A Short History of Nearly Everything-第73章

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ndand　it　sags　in　the　middle；　its　backbone　too　weak　to　support　it。　to　survive　out　of　water；　marinecreatures　needed　to　e　up　with　new　load…bearing　internal　architecture—not　the　sort　ofadjustment　that　happens　overnight。　above　all　and　most　obviously；　any　land　creature　wouldhave　to　develop　a　way　to　take　its　oxygen　directly　from　the　air　rather　than　filter　it　from　water。

these　were　not　trivial　challenges　to　overe。　on　the　other　hand；　there　was　a　powerfulincentive　to　leave　the　water：　it　was　getting　dangerous　down　there。　the　slow　fusion　of　thecontinents　into　a　single　landmass；　pangaea；　meant　there　was　much；　much　less　coastline　thanformerly　and　thus　much　less　coastal　habitat。　so　petition　was　fierce。　there　was　also　anomnivorous　and　unsettling　new　type　of　predator　on　the　scene；　one　so　perfectly　designed　forattack　that　it　has　scarcely　changed　in　all　the　long　eons　since　its　emergence：　the　shark。　neverwould　there　be　a　more　propitious　time　to　find　an　alternative　environment　to　water。

plants　began　the　process　of　land　colonization　about　450　million　years　ago；　acpanied　ofnecessity　by　tiny　mites　and　other　organisms　that　they　needed　to　break　down　and　recycle　deadorganic　matter　on　their　behalf。　larger　animals　took　a　little　longer　to　emerge；　but　by　about　400million　years　ago　they　were　venturing　out　of　the　water；　too。　popular　illustrations　haveencouraged　us　to　envision　the　first　venturesome　land　dwellers　as　a　kind　of　ambitious　fish—something　like　the　modern　mudskipper；　which　can　hop　from　puddle　to　puddle　duringdroughts—or　even　as　a　fully　formed　amphibian。　in　fact；　the　first　visible　mobile　residents　ondry　land　were　probably　much　more　like　modern　wood　lice；　sometimes　also　known　as　pillbugsor　sow　bugs。　these　are　the　little　bugs　（crustaceans；　in　fact）　that　are　monly　thrown　intoconfusion　when　you　upturn　a　rock　or　log。

for　those　that　learned　to　breathe　oxygen　from　the　air；　times　were　good。　oxygen　levels　inthe　devonian　and　carboniferous　periods；　when　terrestrial　life　first　bloomed；　were　as　high　as35　percent　（as　opposed　to　nearer　20　percent　now）。　this　allowed　animals　to　grow　remarkablylarge　remarkably　quickly。

and　how；　you　may　reasonably　wonder；　can　scientists　know　what　oxygen　levels　were　likehundreds　of　millions　of　years　ago？　the　answer　lies　in　a　slightly　obscure　but　ingenious　fieldknown　as　isotope　geochemistry。　the　long…ago　seas　of　the　carboniferous　and　devonianswarmed　with　tiny　plankton　that　wrapped　themselves　inside　tiny　protective　shells。　then；　asnow；　the　plankton　created　their　shells　by　drawing　oxygen　from　the　atmosphere　and　biningit　with　other　elements　（carbon　especially）　to　form　durable　pounds　such　as　calciumcarbonate。　it’s　the　same　chemical　trick　that　goes　on　in　（and　is　discussed　elsewhere　in　relationto）　the　long…term　carbon　cycle—a　process　that　doesn’t　make　for　terribly　exciting　narrative　butis　vital　for　creating　a　livable　planet。

eventually　in　this　process　all　the　tiny　organisms　die　and　drift　to　the　bottom　of　the　sea；where　they　are　slowly　pressed　into　limestone。　among　the　tiny　atomic　structures　theplankton　take　to　the　grave　with　them　are　two　very　stable　isotopes—oxygen…16　and　oxygen…18。

（if　you　have　forgotten　what　an　isotope　is；　it　doesn’t　matter；　though　for　the　record　it’s　an　atomwith　an　abnormal　number　of　neutrons。）　this　is　where　the　geochemists　e　in；　for　theisotopes　accumulate　at　different　rates　depending　on　how　much　oxygen　or　carbon　dioxide　is　inthe　atmosphere　at　the　time　of　their　creation。　by　paring　these　ancient　ratios；　thegeochemists　can　cunningly　read　conditions　in　the　ancient　world—oxygen　levels；　air　and　oceantemperatures；　the　extent　and　timing　of　ice　ages；　and　much　else。　by　bining　their　isotopefindings　with　other　fossil　residues—pollen　levels　and　so　on—scientists　can；　with　considerableconfidence；　re…create　entire　landscapes　that　no　human　eye　ever　saw。

the　principal　reason　oxygen　levels　were　able　to　build　up　so　robustly　throughout　the　periodof　early　terrestrial　life　was　that　much　of　the　world’s　landscape　was　dominated　by　giant　treeferns　and　vast　swamps；　which　by　their　boggy　nature　disrupted　the　normal　carbon　recyclingprocess。　instead　of　pletely　rotting　down；　falling　fronds　and　other　dead　vegetative　matteraccumulated　in　rich；　wet　sediments；　which　were　eventually　squeezed　into　the　vast　coal　bedsthat　sustain　much　economic　activity　even　now。

the　heady　levels　of　oxygen　clearly　encouraged　outsized　growth。　the　oldest　indication　of　asurface　animal　yet　found　is　a　track　left　350　million　years　ago　by　a　millipede…like　creature　on　arock　in　scotland。　it　was　over　three　feet　long。　before　the　era　was　out　some　millipedes　wouldreach　lengths　more　than　double　that。

