19 THE RISE OF LIFE

百度搜索 A Short History of Nearly Everything 天涯 或 A Short History of Nearly Everything 天涯在线书库 即可找到本书最新章节.

    IN 1953, STANLEY Miller, a graduate student at the Uy of Chicago, took twoflasks—one taining a little water to represent a primeval o, the other holding amixture of methane, ammonia, and hydrogen sulphide gases to represeh’s earlyatmosph<cite>９9liｂ?</cite>ere—ected them with rubber tubes, and introduced some electrical sparks as astand-in fhtning. After a few days, the water in the flasks had turned green and yellow iy broth of amino acids, fatty acids, sugars, and anipounds. “If Goddidn’t do it this way,” observed Miller’s delighted supervisor, the Nobel laureate HaroldUrey, “He missed a good bet.”

    Press reports of the time made it sound as if about all that was needed now was forsomebody to give the whole a good shake and life would crawl out. As time has shown, itwasn’t nearly so simple. Despite half a tury of further study, we are no osynthesizing life today than we were in 1953 and much further away from thinking we .

    Stists are now pretty certain that the early atmosphere was nothing like as primed fordevelopment as Miller and Urey’s gaseous stew, but rather was a much less reactive blend ofnitrogen and carbon dioxide. Repeating Miller’s experiments with these more challenginginputs has so far produced only one fairly primitive amino acid. At all events, creating aminoacids is not really the problem. The problem is proteins.

    Proteins are what you get when you string amino acids together, and we need a lot of them.

    No one really knows, but there may be as many as a million types of protein in the humanbody, and eae is a little miracle. By all the laws of probability proteins should.

    To make a protein you o assemble amino acids (which I am obliged by long tradition torefer to here as “the building blocks of life”) in a particular order, in much the same way thatyou assemble letters in a particular order to spell a word. The problem is that words in theamino acid alphabet are often exceedingly long. To spell collagen, the name of a ontype of protein, you e eight letters in the right order. But to make collagen, youe 1,055 amino acids in precisely the right sequence. But—and here’s anobvious but crucial point—you don’t make it. It makes itself, spontaneously, withoutdire, and this is where the unlikelihoods e in.

    The ces of a 1,055-sequence molecule like collagen spontaneously self-assembling are,frankly, nil. It just isn’t going to happen. To grasp what a long shot its existence is, visualize astandard Las Vegas slot mae but broadened greatly—to about y feet, to be precise—to aodate 1,055 spinning wheels instead of the usual three or four, and with twentysymbols on each wheel (one for eaon amino acid).

    1How long would you have topull the handle before all 1,055 symbols came up in the right order? Effectively forever. Evenif you reduced the number of spinning wheels to two hundred, which is actually a moretypiumber of amino acids for a protein, the odds against all two hundred ing up in a1There are actually twenty-two naturally  amino acids known oh, and more may await discovery,but only twenty of them are necessary to produce us and most other living things. The twenty-sed, calledpyrrolysine, was discovered in 2002 by researchers at Ohio State Uy and is found only in a siype ofarchaean (a basi of life that we will discuss a little further on.. iory) called Methanosara barkeri.

    prescribed sequence are 1 in 10260(that is a 1 followed by 260 zeroes). That in itself is a largerhan all the atoms in the universe.

    Proteins, in short, are plex entities. Hemoglobin is only 146 amino acids long, a runt byprotein standards, yet even it offers 10190possible amino acid binations, which is why ittook the Cambridge Uy chemist Max Perutz twenty-three years—a career, more orless—to u. For random events to produce even a single protein would seem astunning improbability—like a whirlwind spinning through a junkyard and leaving behind afully assembled jumbo jet, in the colorful simile of the astronomer Fred Hoyle.

    Yet we are talking about several huhousand types of protein, perhaps a million, eaique and each, as far as we know, vital to the maintenance of a sound and happy you. Andit goes on from there. A protein to be of use must not only assemble amino acids in the rightsequence, but then must engage in a kind of chemical ami and fold itself into a veryspecific shape. Even having achieved this structural plexity, a protein is no good to you ifit ’t reproduce itself, and proteins ’t. For this you need DNA. DNA is a whiz atreplig—it  make a copy of itself in seds—but  do virtually nothing else. So wehave a paradoxical situation. Proteins ’t exist without DNA, and DNA has no purposewithout proteins. Are we to assume then that they arose simultaneously with the purpose ofsupp each other? If so: wow.

