万物简史英文版_比尔·布莱森-第55章
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ever readconcerning the conditions necessary for life involves water鈥攆rom the 鈥渨arm little pond鈥�
where darwin supposed life began to the bubbling sea vents that are now the most popularcandidates for life鈥檚 beginnings鈥攂ut all this overlooks the fact that to turn monomers intopolymers （which is to say； to begin to create proteins） involves what is known to biology as鈥渄ehydration linkages。鈥潯s one leading biology text puts it； with perhaps just a tiny hint ofdisfort； 鈥渞esearchers agree that such reactions would not have been energeticallyfavorable in the primitive sea； or indeed in any aqueous medium； because of the mass actionlaw。鈥潯t is a little like putting sugar in a glass of water and having it bee a cube。 itshouldn鈥檛 happen； but somehow in nature it does。 the actual chemistry of all this is a littlearcane for our purposes here； but it is enough to know that if you make monomers wet theydon鈥檛 turn into polymers鈥攅xcept when creating life on earth。 how and why it happens thenand not otherwise is one of biology鈥檚 great unanswered questions。
one of the biggest surprises in the earth sciences in recent decades was the discovery ofjust how early in earth鈥檚 history life arose。 well into the 1950s； it was thought that life wasless than 600 million years old。 by the 1970s； a few adventurous souls felt that maybe it wentback 2。5 billion years。 but the present date of 3。85 billion years is stunningly early。 earth鈥檚surface didn鈥檛 bee solid until about 3。9 billion years ago。
鈥渨e can only infer from this rapidity that it is not 鈥榙ifficult鈥櫋or life of bacterial grade toevolve on planets with appropriate conditions；鈥潯tephen jay gould observed in the new yorktimes in 1996。 or as he put it elsewhere； it is hard to avoid the conclusion that 鈥渓ife； arising assoon as it could； was chemically destined to be。鈥�
life emerged so swiftly； in fact； that some authorities think it must have had help鈥攑erhapsa good deal of help。 the idea that earthly life might have arrived from space has a surprisinglylong and even occasionally distinguished history。 the great lord kelvin himself raised thepossibility as long ago as 1871 at a meeting of the british association for the advancement ofscience when he suggested that 鈥渢he germs of life might have been brought to the earth bysome meteorite。鈥潯ut it remained little more than a fringe notion until one sunday inseptember 1969 when tens of thousands of australians were startled by a series of sonicbooms and the sight of a fireball streaking from east to west across the sky。 the fireball madea strange crackling sound as it passed and left behind a smell that some likened to methylatedspirits and others described as just awful。
the fireball exploded above murchison； a town of six hundred people in the goulburnvalley north of melbourne； and came raining down in chunks； some weighing up to twelvepounds。 fortunately； no one was hurt。 the meteorite was of a rare type known as acarbonaceous chondrite； and the townspeople helpfully collected and brought in some twohundred pounds of it。 the timing could hardly have been better。 less than two months earlier；the apollo 11 astronauts had returned to earth with a bag full of lunar rocks； so labsthroughout the world were geared up鈥攊ndeed clamoring鈥攆or rocks of extraterrestrial origin。
the murchison meteorite was found to be 4。5 billion years old； and it was studded withamino acids鈥攕eventy…four types in all； eight of which are involved in the formation of earthlyproteins。 in late 2001； more than thirty years after it crashed； a team at the ames researchcenter in california announced that the murchison rock also contained plex strings ofsugars called polyols； which had not been found off the earth before。
a few other carbonaceous chondrites have strayed into earth鈥檚 path since鈥攐ne that landednear tagish lake in canada鈥檚 yukon in january 2000 was seen over large parts of northamerica鈥攁nd they have likewise confirmed that the universe is actually rich in organicpounds。 halley鈥檚 et； it is now thought； is about 25 percent organic molecules。 getenough of those crashing into a suitable place鈥攅arth； for instance鈥攁nd you have the basicelements you need for life。
there are two problems with notions of panspermia； as extraterrestrial theories are known。
the first is that it doesn鈥檛 answer any questions about how life arose； but merely movesresponsibility for it elsewhere。 the other is that panspermia sometimes excites even the mostrespectable adherents to levels of speculation that can be safely called imprudent。 franciscrick； codiscoverer of the structure of dna； and his colleague leslie orgel have suggestedthat earth was 鈥渄eliberately seeded with life by intelligent aliens；鈥潯n idea that gribbin calls鈥渁t the very fringe of scientific respectability鈥濃�攐r； put another way； a notion that would beconsidered wildly lunatic if not voiced by a nobel laureate。 fred hoyle and his colleaguechandra wickramasinghe further eroded enthusiasm for panspermia by suggesting that outerspace brought us not only life but also many diseases such as flu and bubonic plague； ideasthat were easily disproved by biochemists。 hoyle鈥攁nd it seems necessary to insert areminder here that he was one of the great scientific minds of the twentieth century鈥攁lsoonce suggested； as mentioned earlier； that our noses evolved with the nostrils underneath as away of keeping cosmic pathogens from falling into them as they drifted down from space。