with　such　creatures　on　the　prowl；　it　is　perhaps　not　surprising　that　insects　in　the　periodevolved　a　trick　that　could　keep　them　safely　out　of　tongue　shot：　they　learned　to　fly。　some　tookto　this　new　means　of　lootion　with　such　uncanny　facility　that　they　haven’t　changed　theirtechniques　in　all　the　time　since。　then；　as　now；　dragonflies　could　cruise　at　up　to　thirty…fivemiles　an　hour；　instantly　stop；　hover；　fly　backwards；　and　lift　far　more　proportionately　than　anyhuman　flying　machine。　“the　u。s。　air　force；”　one　mentator　has　written；　“has　put　them　inwind　tunnels　to　see　how　they　do　it；　and　despaired。”　they；　too；　gorged　on　the　rich　air。　incarboniferous　forests　dragonflies　grew　as　big　as　ravens。　trees　and　other　vegetation　likewiseattained　outsized　proportions。　horsetails　and　tree　ferns　grew　to　heights　of　fifty　feet；　clubmosses　to　a　hundred　and　thirty。

the　first　terrestrial　vertebrates—which　is　to　say；　the　first　land　animals　from　which　wewould　derive—are　something　of　a　mystery。　this　is　partly　because　of　a　shortage　of　relevantfossils；　but　partly　also　because　of　an　idiosyncratic　swede　named　erik　jarvik　whose　oddinterpretations　and　secretive　manner　held　back　progress　on　this　question　for　almost　half　acentury。　jarvik　was　part　of　a　team　of　scandinavian　scholars　who　went　to　greenland　in　the1930s　and　1940s　looking　for　fossil　fish。　in　particular　they　sought　lobe…finned　fish　of　the　typethat　presumably　were　ancestral　to　us　and　all　other　walking　creatures；　known　as　tetrapods。

most　animals　are　tetrapods；　and　all　living　tetrapods　have　one　thing　in　mon：　four　limbsthat　end　in　a　maximum　of　five　fingers　or　toes。　dinosaurs；　whales；　birds；　humans；　even　fish—all　are　tetrapods；　which　clearly　suggests　they　e　from　a　single　mon　ancestor。　the　clueto　this　ancestor；　it　was　assumed；　would　be　found　in　the　devonian　era；　from　about　400　millionyears　ago。　before　that　time　nothing　walked　on　land。　after　that　time　lots　of　things　did。　luckilythe　team　found　just　such　a　creature；　a　three…foot…long　animal　called　an　ichthyostega。　theanalysis　of　the　fossil　fell　to　jarvik；　who　began　his　study　in　1948　and　kept　at　it　for　the　nextforty…eight　years。　unfortunately；　jarvik　refused　to　let　anyone　study　his　tetrapod。　the　world’spaleontologists　had　to　be　content　with　two　sketchy　interim　papers　in　which　jarvik　noted　thatthe　creature　had　five　fingers　in　each　of　four　limbs；　confirming　its　ancestral　importance。

jarvik　died　in　1998。　after　his　death；　other　paleontologists　eagerly　examined　the　specimenand　found　that　jarvik　had　severely　miscounted　the　fingers　and　toes—there　were　actually　eighton　each　limb—and　failed　to　observe　that　the　fish　could　not　possibly　have　walked。　thestructure　of　the　fin　was　such　that　it　would　have　collapsed　under　its　own　weight。　needless　tosay；　this　did　not　do　a　great　deal　to　advance　our　understanding　of　the　first　land　animals。　todaythree　early　tetrapods　are　known　and　none　has　five　digits。　in　short；　we　don’t　know　quite　wherewe　came　from。

but　e　we　did；　though　reaching　our　present　state　of　eminence　has　not　of　course　alwaysbeen　straightforward。　since　life　on　land　began；　it　has　consisted　of　four　megadynasties；　as　theyare　sometimes　called。　the　first　consisted　of　primitive；　plodding　but　sometimes　fairly　heftyamphibians　and　reptiles。　the　best…known　animal　of　this　age　was　the　dimetrodon；　a　sail…backed　creature　that　is　monly　confused　with　dinosaurs　（including；　i　note；　in　a　picturecaption　in　the　carl　sagan　book　et）。　the　dimetrodon　was　in　fact　a　synapsid。　so；　onceupon　a　time；　were　we。　synapsids　were　one　of　the　four　main　divisions　of　early　reptilian　life；the　others　being　anapsids；　euryapsids；　and　diapsids。　the　names　simply　refer　to　the　number　andlocation　of　small　holes　to　be　found　in　the　sides　of　their　owners’　skulls。　synapsids　had　one　holein　their　lower　temples；　diapsids　had　two；　euryapsids　had　a　single　hole　higher　up。

over　time；　each　of　these　principal　groupings　split　into　further　subdivisions；　of　which　someprospered　and　some　faltered。　anapsids　gave　rise　to　the　turtles；　which　for　a　time；　perhaps　atouch　improbably；　appeared　poised　to　predominate　as　the　planet’s　most　advanced　and　deadlyspecies；　before　an　evolutionary　lurch　let　them　settle　for　durability　rather　than　dominance。　thesynapsids　divided　into　four　streams；　only　one　of　which　survived　beyond　the　permian。

happily；　that　was　the　stream　we　belonged　to；　and　it　evolved　into　a　family　of　protomammalsknown　as　therapsids。　these　formed　megadynasty　2。

unfortunately　for　the　therapsids；　their　cousins　the　diapsids　were　also　productively　evolving；in　their　case　into　dinosaurs　（among　other　things）；　which　gradually　proved　too　much　for　thetherapsids。　unable　to