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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。 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。 鈥渢o a first approximation；鈥潯sdavid raup of the university of chicago likes to say； 鈥渁ll species are extinct。鈥潯or plexorganisms； the average lifespan of a species is only about four million years鈥攔oughly 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。 鈥渢he alternative to extinction is stagnation；鈥潯ays ian tattersall of theamerican museum of natural history； 鈥渁nd stagnation is seldom a good thing in any realm。鈥�
（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鈥檚 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鈥檛 happened just as they did； just whenthey did； we almost certainly wouldn鈥檛 be here now。
earth has seen five major extinction episodes in its time鈥攖he ordovician； devonian；permian； triassic； and cretaceous； in that order鈥攁nd 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 check out； never to return。 evenabout a third of insect species went鈥攖he only occasion on which they were lost en masse。 it isas close as we have ever e to total obliteration。
鈥渋t was； truly； a mass extinction； a carnage of a magnitude that had never troubled the earthbefore；鈥潯ays richard fortey。 the permian event was particularly devastating to sea creatures。
trilobites vanished altogether。 clams and sea urchins nearly went。 virtually all other marineorganisms were staggered。 altogether； on land and in the water； it is thought that earth lost 52percent of its families鈥攖hat鈥檚 the level above genus and below order on the grand scale of life（the subject of the next chapter）鈥攁nd perhaps as many as 96 percent of all its species。 itwould be a long time鈥攁s much as eighty million years by one reckoning鈥攂efore speciestotals recovered。
two points need to be kept in mind。 first； these are all just informed guesses。 estimates forthe number of animal species alive at the end of the permian range from as low as 45；000 toas high as 240；000。 if you don鈥檛 know how many species were alive； you can hardly specifywith conviction the proportion that perished。 moreover； we are talking about the death ofspecies； not individuals。 f