ÿWPCq  E3§)Y‡\–A¾qE·Ü*€šz„Ʋܵ=“¢xq…\r`6Æ7ë‹úжñ•!ƒ{lþŸ¬Žqq/hÁ 3ó>Yè¿oqC VÞv)ÎóÖÚí0ñ|YhÞUWA>B%‡¦ãIx¬/Ž*ùìÑ^“€'ÏÁ ®jʆ¼A¿ѓ,è:NI´§ãD{Ìþm7Ä ‹ ²c-)EŠñ,aµð5=‰B c0‰°9Âã&XùX>ðÞýÊ(ô¥Âõ¿º%jÏX|på¶þi)XÐݨ†¡vùr;ÿ<ÔžL9µÍP Ë>#nUZg"œŠlæÅvH¬“O«-TéHŠ®ÒAZò)Üƅd¿³ç8¨Ô]¯w›stR™´Û“Npû¯â”^f g"åɊ_c¤oç=UzèÝaJ^¥8ÓÓÖêEŽÌ¥áǕäåŒÏ4. nbž WÈM“­ôL{à°¡µלâsŸÌW¤f”¬1“-AC Eb)À­¡r=S¡÷&i‡¿Wę¾û‡Q›®âØAi~2fÆYžÙ)uEÛ`ØaÎõ[¡þ|¤úªÍÓ@2ÿm̌±Q _ù©Æ­§,,Xdâ¤ó5ö€B«–ª êYà4A%#!4UNU %£ 0±©w@Z4š®½ 0ÍÏNœ hžx D+†U>±^ ïû mý B E1 B>3 ˜R5WCB1_15-06,ü,,,,,ü0(ÖÃ9 Z ‹6Times New Roman RegularX(‰Šì$¡¡ÓK€(€X°KÓÔ€Xš…XXXÔÔ€Xš…XXXš…Ô rasDtDu[v[wDx[y[zÀ{gg}'3|x{ Ý ƒ ¯'ÝÝ  ÝÔ_ÔÓ  Óò òòòÔ‡ —‹!XXÔAn€Overview€of€Early€Geologic€Timeó óóóÔ#†XX! —‹=#ÔÐ ° Ðòò(From€the€University€of€California€at€Berkeleyððs€Internet€Web€site)óó(¥3<$§§Ý ƒŠì!ÝÓK€(€X°KÓÔ€XXXXÔÔ€XXXXÔÝ  Ý ˜c:\COREL\SUITE8\TEMPLATE\CUSTOM~1\WEB\WP8WEB.WPTc:\Corel\Suite8\Template\Custom WP Templates\Web\wp8web.wpt'ÈÈÈÈdxd(ÖÃ9 Z‹&Times New Romanâ¥'äÿU‹ÿÿÿÿ d:web3dhrz160 €€€d ÿÿÿdÝ ƒŠì!ÝÓK€(€XKÓÔ€XXXXÔÔ€XXXXÔÝ  ÝÔ_ÔÒX°ÒÖÀÿÿÖò òÑ8ÀXXdìdÈ8ÑÓ  ÓñzñÔ‡°®+°XXÔñzñOVERVIEW€OF€GEOLOGIC€TIMEñzñÔ#†XX°°®+##ÔñzñÐ ÜÜ Ðñ{ñßA€) °°xd†˜Ex@0¬&Aßñ{ññ{ñßA€) °°xpExø0¬&AßÐ °° Ðñ{ñòòó óBecause€of€the€relative€scarcity€of€geological€and€fossil€evidence€from€early€geologic€time,€the€word€ð ðPrecambrianðð€hasÐ    Ðbeen€traditionally€used€to€refer€to€earth€history€before€about€540€million€years€ago.€€€The€following€is€a€brief€description€ofÏthe€subdivisions€of€Precambrian€time.€ò òóóÐ xx ÐI.€Hadean€time€(4.5€to€3.8€billion€years€ago)ó óÐ @@ Ðà @ àHadean€time€is€not€a€geological€period€as€such.€€No€rocks€on€the€Earth€are€this€old€-€except€for€meteorites.€During€HadeanÏtime,€the€Solar€System€was€forming,€probably€within€a€large€cloud€of€gas€and€dust€around€the€sun,€called€an€accretion€disc.ÏThe€relative€abundance€of€heavier€elements€in€the€Solar€System€suggests€that€this€gas€and€dust€was€derived€from€aÏsupernova,€or€supernovas€-€the€explosion€of€an€old,€massive€star.€Heavier€elements€are€generated€within€stars€by€nuclearÏfusion€of€hydrogen,€and€are€otherwise€uncommon.€We€can€see€similar€processes€taking€place€today€in€so-called€diffuseÏnebulae€in€this€and€other€galaxies€-€such€as€the€nebula€M16.Ìà @ àThe€sun€formed€within€such€a€cloud€of€gas€and€dust,€shrinking€in€on€itself€by€gravitational€compaction€until€it€began€toÏundergo€nuclear€fusion€and€give€off€light€and€heat.€Surrounding€particles€began€to€coalesce€by€gravity€into€larger€lumps,€orÏplanetesimals,€which€continued€to€aggregate€into€planets.€"Left-over"€material€formed€asteroids€and€comets.