whatever prompted life to begin； it happened just once。 that is the most extraordinary factin biology； perhaps the most extraordinary fact we know。 everything that has ever lived； plantor animal； dates its beginnings from the same primordial twitch。 at some point in anunimaginably distant past some little bag of chemicals fidgeted to life。 it absorbed somenutrients； gently pulsed； had a brief existence。 this much may have happened before； perhapsmany times。 but this ancestral packet did something additional and extraordinary： it cleaveditself and produced an heir。 a tiny bundle of genetic material passed from one living entity toanother； and has never stopped moving since。 it was the moment of creation for us all。
biologists sometimes call it the big birth。
鈥渨herever you go in the world； whatever animal； plant； bug； or blob you look at； if it isalive； it will use the same dictionary and know the same code。 all life is one；鈥潯ays mattridley。 we are all the result of a single genetic trick handed down from generation togeneration nearly four billion years； to such an extent that you can take a fragment of humangenetic instruction； patch it into a faulty yeast cell； and the yeast cell will put it to work as if itwere its own。 in a very real sense； it is its own。
the dawn of life鈥攐r something very like it鈥攕its on a shelf in the office of a friendlyisotope geochemist named victoria bennett in the earth sciences building of the australiannational university in canberra。 an american； ms。 bennett came to the anu fromcalifornia on a two…year contract in 1989 and has been there ever since。 when i visited her； inlate 2001； she handed me a modestly hefty hunk of rock posed of thin alternating stripesof white quartz and a gray…green material called clinopyroxene。 the rock came from akiliaisland in greenland； where unusually ancient rocks were found in 1997。 the rocks are 3。85billion years old and represent the oldest marine sediments ever found。
鈥渨e can鈥檛 be certain that what you are holding once contained living organisms becauseyou鈥檇 have to pulverize it to find out；鈥潯ennett told me。 鈥渂ut it es from the same depositwhere the oldest life was excavated； so it probably had life in it。鈥潯or would you find actualfossilized microbes； however carefully you searched。 any simple organisms； alas； would havebeen baked away by the processes that turned ocean mud to stone。 instead what we would seeif we crunched up the rock and examined it microscopically would be the chemical residuesthat the organisms left behind鈥攃arbon isotopes and a type of phosphate called apatite； whichtogether provide strong evidence that the rock once contained colonies of living things。 鈥渨ecan only guess what the organism might have looked like；鈥潯ennett said。 鈥渋t was probablyabout as basic as life can get鈥攂ut it was life nonetheless。 it lived。 it propagated。鈥�
and eventually it led to us。
if you are into very old rocks； and bennett indubitably is； the anu has long been a primeplace to be。 this is largely thanks to the ingenuity of a man named bill pston； who isnow retired but in the 1970s built the world鈥檚 first sensitive high resolution ion microprobe鈥攐r shrimp； as it is more affectionately known from its initial letters。 this is amachine that measures the decay rate of uranium in tiny minerals called zircons。 zirconsappear in most rocks apart from basalts and are extremely durable； surviving every naturalprocess but subduction。 most of the earth鈥檚 crust has been slipped back into the oven at somepoint； but just occasionally鈥攊n western australia and greenland； for example鈥攇eologistshave found outcrops of rocks that have remained always at the surface。 pston鈥檚 machineallowed such rocks to be dated with unparalleled precision。 the prototype shrimp was built and machined in the earth science department鈥檚 own workshops； and looked like somethingthat had been built from spare parts on a budget； but it worked great。 on its first formal test； in1982； it dated the oldest thing ever found鈥攁 4。3…billion…year…old rock from westernaustralia。
鈥渋t caused quite a stir at the time；鈥潯ennett told me； 鈥渢o find something so important soquickly with brand…new technology。鈥�
she took me down the hall to see the current model； shrimp ii。 it was a big heavy pieceof stainless…steel apparatus； perhaps twelve feet long and five feet high； and as solidly built asa deep…sea probe。 at a console in front of it； keeping an eye on ever…changing strings offigures on a screen； was a man named bob from canterbury university in new zealand。 hehad been there since 4 a。m。； he told me。 shrimp ii runs twenty…four hours a day； there鈥檚 thatmany rocks to date。 it was just after 9a。m。 and bob had the machine till noon。 ask a pair ofgeochemists how something like this works； and they will start talking about isotopicabundances and ionization levels with an enthusiasm that is more endearing than fathomable。
the upshot of it； however； was that the machine； by bombarding a sample of rock withstreams of charged atoms； is able to detect subtle differences in the amounts of lead anduranium in the zircon samples； by which means the age of rocks can be accurately adduced。
bob told me that it takes about seventeen minutes to read one zircon and it is necessary toread dozens from each rock to make the data reliable。 in practice； the process seemed toinvolve about the same level of scattered activity； and about as much stimulation； as a trip to alaundromat。 bob seemed very happy； however； but then people from new zealand verygenerally do。
the earth sciences pound was an odd bination of things鈥攑art offices； part labs；part machine shed。 鈥渨e used to build everything here；鈥潯ennett said。 鈥渨e even had o
