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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 species that have ever lived are no longer with us。 “to a first approximation；” asdavid raup of the university of chicago likes to say； “all species are extinct。” for plexorganisms； the average lifespan of a species is only about four million years—roughly aboutwhere we are now。
extinction is always bad news for the victims； of course； but it appears to be a good thingfor a dynamic planet。 “the alternative to extinction is stagnation；” says ian tattersall of theamerican museum of natural history； “and stagnation is seldom a good thing in any realm。”
（i should perhaps note that we are speaking here of extinction as a natural； long…term process。
extinction brought about by human carelessness is another matter altogether。）crises in earth’s history are invariably associated with dramatic leaps afterward。 the fall ofthe ediacaran fauna was followed by the creative outburst of the cambrian period。 theordovician extinction of 440 million years ago cleared the oceans of a lot of immobile filterfeeders and； somehow； created conditions that favored darting fish and giant aquatic reptiles。
these in turn were in an ideal position to send colonists onto dry land when another blowoutin the late devonian period gave life another sound shaking。 and so it has gone at scatteredintervals through history。 if most of these events hadn’t happened just as they did； just whenthey did； we almost certainly wouldn’t be here now。
earth has seen five major extinction episodes in its time—the ordovician； devonian；permian； triassic； and cretaceous； in that order—and many smaller ones。 the ordovician（440 million years ago） and devonian （365 million） each wiped out about 80 to 85 percent ofspecies。 the triassic （210 million years ago） and cretaceous （65 million years） each wipedout 70 to 75 percent of species。 but the real whopper was the permian extinction of about 245million years ago； which raised the curtain on the long age of the dinosaurs。 in the permian； at least 95 percent of animals known from the fossil record ch