    And there is more still. DNA, proteins, and the other pos of life couldn’t prosperwithout some sort of membrao tain them. No atom or molecule has ever achieved lifeindepely. Pluy atom from your body, and it is no more alive than is a grain of sand.

    It is only when they e together within the nurturing refuge of a cell that these diversematerials  take part in the amazing dahat we call life. Without the cell, they arenothing more than iing chemicals. But without the chemicals, the cell has no purpose.

    As the physicist Paul Davies puts it, “If everything needs everything else, how did theunity of molecules ever arise in the first place?” It is rather as if all the ingredients inyour kit somehow got together and baked themselves into a cake—but a cake that oreover divide when necessary to produce more cakes. It is little wohat we call it themiracle of life. It is also little wohat we have barely begun to uand it.

    So what ats for all this wondrous plexity? Well, one possibility is that perhaps itisn’t quite—not quite—so wondrous as at first it seems. Take those amazingly improbableproteins. The wonder we see in their assembly es in assuming that they arrived on these fully formed. But what if the protein s didn’t assemble all at once? What if, in thegreat slot mae of creation, some of the wheels could be held, as a gambler might hold anumber of promising cherries? What if, in other words, proteins didn’t suddenly burst intobeing, but evolved .

    Imagine if you took all the pos that make up a human being—carbon, hydrogen,oxygen, and so on—and put them in a tainer with some water, gave it a vigorous stir, andout stepped a pleted person. That would be amazing. Well, that’s essentially what Hoyleand others (including many ardent creationists) argue when they suggest that proteinsspontaneously formed all at ohey didn’t—they ’t have. As Richard Dawkins arguesin The Blind Watchmaker, there must have been some kind of cumulative sele processthat allowed amino acids to assemble in ks. Perhaps two or three amino acids linked upfor some simple purpose and then after a time bumped into some other similar small clusterand in so doing “discovered” some additional improvement.

    Chemical  reas  of  the  sort  associated with life are actually something of aonplace. It may be beyond us to cook them up in a lab, à la Stanley Miller and HaroldUrey, but the universe does it readily enough. Lots of molecules in nature get together to formlong s called polymers. Sugars stantly assemble to form starches. Crystals  do anumber of lifelike things—replicate, respond to enviroal stimuli, take on a patternedplexity. They’ve never achieved life itself, of course, but they demonstrate repeatedly thatplexity is a natural, spontaneous, entirely onplace event. There may or may not be agreat deal of life in the universe at large, but there is no she of ordered self-assembly, ihing from the transfixing symmetry of snowflakes to the ely rings of Saturn.

    So powerful is this natural impulse to assemble that many stists now believe that lifemay be more iable thahink—that it is, in the words of the Belgian biochemist andNobel laureate Christian de Duve, “an obligatory maion of matter, bound to arisewherever ditions are appropriate.” De Duve thought it likely that such ditions would beentered perhaps a million times in every galaxy.

    Certainly there is nothing terribly exoti the chemicals that animate us. If you wished tocreate another living object, whether a goldfish or a head of lettuce or a human being, youwould need really only four principal elements, carbon, hydrogen, oxygen, and nitrogen, plussmall amounts of a few others, principally sulfur, phosphorus, calcium, and iron. Put thesetogether in three dozen or so binations to form some sugars, acids, and other basipounds and you  build anything that lives. As Dawkins notes: “There is nothingspecial about the substances from which living things are made. Living things are collesof molecules, like everything else.”

    The bottom line is that life is amazing and gratifying, perhaps even miraculous, but hardlyimpossible—as we repeatedly attest with our own modest existeo be sure, many of thedetails of life’s beginnings remaiy imponderable. Every sario you have ever reading the ditions necessary for life involves water—from the “warm little pond”

    where Darwin supposed life began to the bubbling sea vents that are now the most populardidates for life’s beginnings—but 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“dehydration linkages.” As one leading biology text puts it, with perhaps just a tiny hint ofdisfort, “Researchers agree that such reas would not have beeicallyfavorable in the primitive sea, or indeed in any aqueous medium, because of the mass alaw.” It is a little like putting sugar in a glass of water and having it bee a cube. Itshouldn’t happen, but somehow in nature it does. The actual chemistry of all this is a littleare for our purposes here, but it is enough to know that if you make monomers wet theydon’t turn into polymers—except wheing life oh. How and why it happens thenand not otherwise is one of biology’s great unanswered questions.

