A Short History of Nearly Everything 精彩片段：
26 THE STUFF OF LIFE
IF YOUR TWO parents hadn’t bonded just when they did—possibly to the second, possiblyto the nanosecond—you wouldn’t be here. And if their parents hadn’t bonded in a preciselytimely manner, you wouldn’t be here either. And if their parents hadn’t done likewise, andtheir parents before them, and so on, obviously and indefinitely, you wouldn’t 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—remember, these aren’t cousins and aunts and other incidentalrelatives, but only parents and parents of parents in a line leading ineluctably to you—is 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’t be here without a little incest—actually quitea lot of incest—albeit 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’sside of the family procreated with some distant cousin from your father’s 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 café 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: “Me, too!” In the most literal andfundamental sense we are all family.
We are also uncannily alike. Compare your genes with any other human being’s 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—“roughly one nucleotide base in every thousand,”
to quote the British geneticist and recent Nobel laureate John Sulston—are what endow uswith our individuality. Much has been made in recent years of the unraveling of the humangenome. In fact, there is no such thing as “the” human genome. Every human genome isdifferent. Otherwise we would all be identical. It is the endless recombinations of ourgenomes—each nearly identical, but not quite—that make us what we are, both as individualsand as a species.
But what exactly is this thing we call the genome? And what, come to that, are genes?
Well, start with a cell again. Inside the cell is a nucleus, and inside each nucleus are thechromosomes—forty-six little bundles of complexity, of which twenty-three come from yourmother and twenty-three from your father. With a very few exceptions, every cell in yourbody—99.999 percent of them, say—carries the same complement of chromosomes. (Theexceptions are red blood cells, some immune system cells, and egg and sperm cells, which forvarious organizational reasons don’t carry the full genetic package.) Chromosomes constitutethe complete set of instructions necessary to make and maintain you and are made of longstrands of the little wonder chemical called deoxyribonucleic acid or DNA—“the mostextraordinary molecule on Earth,” as it has been called.
DNA exists for just one reason—to create more DNA—and you have a lot of it inside you:
about six feet of it squeezed into almost every cell. Each length of DNA comprises some 3.2billion letters of coding, enough to provide 103,480,000,000possible combinations, “guaranteed tobe unique against all conceivable odds,” in the words of Christian de Duve. That’s a lot ofpossibility—a one followed by more than three billion zeroes. “It would take more than fivethousand average-size books just to print that figure,” notes 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 compacted 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’t live. Yet DNA is notitself alive. No molecule is, but DNA is, as it were, especially unalive. It is “among the mostnonreactive, chemically inert molecules in the living world,” in 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—so, in aword, lifeless—could 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 Tübingen in Germany. While delving microscopically through the pus in surgicalbandages, Miescher found a substance he didn’t 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’s scientific requirements that itattracted no attention at all.
For most of the next half century the common assumption was that the material—nowcalled deoxyribonucleic acid, or DNA—had at most a subsidiary role in matters of heredity. Itwas too simple. It had just four basic components, called nucleotides, which was like havingan alphabet of just four letters. How could you possibly write the story of life with such arudimentary alphabet? (The answer is that you do it in much the way that you create complexmessages with the simple dots and dashes of Morse code—by combining them.) DNA didn’tdo anything at all, as far as anyone could tell. It just sat there in the nucleus, possibly bindingthe chromosome in some way or adding a splash of acidity on command or fulfilling someother trivial task that no one had yet thought of. The necessary complexity, it was thought,had to exist in proteins in the nucleus.