s realize that actually life of a type could range much farther than that…even； perhaps； beneath theicy skin of pluto。 what we are talking about here are the conditions that would produce reasonably plexsurface creatures。
we’re a twin planet。not many of us normally think of the moon as a panion planet；but that is in effect what it is。 most moons are tiny in relation to their master planet。 themartian satellites of phobos and deimos； for instance； are only about ten kilometers indiameter。 our moon； however； is more than a quarter the diameter of the earth； which makesours the only planet in the solar system with a sizeable moon in parison to itself （exceptpluto； which doesn’t really count because pluto is itself so small）； and what a difference thatmakes to us。
without the moon’s steadying influence； the earth would wobble like a dying top； withgoodness knows what consequences for climate and weather。 the moon’s steady gravitationalinfluence keeps the earth spinning at the right speed and angle to provide the sort of stabilitynecessary for the long and successful development of life。 this won’t go on forever。 themoon is slipping from our grasp at a rate of about 1。5 inches a year。 in another two billionyears it will have receded so far that it won’t keep us steady and we will have to e up withsome other solution； but in the meantime you should think of it as much more than just apleasant feature in the night sky。
for a long time； astronomers assumed that the moon and earth either formed together orthat the earth captured the moon as it drifted by。 we now believe； as you will recall from anearlier chapter； that about 4。5 billion years ago a mars…sized object slammed into earth；blowing out enough material to create the moon from the debris。 this was obviously a verygood thing for us—but especially so as it happened such a long time ago。 if it had happened in1896 or last wednesday clearly we wouldn’t be nearly so pleased about it。 which brings us toour fourth and in many ways most crucial consideration：
timing。the universe is an amazingly fickle and eventful place； and our existence within itis a wonder。 if a long and unimaginably pleequence of events stretching back 4。6billion years or so hadn’t played out in a particular manner at particular times—if； to take justone obvious instance； the dinosaurs hadn’t been wiped out by a meteor when they were—youmight well be six inches long； with whiskers and a tail； and reading this in a burrow。
we don’t really know for sure because we have nothing else to pare our own existenceto； but it seems evident that if you wish to end up as a moderately advanced； thinking society；you need to be at the right end of a very long chain of outes involving reasonable periodsof stability interspersed with just the right amount of stress and challenge （ice ages appear tobe especially helpful in this regard） and marked by a total absence of real cataclysm。 as weshall see in the pages that remain to us； we are very lucky to find ourselves in that position。
and on that note； let us now turn briefly to the elements that made us。
there are ninety…two naturally occurring elements on earth； plus a further twenty or so thathave been created in labs； but some of these we can immediately put to one side—as； in fact；chemists themselves tend to do。 not a few of our earthly chemicals are surprisingly littleknown。 astatine； for instance； is practically unstudied。 it has a name and a place on theperiodic table （next door to marie curie’s polonium）； but almost nothing else。 the problem isn’t scientific indifference； but rarity。 there just isn’t much astatine out there。 the mostelusive element of all； however； appears to be francium； which is so rare that it is thought thatour entire planet may contain； at any given moment； fewer than twenty francium atoms。
altogether only about thirty of the naturally occurring elements are widespread on earth； andbarely half a dozen are of central importance to life。
as you might expect； oxygen is our most abundant element； accounting for just under 50percent of the earth’s crust； but after that the relative abundances are often surprising。 whowould guess； for instance； that silicon is the second most mon element on earth or thattitanium is tenth？ abundance has little to do with their familiarity or utility to us。 many of themore obscure elements are actually more mon than the better…known ones。 there is morecerium on earth than copper； more neodymium and lanthanum than cobalt or nitrogen。 tinbarely makes it into the top fifty； eclipsed by such relative obscurities as praseodymium；samarium； gadolinium； and dysprosium。
abundance also has little to do with ease of detection。 aluminum is the fourth mostmon element on earth； accounting for nearly a tenth of everything that’s underneath yourfeet； but its existence wasn’t even suspected until it was discovered in the nineteenth centuryby humphry davy； and for a long time after that it was treated as rare and precious。 congressnearly put a shiny lining of aluminum foil atop the washington monument to show what aclassy and prosperous nation we had bee； and the french imperial family in the sameperiod discarded the state silver dinner service and replaced it with an aluminum one。 thefashion was cutting edge even if the knives weren’t。
nor does abundance necessarily relate to importance。 carbon is only the fifteenth mostmon element； accounting for a very modest 0。048 percent of earth’s crust； but we wouldbe lost without it。 what sets the carbon atom apart is that it is shamelessly promiscuous。 it isthe party animal of the atomic world； latching on to many other atoms （including itself） andholding tight； forming molecular conga lines of hearty robustness—the very trick of naturenecessary to build proteins and dna。 as paul davies has written： “if it wasn’t for carbon； lifeas we know it would be impossible。 probably any sort of life would be impossible。” yetcarbon is not all that plentiful even in humans； who so vitally depend on it。 of every 200atoms in your body； 126 are hydrogen； 51 are oxygen； and just 19 are carbon。
2other elements are critical not for creating life but for sustaining it。 we need iron tomanufacture hemoglobin； and without it we would die。 cobalt is necessary for the creation ofvitamin b12。 potassium and a very little sodium are literally good for your nerves。
molybdenum； manganese； and vanadium help to keep your enzymes purring。 zinc—bless it—oxidizes alcohol。
we have evolved to utilize or tolerate these things—we could hardly be here otherwise—but even then we live within narrow ranges of acceptance。 selenium is vital to all of us； buttake in just a little too much and it will be the last thing you ever do。 the degree to whichorganisms require or tolerate certain elements is a relic of their evolution。 sheep and cattlenow graze side by side； but actually have very different mineral requirements。 modern cattleneed quite a lot of copper because they evolved in parts of europe and africa where copperwas abundant。 sheep； on the other hand； evolved in copper…poor areas of asia minor。 as arule； and not surprisingly； our tolerance for elements is directly proportionate to their2of the remaining four； three are nitrogen and the remaining atom is divided among all the other elements。
abundance in the earth’s crust。 we have evolved to expect； and in some cases actually need；the tiny amounts of rare elements that accumulate in the flesh or fiber that we eat。 but step upthe doses； in some cases by only a tiny amount； and we can soon cross a threshold。 much ofthis is only imperfectly understood。 no one knows； for example； whether a tiny amount ofarsenic is necessary for our well…being or not。 some authorities say it is； some not。 all that iscertain is that too much of it will kill you。
the properties of the elements can bee more curious still when they are bined。
oxygen and hydrogen； for instance； are two of the most bustion…friendly elements around；but put them together and they make inbustible water。
3odder still in bination aresodium； one of the most unstable of all elements； and chlorine； one of the most toxic。 drop asmall lump of pure sodium into ordinary water and it will explode with enough force to kill。
chlorine is even more notoriously hazardous。 though useful in small concentrations forkilling microorganisms （it’s chlorine you smell in bleach）； in larger volumes it is lethal。
chlorine was the element of choice for many of the poison gases of the first world war。 and；as many a sore…eyed swimmer will attest； even in exceedingly dilute form the human bodydoesn’t appreciate it。 yet put these two nasty elements together and what do you get？ sodiumchloride—mon table salt。
by and large； if an element doesn’t naturally find its way into our systems—if it isn’tsoluble in water； say—we tend to be intolerant of it。 lead poisons us because we were neverexposed to it until we began to fashion it into food vessels and pipes for plumbing。 （notincidentally； lead’s symbol is pb； for the latin plumbum； the source word for our modernplumbing。） the romans also flavored their wine with lead； which may be part of the reasonthey are not the force they used to be。