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whether there are many other animals whichhave played so important a part in the history of the world；鈥潯e wrote in his masterwork on thesubject； the formation of vegetable mould through the action of worms （1881）； which wasactually more popular thanon the origin of species had ever been。 among his other bookswere on the various contrivances by which british and foreign orchids are fertilised byinsects （1862）； expressions of the emotions in man and animals （1872）； which sold almost5；300 copies on its first day； the effects of cross and self fertilization in the vegetablekingdom （1876）鈥攁 subject that came improbably close to mendel鈥檚 own work； withoutattaining anything like the same insights鈥攁nd his last book； the power of movement inplants。 finally； but not least； he devoted much effort to studying the consequences ofinbreeding鈥攁 matter of private interest to him。 having married his own cousin； darwinglumly suspected that certain physical and mental frailties among his children arose from alack of diversity in his family tree。
darwin was often honored in his lifetime； but never for on the origin of species ordescentof man。 when the royal society bestowed on him the prestigious copley medal it was for hisgeology； zoology； and botany； not evolutionary theories； and the linnaean society wassimilarly pleased to honor darwin without embracing his radical notions。 he was neverknighted； though he was buried in westminster abbey鈥攏ext to newton。 he died at down inapril 1882。 mendel died two years later。
darwin鈥檚 theory didn鈥檛 really gain widespread acceptance until the 1930s and 1940s； withthe advance of a refined theory called； with a certain hauteur； the modern synthesis；bining darwin鈥檚 ideas with those of mendel and others。 for mendel； appreciation wasalso posthumous； though it came somewhat sooner。 in 1900； three scientists workingseparately in europe rediscovered mendel鈥檚 work more or less simultaneously。 it was onlybecause one of them； a dutchman named hugo de vries； seemed set to claim mendel鈥檚insights as his own that a rival made it noisily clear that the credit really lay with the forgottenmonk。
the world was almost ready； but not quite； to begin to understand how we got here鈥攈owwe made each other。 it is fairly amazing to reflect that at the beginning of the twentiethcentury； and for some years beyond； the best scientific minds in the world couldn鈥檛 actuallytell you where babies came from。
and these； you may recall； were men who thought science was nearly at an end。
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26 THE STUFF OF LIFE
灏忥伎璇磘…x…t锛垮ぉ/鍫�
if your two parents hadn鈥檛 bonded just when they did鈥攑ossibly to the second； possiblyto the nanosecond鈥攜ou wouldn鈥檛 be here。 and if their parents hadn鈥檛 bonded in a preciselytimely manner； you wouldn鈥檛 be here either。 and if their parents hadn鈥檛 done likewise； andtheir parents before them； and so on； obviously and indefinitely； you wouldn鈥檛 be here。
push backwards through time and these ancestral debts begin to add up。 go back just eightgenerations to about the time that charles darwin and abraham lincoln were born； andalready there are over 250 people on whose timely couplings your existence depends。
continue further； to the time of shakespeare and the mayflower pilgrims； and you have nofewer than 16；384 ancestors earnestly exchanging genetic material in a way that would；eventually and miraculously； result in you。
at twenty generations ago； the number of people procreating on your behalf has risen to1；048；576。 five generations before that； and there are no fewer than 33；554；432 men andwomen on whose devoted couplings your existence depends。 by thirty generations ago； yourtotal number of forebears鈥攔emember； these aren鈥檛 cousins and aunts and other incidentalrelatives； but only parents and parents of parents in a line leading ineluctably to you鈥攊s overone billion （1；073；741；824； to be precise）。 if you go back sixty…four generations； to the time ofthe romans； the number of people on whose cooperative efforts your eventual existencedepends has risen to approximately 1；000；000；000；000；000；000； which is several thousandtimes the total number of people who have ever lived。