    One of the biggest surprises in the earth sces i decades was the discovery ofjust how early ih’s 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’ssurface didn’t bee solid until about 3.9 billion years ago.

    “We  only infer from this rapidity that it is not ‘difficult’ for life of bacterial grade toevolve on plas with appropriate ditions,” Stephen Jay Gould observed in the New YorkTimes in 1996. Or as he put it elsewhere, it is hard to avoid the clusion that “life, arising assoon as it could, was chemically destio be.”

    Life emerged so swiftly, in fact, that some authorities think it must have had help—perhapsa 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 Adva ofSce when he suggested that “the germs of life might have been brought to the earth bysome meteorite.” But it remained little more than a friion until one Sunday iember 1969 when tens of thousands of Australians were startled by a series of sonis and the sight of a fireball streaking from east to west across the sky. The fireball madea strange crag sound as it passed a behind a smell that some likeo 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 ks, some weighing up to twelvepounds. Fortunately, no one was hurt. The meteorite was of a rare type known as acarbonaceous drite, and the townspeople helpfully collected and brought in some twohundred pounds of it. The timing could hardly have beeer. Less than two months earlier,the Apollo 11 astronauts had returo Earth with a bag full of lunar rocks, so labsthroughout the world were geared up—indeed clam—for rocks of extraterrestrial in.

    The Mureteorite was found to be 4.5 billion years old, and it was studded withamino acids—seventy-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 Researter in California annouhat the Murchison rock also tained plex strings ofsugars called polyols, which had not been found off the Earth before.

    A few other carbonaceous drites have strayed ih’s path sine that landedagish Lake in ada’s Yukon in January 2000 was seen over large parts of NorthAmerid they have likewise firmed that the universe is actually ri anipounds. Halley’s et, it is now thought, is about 25 pert anic molecules. Getenough of those crashing into a suitable place—Earth, for instand 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’t 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  be safely called imprudent. FrancisCrick, codiscoverer of the structure of DNA, and his colleague Leslie el have suggestedthat Earth was “deliberately seeded with life by intelligent aliens,” ahat Gribbin calls“at the very fringe of stific respectability”—or, put another way, a notion that would besidered wildly lunatic if not v<mark></mark>oiced by a Nobel laureate. Fred Hoyle and his colleaguedra 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—and it seems necessary to i areminder here that he was one of the great stifids of the tweh tury—alsoonce suggested, as mentioned earlier, that our noses evolved with the nostrils underh as away of keeping ic pathogens from falling into them as they drifted down from space.

    Whatever prompted life to begin, it happened just ohat is the most extraordinary fa 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 existehis much may have happened before, perhapsmany times. But this aral packet did something additional araordinary: it cleaveditself and produced an heir. A tiny bundle of geic material passed from one liviy toanother, and has opped moving si was the moment of creation for us all.

    Biologists sometimes call it the Big Birth.

    “Wherever you go in the world, whatever animal, plant, bug, or blob you look at, if it isalive, it will use the same diary and know the same code. All life is one,” says MattRidley. We are all the result of a single geic trick handed down from geion togeion nearly four billion years, to su extent that you  take a fragment of humaistru, 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 se is its own.

    The dawn of life—or something very like it—sits on a shelf in the office of a friendlyisotope geochemist named Victoria Be in the Earth Sces building of the AustralianNational Uy in berra. An Ameri, Ms. Be came to the ANU fromCalifornia on a two-year tra 1989 and has been there ever since. When I visited her, inlate 2001, she handed me a modestly hefty hunk of roposed of thin alternating stripesof white quartz and a gray-green material called opyroxehe rock came from AkiliaIsland in Greenland, where unusually a rocks were found in 1997. The rocks are 3.85billion years old and represent the oldest marine sediments ever found.

