliticalopinionshouldnotextendtodenyingor distorting scientific fact. Carrying the burden of ignorance in the upper echelons ofpowerconstitutesanenormousdragonsociety’sabilitytorespondtothemostprofoundeconomicandsecuritychallengesofourtime.

Afterall,as thesayinggoes:“Wedonot inherit the landfromourancestors,weborrowitfromourchildren,”885orinthecaseofclimatechange,ourgrandchildren.

###

[A] Incidentally,my fatherwas at the first excavation ofKenyapithecuswickeriby Louis Leakey at Fort Ternan inKenya.

[B]Iwouldn’tblameyouforbeingskepticalofthesefigures,sohere’swheretheycamefrom.TheenergybuildupintheatmospherecomesfromNOAA’sAnnualGreenhouseGasIndex(AGGI).741For2013theadditionalheatenergysince1750wascalculatedbyNOAAtobe2.916wattsforeverysquaremeterontheplanet,whichisthesameas2.916joulesper squaremeterper second.All that remains is tomultiply thatby thenumberof squaremeterscovering theEarthandthenumberofsecondsinayeartoget4.69x1022extrajoulesgloballyoverayear.TheHiroshimabombwasequivalenttoabout0.02megatonsofTNT,whichinjoulesis8.37x1013.DividingNOAA’sextraenergyfigurebythisnumberarrivesatabout561millionHiroshimabombs.Thisdoesnotallowforthecoolingbyvariousfeedbacks–doingsoreducestheenergyimbalancetosomethinglike180millionHiroshimabombsperyear.Stillalot.

[C]Usingroughrepresentativevaluesof1,000MWfornuclear,150MWforsolarthermal,5MWforwindturbine

AcknowledgementIamindebtedtoanumberofpeoplewhoansweredquestions,suggestedpapersto

include, or provided feedback on early drafts, including: Emily Pope, Bill Ruddiman,Eelco Rohling, Oliver Jagoutz, RachelWood, John Cook, Luis Aldamiz, PietMartens,DuncanMurdock,RobinWordsworth,DanielBreecker,ScottJasechko,GuillaumeSuan,RobertGaines,EberhardFaust,HartmutAumann,GraemeStephens,RogerPielke,ErikAsphaug, Dana Royer, Clara L. Blättler, Lev Horodyskyj, Richard Roberts, MargaretCollinson,HenrikSvensen,AndyRidgwell,LindyElkins-Tanton,andGertaKeller.

Icouldn’thavewrittenthisatallwithouttheencouragementofHenryKavett,andwithoutthepatientsupportofmywife,Mojgan.

SourcesFirstEdition.Visitylape.comforcitationlistandmostup-to-datelinks.

Geologicaltimescale:

InternationalStratigraphicChart2012,InternationalCommissiononStratigraphy,Availableat:http://www.stratigraphy.org/index.php/ics-chart-timescale

Globalchronostratigraphicalcorrelationtableforthelast2.7millionyears,v2010,International Commission on Stratigraphy, Available at:http://www.stratigraphy.org/index.php/ics-chart-timescale

Somedevicestruncatethistableofreferences.Ifyouareexperiencingthisproblem,pleaserotatetolandscapeorientationand/ortryasmallerfont.

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AboutTheAuthor

HOWARDLEEB.ScM.ScFGS

HowardisafreelanceEarthsciencewriterandbloggeronpastclimates.HeearnedhisB.ScinGeologyandhisM.ScinRemoteSensingattheUniversityofLondon,andheis a Fellow of the Geological Society of London (FGS). Howard draws upon years ofexperience in diverse Earth Science industries (seismic hazard, geothermal exploration,miningandradioactivewastedisposal,oilandgasexploration)forhiswriting.

HedecidedtowriteYourLifeasPlanetEarthafteraraftingtripthroughtheGrandCanyon with his son’s Boy Scout troop. In explaining the geological wonders thatunfoldedaroundthemeveryday,herealizedtherewasaneedtopresentthestoryoftheplanetinanapproachableway,connectingthepasttotheworldoftoday,particularlywithrespecttoclimatechange.

Hisother interest isAncientPersia,andhis firstbook forchildren:JamshidandtheLostMountainofLight, earnedHoward a “PersianGoldenLionessAward” asBestChildren’sBook.

HowardisaKenya-bornBritish-AmericanwholivesinNewJerseywithhiswifeandtheirtwosons.