clearly something has gone wrong with our math here。 the answer； it may interest you tolearn； is that your line is not pure。 you couldn鈥檛 be here without a little incest鈥攁ctually quitea lot of incest鈥攁lbeit at a genetically discreet remove。 with so many millions of ancestors inyour background； there will have been many occasions when a relative from your mother鈥檚side of the family procreated with some distant cousin from your father鈥檚 side of the ledger。 infact； if you are in a partnership now with someone from your own race and country； thechances are excellent that you are at some level related。 indeed； if you look around you on abus or in a park or caf茅 or any crowded place； most of the people you see are very probablyrelatives。 when someone boasts to you that he is descended from william the conqueror orthe mayflower pilgrims； you should answer at once： 鈥渕e； too！鈥潯n the most literal andfundamental sense we are all family。
we are also uncannily alike。 pare your genes with any other human being鈥檚 and onaverage they will be about 99。9 percent the same。 that is what makes us a species。 the tinydifferences in that remaining 0。1 percent鈥斺�渞oughly one nucleotide base in every thousand；鈥�
to quote the british geneticist and recent nobel laureate john sulston鈥攁re what endow uswith our individuality。 much has been made in recent years of the unraveling of the human genome。 in fact； there is no such thing as 鈥渢he鈥潯uman genome。 every human genome isdifferent。 otherwise we would all be identical。 it is the endless rebinations of ourgenomes鈥攅ach nearly identical； but not quite鈥攖hat make us what we are； both as individualsand as a species。
but what exactly is this thing we call the genome？ and what； e to that； are genes？
well； start with a cell again。 inside the cell is a nucleus； and inside each nucleus are thechromosomes鈥攆orty…six little bundles of plexity； of which twenty…three e from yourmother and twenty…three from your father。 with a very few exceptions； every cell in yourbody鈥�99。999 percent of them； say鈥攃arries the same plement of chromosomes。 （theexceptions are red blood cells； some immune system cells； and egg and sperm cells； which forvarious organizational reasons don鈥檛 carry the full genetic package。） chromosomes constitutethe plete set of instructions necessary to make and maintain you and are made of longstrands of the little wonder chemical called deoxyribonucleic acid or dna鈥斺�渢he mostextraordinary molecule on earth；鈥潯s it has been called。
dna exists for just one reason鈥攖o create more dna鈥攁nd you have a lot of it inside you：
about six feet of it squeezed into almost every cell。 each length of dna prises some 3。2billion letters of coding； enough to provide 103；480；000；000possible binations； 鈥済uaranteed tobe unique against all conceivable odds；鈥潯n the words of christian de duve。 that鈥檚 a lot ofpossibility鈥攁 one followed by more than three billion zeroes。 鈥渋t would take more than fivethousand average…size books just to print that figure；鈥潯otes de duve。 look at yourself in themirror and reflect upon the fact that you are beholding ten thousand trillion cells； and thatalmost every one of them holds two yards of densely pacted dna； and you begin toappreciate just how much of this stuff you carry around with you。 if all your dna werewoven into a single fine strand； there would be enough of it to stretch from the earth to themoon and back not once or twice but again and again。 altogether； according to onecalculation； you may have as much as twenty million kilometers of dna bundled up insideyou。
your body； in short； loves to make dna and without it you couldn鈥檛 live。 yet dna is notitself alive。 no molecule is； but dna is； as it were； especially unalive。 it is 鈥渁mong the mostnonreactive； chemically inert molecules in the living world；鈥潯n the words of the geneticistrichard lewontin。 that is why it can be recovered from patches of long…dried blood or semenin murder investigations and coaxed from the bones of ancient neandertals。 it also explainswhy it took scientists so long to work out how a substance so mystifyingly low key鈥攕o； in aword； lifeless鈥攃ould be at the very heart of life itself。
as a known entity； dna has been around longer than you might think。 it was discoveredas far back as 1869 by johann friedrich miescher； a swiss scientist working at the universityof t眉bingen in germany。 while delving microscopically through the pus in surgicalbandages； miescher found a substance he didn鈥檛 recognize and called it nuclein （because itresided in the nuclei of cells）。 at the time； miescher did little more than note its existence； butnuclein clearly remained on his mind； for twenty…three years later in a letter to his uncle heraised the possibility that such molecules could be the agents behind heredity。 this was anextraordinary insight； but one so far in advance of the day鈥檚 scientific requirements that itattracted no attention at all。
for most of the next half century the m