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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 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。 thebiggest of them grew no larger than a house cat； and most were no bigger than mice。
eventually； this would prove their salvation； but they would have to wait nearly 150 millionyears for megadynasty 3； the age of dinosaurs； to e to an abrupt end and make room formegadynasty 4 and our own age of mammals。
each of these massive transformations； as well as many smaller ones between and since；was dependent on that paradoxically important motor of progress： extinction。 it is a curiousfact that on earth species death is； in the most literal sense； a way of life。 no one knows howmany species of organisms have existed since life began。 thirty billion is a monly citedfigure； but the number has been put as high as 4；000 billion。 whatever the actual total； 99。99percent of all specie