    “We ’t be certain that what you are holding once tained living anisms becauseyou’d have to pulverize it to find out,” Beold me. “But it es from the same depositwhere the oldest life was excavated, so it probably had life in it.” Nor would you find actualfossilized microbes, however carefully you searched. Any simple anisms, alas, would havebeen baked away by the processes that turned o mud to stone. Instead what we would seeif we ched up the rod exami microscopically would be the chemical residuesthat the anisms left behind—carbon isotopes and a type of phosphate called apatite, whichtogether provide strong evidehat the roce tained ies of living things. “We only guess what the anism might have looked like,” Be said. “It robablyabout as basic as life  get—but it was life heless. It lived. It propagated.”

    Aually it led to us.

    If you are into very old rocks, at indubitably is, the ANU has long been a primeplace to be. This is largely thanks to the iy of a man named Bill pston, who isnow retired but in the 1970s built the world’s first Sensitive High Resolution Ion MicroProbe—or SHRIMP, as it is more affeately known from its initial letters. This is amae that measures the decay rate of uranium in tiny minerals called zirs. Zirsappear in most rocks apart from basalts and are extremely durable, surviving every naturalprocess but subduost of the Earth’s crust has been slipped bato the oven at somepoint, but just occasionally—iern Australia and Greenland, for example—geologistshave found outcrops of rocks that have remained always at the surface. pston’s maeallowed such rocks to be date<var>藏书网</var>d with unparalleled precision. The prototype SHRIMP was builtand maed in the Earth Sce department’s 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—a 4.3-billion-year-old  rock from WesternAustralia.

    “It caused quite a stir at the time,” Beold me, “to find something so important soquickly with braeology.”

    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 sole in front of it, keeping an eye on ever-ging strings offigures on a s, was a man named Bob from terbury Uy in New Zealand. Hehad been there since 4 A.M., he told me. SHRIMP II runs twenty-four hours a day; there’s thatmany rocks to date. It was just after 9A.M. and Bob had the mae till noon. Ask a pair ofgeochemists how something like this works, and they will start talking about isotopidances and ionization levels with ahusiasm that is more endearing than fathomable.

    The upshot of it, however, was that the mae, by b a sample of rock withstreams of charged atoms, is able to detect subtle differences in the amounts of <big></big>lead anduranium in the zir samples, by which means the age of rocks  be accurately adduced.

    Bob told me that it takes about seventeen mio read one zir and it is necessary toread dozens from each roake 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 Sces pound was an odd bination of things—part offices, part labs,part mae shed. “We used to build everything here,” Be said. “We even had our ownglassblower, but he’s retired. But we still have two full-time rock crushers.” She caught mylook of mild surprise. “We get through a lot of rocks. And they have to be very carefullyprepared. You have to make sure there is no ination from previous samples—no dustor anything. It’s quite a meticulous process.” She showed me the rock-crushing maes,which were indeed pristihough the rock crushers had apparently gone for coffee. Besidethe maes were large boxes taining rocks of all shapes and sizes. They do indeed getthrough a lot of rocks at the ANU.

    Ba Be’s office after our tour, I noticed hanging on her wall a piving anartist’s colorfully imaginative interpretation of Earth as it might have looked 3.5 billion yearsago, just when life was getting going, in the a period known to earth sce as theArchaean. The poster showed an alien landscape of huge, very active voloes, and asteamy, copper-colored sea beh a harsh red sky. Stromatolites, a kind of bacterial rock,filled the shallows in the fround. It didn’t look like a very promising place to create andnurture life. I asked her if the painting was accurate.

    “Well, one school of thought says it was actually cool then because the sun was muchweaker.” (I later learhat biologists, when they are feeling jocose, refer to this as the“ese restaurant problem”—because we had a dim sun.) “Without an atmosphereultraviolet rays from the sun, even from a weak sun, would have teo break apart anyincipient bonds made by molecules. A right there”—she tapped the stromatolites—“youhave anisms almost at the surface. It’s a puzzle.”

    “So we don’t know what the world was like back then?”

    “Mmmm,” she agreed thoughtfully.

    “Either way it doesn’t seem very ducive to life.”

    She nodded amiably. “But there must have been something that suited life. Otherwise wewouldn’t be here.”

