19 THE RISE OF LIFE
《万物简史英文版》章节: 19 THE RISE OF LIFE,宠文网网友提供全文无弹窗免费在线阅读。!
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 century of further study, we are no nearer tosynthesizing life today than we were in 1953 and much further away from thinking we can.
Scientists 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 each one is a little miracle. By all the laws of probability proteins shouldn’t exist.
To make a protein you need to 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 commontype of protein, you need to arrange eight letters in the right order. But to make collagen, youneed to arrange 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, withoutdirection, and this is where the unlikelihoods come in.
The chances 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 machine but broadened greatly—to about ninety feet, to be precise—to accommodate 1,055 spinning wheels instead of the usual three or four, and with twentysymbols on each wheel (one for each common 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 moretypical number of amino acids for a protein, the odds against all two hundred coming up in a1There are actually twenty-two naturally occurring amino acids known on Earth, and more may await discovery,but only twenty of them are necessary to produce us and most other living things. The twenty-second, calledpyrrolysine, was discovered in 2002 by researchers at Ohio State University and is found only in a single type ofarchaean (a basic form of life that we will discuss a little further on in the story) called Methanosarcina barkeri.
prescribed sequence are 1 in 10260(that is a 1 followed by 260 zeroes). That in itself is a largernumber than all the atoms in the universe.
Proteins, in short, are complex entities. Hemoglobin is only 146 amino acids long, a runt byprotein standards, yet even it offers 10190possible amino acid combinations, which is why ittook the Cambridge University chemist Max Perutz twenty-three years—a career, more orless—to unravel it. 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 hundred thousand types of protein, perhaps a million, eachunique 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 origami and fold itself into a veryspecific shape. Even having achieved this structural complexity, a protein is no good to you ifit can’t reproduce itself, and proteins can’t. For this you need DNA. DNA is a whiz atreplicating—it can make a copy of itself in seconds—but can do virtually nothing else. So wehave a paradoxical situation. Proteins can’t exist without DNA, and DNA has no purposewithout proteins. Are we to assume then that they arose simultaneously with the purpose ofsupporting each other? If so: wow.
And there is more still. DNA, proteins, and the other components of life couldn’t prosperwithout some sort of membrane to contain them. No atom or molecule has ever achieved lifeindependently. Pluck any atom from your body, and it is no more alive than is a grain of sand.
It is only when they come together within the nurturing refuge of a cell that these diversematerials can take part in the amazing dance that we call life. Without the cell, they arenothing more than interesting chemicals. But without the chemicals, the cell has no purpose.
As the physicist Paul Davies puts it, “If everything needs everything else, how did thecommunity of molecules ever arise in the first place?” It is rather as if all the ingredients inyour kitchen somehow got together and baked themselves into a cake—but a cake that couldmoreover divide when necessary to produce more cakes. It is little wonder that we call it themiracle of life. It is also little wonder that we have barely begun to understand it.
So what accounts for all this wondrous complexity? 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 comes in assuming that they arrived on thescene fully formed. But what if the protein chains didn’t assemble all at once? What if, in thegreat slot machine 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 components that make up a human being—carbon, hydrogen,oxygen, and so on—and put them in a container with some water, gave it a vigorous stir, andout stepped a completed 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 once. They didn’t—they can’t have. As Richard Dawkins arguesin The Blind Watchmaker, there must have been some kind of cumulative selection processthat allowed amino acids to assemble in chunks. Perhaps two or three amino acids linked up for some simple purpose and then after a time bumped into some other similar small clusterand in so doing “discovered” some additional improvement.
Chemical reactions of the sort associated with life are actually something of acommonplace. 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 chains called polymers. Sugars constantly assemble to form starches. Crystals can do anumber of lifelike things—replicate, respond to environmental stimuli, take on a patternedcomplexity. They’ve never achieved life itself, of course, but they demonstrate repeatedly thatcomplexity is a natural, spontaneous, entirely commonplace event. There may or may not be agreat deal of life in the universe at large, but there is no shortage of ordered self-assembly, ineverything from the transfixing symmetry of snowflakes to the comely rings of Saturn.
So powerful is this natural impulse to assemble that many scientists now believe that lifemay be more inevitable than we think—that it is, in the words of the Belgian biochemist andNobel laureate Christian de Duve, “an obligatory manifestation of matter, bound to arisewherever conditions are appropriate.” De Duve thought it likely that such conditions would beencountered perhaps a million times in every galaxy.
Certainly there is nothing terribly exotic in 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 combinations to form some sugars, acids, and other basiccompounds and you can build anything that lives. As Dawkins notes: “There is nothingspecial about the substances from which living things are made. Living things are collectionsof 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 existences. To be sure, many of thedetails of life’s beginnings remain pretty imponderable. Every scenario you have ever readconcerning the conditions 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 popularcandidates 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 ofdiscomfort, “Researchers agree that such reactions would not have been energeticallyfavorable in the primitive sea, or indeed in any aqueous medium, because of the mass actionlaw.” It is a little like putting sugar in a glass of water and having it become a cube. Itshouldn’t 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’t turn into polymers—except when creating life on Earth. How and why it happens thenand not otherwise is one of biology’s great unanswered questions.