Ìà @ àBecause€collisions€between€large€planetesimals€release€a€lot€of€heat,€the€Earth€and€other€planets€would€have€been€molten€atÏthe€beginning€of€their€histories.€Radiometric€dating€of€rocks€dates€igneous€rocks€from€the€time€of€solidification;€thisÏexplains€why€the€oldest€meteorites€and€lunar€rocks€are€about€4.5€billion€years€old,€but€the€oldest€Earth€rocks€currentlyÏknown€are€3.8€billion€years€old.€€During€the€first€800€million€or€so€years€of€its€history,€the€Earth€was€constantly€beingÏbombarded€by€planetesimals€and€would€have€been€molten.€The€age€of€the€oldest€Earth€rock€corresponds€with€the€age€of€theÏlast€major€episode€of€bombardment€of€the€Moon.€Once€most€of€the€planetesimals€had€been€swept€up€by€the€growingÏplanets,€solid€rock€formed€on€the€Earth,€and€its€own€geological€history€began€--€a€time€known€as€the€Archaean.ÌßA€) °°xd&0Ex¤0¬&AßÐ \\ Ðò òII.€Archaean€Era€(3.8€to€2.5€billion€years€ago)ó óÐ (( Ðà @ àIf€you€were€able€to€travel€back€to€visit€the€Earth€during€the€Archaean€Era,€you€would€likely€not€recognize€it€is€the€sameÏplanet€we€inhabit€today.€The€atmosphere€was€very€different€from€what€we€breathe€today;€at€that€time,€it€was€likely€aÏreducing€atmosphere€of€methane,€ammonia,€and€other€gases€which€would€be€toxic€to€most€life€on€our€planet€today.€AlsoÏduring€this€time,€the€Earth's€crust€cooled€enough€that€rocks€and€continental€plates€began€to€form.Ìà @ àIt€was€early€in€the€Archaean€that€life€first€appeared€on€Earth.€Our€oldest€fossils€date€to€roughly€3.5€billion€years€ago,€andÏconsist€of€bacteria€microfossils.€In€fact,€all€life€during€the€more€than€one€billion€years€of€this€Era€was€bacterial.€€Dating€backÏas€far€as€3.5€billion€years,€mound„like€rock€formations€called€Ô_ÔstromatolitesÔ_Ô,€made€by€ancient€colonies€of€Ô_ÔphotosyntheticÔ_ÔÏbacteria,€have€been€found€as€fossils€in€Early€Archaean€rocks€of€South€Africa€and€Western€Australia.€Ô_ÔStromatolitesÔ_ԀincreasedÏin€abundance€throughout€the€Archaean,€but€began€to€decline€during€the€Proterozoic.€They€are€not€common€today.ÌßA€) °°xd&0Ex<(0¬&AßÐ ô'ô'! Ðò òIII.€Proterozoic€Era:€(2.5€billion€„€544€million€years€ago)Ð À)À)" Ðó óà @ àThe€period€of€Earth's€history€that€began€2.5€billion€years€ago€and€ended€544€million€years€ago€is€known€as€the€Proterozoic;Ð ˆ+ˆ+# Ðit€is€divided€up,€rather€arbitrarily,€into€the€Ô_ÔPaleoproterozoicÔ_Ԁ(2.5€to€1.6€billion€years€ago),€Ô_ÔMesoproterozoicÔ_Ԁ(1.6€billion€toÏ900€million€years€ago)€and€Ô_ÔNeoproterozoicÔ_Ԁ(900€to€544€million€years€ago).€Near€the€beginning€of€the€Proterozoic,€stableÏcontinents€first€appeared€and€began€to€accrete,€along€process€taking€about€a€billion€years.