    It certainly wouldn’t have suited us. If you were to step from a time mae into thata Archaean world, you would very swiftly scamper baside, for there was no moreoxygen to breathe oh back then than there is on Mars today. It was also full of noxiousvapors from hydrochlorid sulfuric acids powerful enough to eat through clothing andblister skin. Nor would it have provided the  and glowing vistas depicted in the poster inVictoria Be’s office. The chemical stew that was the atmosphere then would haveallowed little sunlight to reach the Earth’s surface. What little you could see would beillumined only briefly by bright and frequent lightning flashes. In short, it was Earth, but ah we wouldn’t reize as our own.

    Anniversaries were few and far between in the Archaean world. For two billion yearsbacterial anisms were the only forms of life. They lived, they reproduced, they swarmed,but they didn’t shoarticular ination to move on to another, more challenging levelof existe some point in the first billion years of life, obacteria, or blue-green algae,learo tap into a freely available resource—the hydrogen that exists iacularabundan water. They absorbed water molecules, supped on the hydrogen, and releasedthe oxygen as waste, and in so doing ied photosynthesis. As Margulis and Sagan note,photosynthesis is “undoubtedly the most important siaboliovation in the historyof life on the pla”—and it was ied not by plants but by bacteria.

    As obacteria proliferated the world began to fill with O2to the sternation of thanisms that found it poisonous—whi those days was all of them. In an anaerobic (or anon-oxygen-using) world, oxygen is extremely poisonous. Our white cells actually useoxygen to kill invading bacteria. That oxygen is fually toxic often es as a surpriseto those of us who find it so vivial to our well-being, but that is only because we haveevolved to exploit it. To other things it is a terror. It is what turns butter rancid and makes ironrust. Even we  tolerate it only up to a point. The oxygen level in our cells is only about atenth the level found imosphere.

    The new oxygen-using anisms had two advantages. Oxygen was a more effit way toproduergy, and it vanquished petitanisms. Some retreated into the oozy,anaerobic world of bogs and lake bottoms. Others did likewise but then later (much later)migrated to the digestive tracts of beings like you and me. Quite a number of these primevalentities are alive inside your bht now, helping to digest your food, but abh evei hint of O2. Untold numbers of others failed to adapt and died.

    The obacteria were a runaway success. At first, the extra oxygen they produced didn’taccumulate imosphere, but bined with iron to form ferric oxides, which sank to thebottom of primitive seas. For millions of years, the world literally rusted—a phenomenonvividly recorded in the banded iron deposits that provide so much of the world’s irooday. For many tens of millions of years not a great deal more than this happened. If youwent back to that early Proterozoic world you wouldn’t find many signs of promise forEarth’s future life. Perhaps here and there iered pools you’d enter a film of livingscum or a coating of glossy greens and browns on shoreline rocks, but otherwise life remainedinvisible.

    But about 3.5 billion years ago something more emphatic became apparent. Wherever theseas were shallow, visible structures began to appear. As they went through their chemicalroutihe obacteria became very slightly tacky, and that taess trappedmicroparticles of dust and sand, which became bound together to form slightly weird but solidstructures—the stromatolites that were featured in the shallows of the poster on VictoriaBe’s office wall. Stromatolites came in various shapes and sizes. Sometimes they lookedlike enormous cauliflowers, sometimes like fluffy mattresses (stromatolite es from theGreek for “mattress”), sometimes they came in the form of ns, rising tens of metersabove the surface of the water—sometimes as high as a hundred meters. In all theirmaions, they were a kind of living rock, and they represehe world’s firstcooperative venture, with some varieties of primitive anism living just at the surfadothers living just underh, each taking advantage of ditions created by the other. Theworld had its first ecosystem.

    For many years, stists knew about stromatolites from fossil formations, but in 1961they got a real surprise with the discovery of a unity of living stromatolites at SharkBay on the remote northwest coast of Australia. This was most ued—so ued,in fact, that it was some years before stists realized quite what they had found. Today,however, Shark Bay is a tourist attra—or at least as much of a tourist attra as a pladreds of miles from anywhere mud dozens of miles from anywhere at all  ever be.