One of the biggest surprises in the earth sciences in recent decades was the discovery ofjust how early in Earth’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 become solid until about 3.9 billion years ago.
“We can only infer from this rapidity that it is not ‘difficult’ for life of bacterial grade toevolve on planets with appropriate conditions,” Stephen Jay Gould observed in the New YorkTimes in 1996. Or as he put it elsewhere, it is hard to avoid the conclusion that “life, arising assoon as it could, was chemically destined to 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 Advancement ofScience when he suggested that “the germs of life might have been brought to the earth bysome meteorite.” But 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—indeed clamoring—for rocks of extraterrestrial origin.
The Murchison meteorite 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 ResearchCenter in California announced that the Murchison rock also contained complex strings ofsugars called polyols, which had not been found off the Earth before.
A few other carbonaceous chondrites have strayed into Earth’s path since—one that landednear Tagish Lake in Canada’s Yukon in January 2000 was seen over large parts of NorthAmerica—and they have likewise confirmed that the universe is actually rich in organiccompounds. Halley’s comet, it is now thought, is about 25 percent organic molecules. Getenough of those crashing into a suitable place—Earth, for instance—and 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 can be safely called imprudent. FrancisCrick, codiscoverer of the structure of DNA, and his colleague Leslie Orgel have suggestedthat Earth was “deliberately seeded with life by intelligent aliens,” an idea that Gribbin calls“at the very fringe of scientific respectability”—or, 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—and it seems necessary to insert areminder here that he was one of the great scientific minds of the twentieth century—alsoonce 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.
“Wherever 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,” says 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—or something very like it—sits 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 composed 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.
“We can’t be certain that what you are holding once contained living organisms becauseyou’d have to pulverize it to find out,” Bennett told me. “But it comes 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 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—carbon isotopes and a type of phosphate called apatite, whichtogether provide strong evidence that the rock once contained colonies of living things. “Wecan only guess what the organism might have looked like,” Bennett said. “It was probablyabout as basic as life can get—but it was life nonetheless. It lived. It propagated.”
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 Compston, who isnow retired but in the 1970s built the world’s first Sensitive High Resolution Ion MicroProbe—or 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’s crust has been slipped back into the oven at somepoint, but just occasionally—in Western Australia and Greenland, for example—geologistshave found outcrops of rocks that have remained always at the surface. Compston’s machineallowed such rocks to be dated with unparalleled precision. The prototype SHRIMP was built and machined in the Earth Science 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,” Bennett told me, “to find something so important soquickly with brand-new technology.”
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’s 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 compound was an odd combination of things—part offices, part labs,part machine shed. “We used to build everything here,” Bennett 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 contamination from previous samples—no dustor anything. It’s quite a meticulous process.” She showed me the rock-crushing machines,which were indeed pristine, though the rock crushers had apparently gone for coffee. Besidethe machines were large boxes containing rocks of all shapes and sizes. They do indeed getthrough a lot of rocks at the ANU.
Back in Bennett’s office after our tour, I noticed hanging on her wall a poster giving anartist’s colorfully imaginative interpretation of Earth as it might have looked 3.5 billion yearsago, just when life was getting going, in the ancient period known to earth science as theArchaean. The poster showed an alien landscape of huge, very active volcanoes, and asteamy, copper-colored sea beneath a harsh red sky. Stromatolites, a kind of bacterial rock,filled the shallows in the foreground. 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 learned that biologists, when they are feeling jocose, refer to this as the“Chinese restaurant problem”—because we had a dim sun.) “Without an atmosphereultraviolet rays from the sun, even from a weak sun, would have tended to break apart anyincipient bonds made by molecules. And yet right there”—she tapped the stromatolites—“youhave organisms 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 conducive 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 machine into thatancient Archaean world, you would very swiftly scamper back inside, for there was no moreoxygen to breathe on Earth back then than there is on Mars today. It was also full of noxiousvapors from hydrochloric and sulfuric acids powerful enough to eat through clothing andblister skin. Nor would it have provided the clean and glowing vistas depicted in the poster inVictoria Bennett’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 anEarth we wouldn’t recognize as our own.
Anniversaries were few and far between in the Archaean world. For two billion yearsbacterial organisms were the only forms of life. They lived, they reproduced, they swarmed,but they didn’t show any particular inclination to move on to another, more challenging levelof existence. At some point in the first billion years of life, cyanobacteria, or blue-green algae,learned to tap into a freely available resource—the hydrogen that exists in spectacularabundance in water. They absorbed water molecules, supped on the hydrogen, and releasedthe oxygen as waste, and in so doing invented photosynthesis. As Margulis and Sagan note,photosynthesis is “undoubtedly the most important single metabolic innovation in the historyof life on the planet”—and it was invented not by plants but by bacteria.