Ìòòò òAncient€Global€Pollution€(Creation€of€Atmospheric€Oxygen)ó óóóÐ 00' Ðà @ àThe€first€"pollution€crisis"€hit€the€Earth€about€2.2€billion€years€ago.€€Several€pieces€of€evidence€--€the€presence€of€ironÐ Ø1Ø1( Ðoxides€in€Ô_ÔpaleosolsÔ_Ԁ(fossil€soils),€the€appearance€of€"red€beds"€containing€metal€oxides,€and€others€-€point€to€a€fairly€rapidÏincrease€in€levels€of€oxygen€in€the€atmosphere€at€about€this€time.€Oxygen€levels€in€the€Archaean€had€been€less€that€1%€ofÏpresent€levels€in€the€atmosphere,€but€by€about€1.8€billion€years€ago,€oxygen€levels€were€greater€than€15%€of€present€levelsÏand€rising.€€€(Holland,€1994)€It€may€seem€strange€to€call€this€a€"pollution€crisis,"€since€most€of€the€organisms€that€we€areÏfamiliar€with€not€only€tolerate€but€require€oxygen€to€live.€However,€oxygen€is€a€powerful€degrader€of€organic€compounds.ÏEven€today,€many€bacteria€and€protists€(single„celled€organisms)€are€killed€by€oxygen.€€Organisms€had€to€evolveÏbiochemical€methods€for€rendering€oxygen€harmless;€one€of€these€methods,€oxidative€respiration,€had€the€advantage€ofÏproducing€large€amounts€of€energy€for€the€cell,€and€is€now€found€in€most€Ô_ÔeukaryotesÔ_Ô.Ìà @ àWhere€was€this€oxygen€coming€from?€Cyanobacteria€(ð ðblue„green€algaeðð),€Ô_ÔphotosyntheticÔ_Ԁorganisms€that€produce€oxygenÏas€a€by„product,€had€first€appeared€3.5€billion€years€ago,€but€became€common€and€widespread€in€the€Proterozoic.€TheirÏÔ_ÔphotosyntheticÔ_Ԁactivity€was€primarily€responsible€for€the€rise€in€atmospheric€oxygen.€€òòThe€transformation€to€an€oxygenÐ ð ð  Ðatmosphere€enabled€the€eventual€formation€of€an€ozone€layer€in€the€upper€atmosphere,€which€filtered€out€deadly€ultravioletÏrays€from€the€sun€and€made€life€on€land€(a€much€later€development)€possible.€óóÐ È È  ÐßA€) °°xd&0ExØ0¬&AßÐ   Ðòòò òThe€Ô_ÔVendianÔ_Ԁ(Ô_ÔEdicarianÔ_Ô)€Period€(650€to€540€Million€Years€Ago)óóó óÐ \\ Ðà @ àWhen€Charles€Darwin€wrote€ð ðOn€the€Origin€of€Speciesðð€in€1859,€€he€and€most€paleontologists€believed€that€the€oldestÏanimal€fossils€were€the€trilobites€and€brachiopods€of€the€Cambrian€Period,€now€known€to€be€about€540€million€years€old.ÏMany€paleontologists€believed€that€simpler€forms€of€life€must€have€existed€before€this€but€that€they€left€no€fossils.€A€fewÏbelieved€that€the€Cambrian€fossils€represented€the€moment€of€God's€creation€of€animals,€or€the€first€deposits€laid€down€byÏthe€biblical€Flood.