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ring out of the water； too。 popular illustrations haveencouraged us to envision the first venturesome land dwellers as a kind of ambitious fish鈥攕omething like the modern mudskipper； which can hop from puddle to puddle duringdroughts鈥攐r 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鈥檚 the same chemical trick that goes on in （and is discussed elsewhere in relationto） the long…term carbon cycle鈥攁 process that doesn鈥檛 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鈥攐xygen…16 and oxygen…18。
（if you have forgotten what an isotope is； it doesn鈥檛 matter； though for the record it鈥檚 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鈥攐xygen levels； air and oceantemperatures； the extent and timing of ice ages； and much else。 by bining their isotopefindings with other fossil residues鈥攑ollen levels and so on鈥攕cientists 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鈥檚 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鈥檛 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。 鈥渢he u。s。 air force；鈥潯ne mentator has written； 鈥渉as put them inwind tunnels to see how they do it； and despaired。鈥潯hey； 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鈥攚hich is to say； the first land animals from which wewould derive鈥攁re 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鈥攁ll 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鈥檚paleontologists 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鈥攖here were actually eighton each limb鈥攁nd 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鈥檛 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鈥櫋kulls。 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鈥檚 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 pete head to head with these aggressive new creatures； thetherapsids by and large vanished from the record。 a very few； however； evolved into small；furry； burrowing beings that bided their time for a very long while as little mammals。 thebigges