As cyanobacteria proliferated the world began to fill with O2to the consternation of thoseorganisms that found it poisonous—which in 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 fundamentally toxic often comes as a surpriseto those of us who find it so convivial 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 can tolerate it only up to a point. The oxygen level in our cells is only about atenth the level found in the atmosphere.
The new oxygen-using organisms had two advantages. Oxygen was a more efficient way toproduce energy, and it vanquished competitor organisms. 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 body right now, helping to digest your food, but abhorring eventhe tiniest hint of O2. Untold numbers of others failed to adapt and died.
The cyanobacteria were a runaway success. At first, the extra oxygen they produced didn’taccumulate in the atmosphere, but combined 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 iron oretoday. 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 for Earth’s future life. Perhaps here and there in sheltered pools you’d encounter 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 chemicalroutines, the cyanobacteria became very slightly tacky, and that tackiness 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 VictoriaBennett’s office wall. Stromatolites came in various shapes and sizes. Sometimes they lookedlike enormous cauliflowers, sometimes like fluffy mattresses (stromatolite comes from theGreek for “mattress”), sometimes they came in the form of columns, rising tens of metersabove the surface of the water—sometimes as high as a hundred meters. In all theirmanifestations, they were a kind of living rock, and they represented the world’s firstcooperative venture, with some varieties of primitive organism living just at the surface andothers living just underneath, each taking advantage of conditions created by the other. Theworld had its first ecosystem.
For many years, scientists knew about stromatolites from fossil formations, but in 1961they got a real surprise with the discovery of a community of living stromatolites at SharkBay on the remote northwest coast of Australia. This was most unexpected—so unexpected,in fact, that it was some years before scientists realized quite what they had found. Today,however, Shark Bay is a tourist attraction—or at least as much of a tourist attraction as a placehundreds of miles from anywhere much and dozens of miles from anywhere at all can ever be.
Boardwalks have been built out into the bay so that visitors can stroll over the water to get agood look at the stromatolites, quietly respiring just beneath 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 attuned to its real wonders this sight would be as well-known as the pyramids of Giza.”
Although you’d never guess it, these dull rocks swarm with life, with an estimated (well,obviously estimated) three billion individual organisms on every square yard of rock.
Sometimes when you look carefully you can see tiny strings of bubbles rising to the surface asthey give up their oxygen. In two billion years such tiny exertions raised the level of oxygenin Earth’s atmosphere to 20 percent, preparing the way for the next, more complex chapter inlife’s history.
It has been suggested that the cyanobacteria at Shark Bay are perhaps the slowest-evolvingorganisms on Earth, and certainly now they are among the rarest. Having prepared the way formore complex life forms, they were then grazed out of existence nearly everywhere by thevery organisms whose existence they 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 complex was that the world had to wait until thesimpler organisms had oxygenated the atmosphere sufficiently. “Animals could not summonup the energy to work,” as Fortey has put it. It took about two billion years, roughly 40percent of Earth’s history, for oxygen levels to reach more or less modern levels ofconcentration in the atmosphere. But once the stage was set, and apparently quite suddenly, anentirely new type of cell arose—one with a nucleus and other little bodies collectively calledorganelles (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 some other bacterium and it turned out that this suited them both. The captive bacterium became, itis thought, a mitochondrion. This mitochondrial invasion (or endosymbiotic event, asbiologists like to term it) made complex life possible. (In plants a similar invasion producedchloroplasts, which enable plants to photosynthesize.)Mitochondria manipulate oxygen in a way that liberates energy from foodstuffs. Withoutthis niftily facilitating trick, life on Earth today would be nothing more than a sludge ofsimple microbes. Mitochondria are very tiny—you could pack a billion into the spaceoccupied by a grain of sand—but also very hungry. Almost every nutriment you absorb goesto feeding them.
We couldn’t live for two minutes without them, yet even after a billion years mitochondriabehave 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 antibiotics in the way bacteria do. In short, they keep theirbags packed. They don’t even speak the same genetic language 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 new type of cell is known as a eukaryote (meaning “truly nucleated”), as contrastedwith the old type, which is known as a prokaryote (“prenucleated”), 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.
Compared 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—organisms 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 disdained. Today the common term for them is protists . Compared 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, contains 400million bits of genetic information in its DNA—enough, as Carl Sagan noted, to 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 learned to formtogether into complex multicellular beings. Thanks to this innovation, big, complicated,visible entities like us were possible. Planet Earth was ready to move on to its next ambitiousphase.
But before we get too excited