€Darwin€wrote,€"the€difficulty€of€assigning€any€good€reason€for€the€absence€of€vast€piles€of€strata€rich€inÏfossils€beneath€the€Cambrian€system€is€very€great,"€yet€he€expressed€hope€that€such€fossils€would€be€found,€noting€thatÏ"only€a€small€portion€of€the€world€is€known€with€accuracy."€€Since€Darwin's€time,€the€fossil€history€of€life€on€Earth€hasÏbeen€pushed€back€to€3.5€billion€years€before€the€present.€Most€of€these€fossils€are€microscopic€bacteria€and€algae.€However,Ïin€the€latest€Proterozoic€-€a€€time€period€now€called€the€Ô_ÔVendianÔ_Ô,€or€the€Ô_ÔEdiacaranÔ_Ô,€and€lasting€from€about€650€to€540Ïmillion€years€ago€-€macroscopic€fossils€of€soft-bodied€organisms€can€be€found€in€a€few€localities€around€the€world,Ïconfirming€Darwin's€expectations.òò€€€(The€Ô_ÔVendianÔ_ԀPeriod€was€thought€to€be€a€€period€inhabited€by€very€unusual€soft„bodiedÐ XX Ðcreatures,€most€of€which€ultimately€died€out€and€left€no€descendants,€but€this€period€also€spawned€the€earliest€ancestors€ofÏlife€on€the€present„day€Earth.)óóÐ 00 ÐñrñßA€) °°xd&0Ex@0¬&AßñrñÐ øø Ðò òIV.€Paleozoic€Era€(544€to€240€million€years€ago)ó óÐ Ä Ä  ÐñsñÌñsñà @ àòòThe€ñuñò òñuñPaleozoic€Erañuñó óñuñ€(ð ðera€of€ancient€lifeðð)€begins€with€the€Cambrian€Period,€beginning€about€544€million€years€ago.óó€€TheÐ Œ"Œ" ÐCambrian€Period€marks€an€important€point€in€the€history€of€life€on€earth;€it€is€the€time€when€most€of€the€major€groups€ofÏanimals€first€appear€in€the€fossil€record.€This€event€is€sometimes€called€the€"Cambrian€Explosion",€because€of€the€relativelyÏshort€time€over€which€this€diversity€of€forms€appears.€It€was€once€thought€that€the€Cambrian€rocks€contained€the€first€andÏoldest€fossil€animals,€but€these€are€now€to€be€found€in€the€earlier€Ô_ÔVendianÔ_Ԁstrata.ÌñtñÌñtñà @ àòòThe€most€important€change€in€the€evolution€of€life€that€first€appeared€during€the€Cambrian€was€the€€appearance€ofñvñò òñvñ€óó€hardÐ ((# Ðpartsñvñó óñvñòò,€which€were€much€better€preserved€as€fossils€than€were€soft„bodied€creatures€from€earlier€times.€€€This€is€the€primaryÐ ô(ô($ Ðreason€why€Precambrian€fossils€are€so€rare€„€itððs€not€because€they€werenððt€there€„€they€simply€left€no€traces€of€theirÏexistence€(except€through€the€most€extraordinary€good€fortune)€due€to€the€lack€of€hard€parts.€€ÌñwñÌñwñà @ àPaleozoic€life€lasted€for€more€than€300€million€years,€until€the€end€of€the€Permian€Period€(240€million€years€ago),€whichÏresulted€in€the€greatest€known€mass€extinction€of€life€on€Earth€„€it€is€thought€that€more€than€90%€of€all€life€perished€beforeÏthe€survivors€and€their€descendants€lived€during€the€next€great€geologic€era,€the€ñyñò òñyñMesozoicñyñó óñyñ,€also€known€as€the€ñxñò òñxñAge€ofÐ p.p.) ÐReptiles.ñxñó óñxñóó