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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