    Boardwalks have been built out into the bay so that visitors  stroll over the water to get agood look at the stromatolites, quietly respiring just beh the surface. They are lusterlessand gray and look, as I recorded in an earlier book, like very large cow-pats. But it is acuriously giddying moment to find yourself staring at living remnants of Earth as it was 3.5billion years ago. As Richard Fortey has put it: “This is truly time traveling, and if the worldwere attuo its real wohis sight would be as well-known as the pyramids of Giza.”

    Although you’d never guess it, these dull rocks swarm with life, with aimated (well,obviously estimated) three billion individual anisms on every square yard of rock.

    Sometimes when you look carefully you  see tiny strings of bubbles rising to the surface asthey give up their oxygen. In two billion years such tiions raised the level of oxygeh’s atmosphere to 20 pert, preparing the way for the , more plex chapter inlife’s history.

    It has been suggested that the obacteria at Shark Bay are perhaps the slowest-evolvinganisms oh, aainly now they are among the rarest. Having prepared the way formore plex life forms, they were then grazed out of existenearly everywhere by thevery anisms whose existehey had made possible. (They exist at Shark Bay becausethe waters are too saline for the creatures that would normally feast on them.)One reason life took so long to grow plex was that the world had to wait until thesimpler anisms had oxygehe atmosphere suffitly. “Animals could not summonup the energy to work,” as Fortey has put it. It took about two billion years, roughly 40pert of Earth’s history, for oxygen levels to reach more or less modern levels oftration imosphere. But ohe stage was set, and apparently quite suddenly, airely ype of cell arose—oh a nucleus and other little bodies collectively calledanelles (from a Greek word meaning “little tools”). The process is thought to have startedwhen some blundering or adventuresome bacterium either invaded or was captured by someother bacterium and it turned out that this suited them both. The captive bacterium became, itis thought, a mitodrion. This mitodrial invasion (or endosymbiotic event, asbiologists like to term it) made plex life possible. (In plants a similar invasion producedchloroplasts, whiable plants to photosynthesize.)Mitodria manipulate oxygen in a way that liberates energy from foodstuffs. Withoutthis niftily facilitating trick, life oh today would be nothing more than a sludge ofsimple microbes. Mitodria are very tiny—you could pack a billion into the spaceoccupied by a grain of sand—but also very hungry. Almost every nutriment you absoesto feeding them.

    We couldn’t live for two minutes without them, yet even after a billion years mitodriabehave as if they think things might not work out between us. They maintain their own DNA.

    They reproduce at a different time from their host cell. They look like bacteria, divide likebacteria, and sometimes respond to antibioti the way bacteria do. In short, they keep theirbags packed. They don’t evehe same geiguage as the cell in which they live.

    It is like having a stranger in your house, but one who has been there for a billion years.

    The ype of cell is known as a eukaryote (meaning “truly ed”), as trastedwith the old type, which is known as a prokaryote (“preed”), and it seems to havearrived suddenly in the fossil record. The oldest eukaryotes yet known, called Grypania, werediscovered in iron sediments in Michigan in 1992. Such fossils have been found just once, andthen no more are known for 500 million years.

    pared with the new eukaryotes the old prokaryotes were little more than “bags ofchemicals,” in the words of the geologist Stephen Drury. Eukaryotes were bigger—eventuallyas much as ten thousand times bigger—than their simpler cousins, and carried as much as athousand times more DNA. Gradually a system evolved in which life was dominated by twotypes of form—anisms that expel oxygen (like plants) and those that take it in (you andme).

    Single-celled eukaryotes were once called protozoa (“pre-animals”), but that term isincreasingly disdaioday the on term for them is protists . pared with thebacteria that had gone before, these new protists were wonders of design and sophistication.

    The simple amoeba, just one cell big and without any ambitions but to exist, tains 400million bits of geiformation in its DNA—enough, as Carl Sagan o fill eightybooks of five hundred pages.

    Eventually the eukaryotes learned an even more singular trick. It took a long time—abillion years or so—but it was a good one when they mastered it. They learether into plex multicellular beings. Thanks to this innovation, big, plicated,visible entities like us were possible. Plah was ready to move on to its  ambitiousphase.

    But before we get too excited about that, it is worth remembering that the world, as we areabout to see, still belongs to the very small.

百度搜索 A Short History of Nearly Everything 天涯 或 A Short History of Nearly Everything 天涯在线书库 即可找到本书最新章节.