Chapter 6 Chapter 5 The Replication Big Bang of Life

Most stars Our sun is a typical star that has been burning steadily for billions of years.Sometimes, somewhere in a galaxy, a star suddenly increases in brightness without any obvious warning, becoming a very rare supernova. Over the course of a few weeks, the supernova becomes billions of times brighter, and then it dies off, leaving only a faint remnant.In the few days when a star goes supernova and is at its climax, it emits more energy than a normal star has emitted over the past billions of years combined.If our sun went supernova, the entire solar system would vanish in an instant.Fortunately, this kind of thing is not easy to happen.In our galaxy, which contains hundreds of billions of stars, astronomers have recorded only three supernova explosions so far, in 1054, 1572 and 1604.The Crab Nebula is the remnant of a supernova explosion in 1054.This supernova was discovered and recorded by Chinese astronomers.Of course, when I speak of the 1054 event here, I mean that information about the event reached Earth in 1054, and the event itself occurred 6,000 years earlier than that.The bright front of the supernova reached Earth in 1054.Since 1604, only supernovae in other galaxies have been observed.

There is another class of explosions that stars can sustain.This kind of explosion does not form a supernova, but produces information.This kind of explosion is much slower than the explosion of supernova, and the long time of formation is unmatched by supernova.We can call it the information explosion or the replication explosion.Why is it called a replication explosion, we will talk about it below.In the first few billion years of the formation of the information explosion, only in close proximity can it be possible to detect the replication explosion.Eventually, this looming evidence of the explosion leaked farther out in the universe, and it became (at least likely) detectable from afar.We don't yet know how this explosion will end.Perhaps, eventually it will fade away like a supernova, but first we don't yet know how far this explosion will go.Perhaps the ultimate outcome is a violent, self-destructive catastrophe; perhaps, the ultimate outcome is a gentler, repeated ejection of matter, with matter leaving the planet and entering The distant cosmic space, where it may affect other galaxies to explode with the same tendency.

The reason why we know so little about replicating explosions in the universe is that we have seen only one example so far; and one example is not sufficient as an inductive criterion for any phenomenon.The history of our example is still developing.So far, it has gone through a history of three to four billion years, and has just begun to leak to the next star.The star we're talking about is a yellow main-sequence dwarf near the edge of our own Milky Way, in one of the Milky Way's spiral arms.We call it the sun.The explosion actually originated from a planet in a perihelion orbit, but the energy that caused the explosion came entirely from the sun.Of course, this planet is the earth, and this four billion-year explosion (or called the replication explosion) is the replication explosion of life.We humans are an extremely important embodiment of the reproduction explosion, because it is through us through our brains, our symbolic culture, and our technology that this explosion can progress to the next stage, echoing in the depths of the universe .

I said earlier that our replica bomb is the only replica bomb we have known so far in the universe, but this does not mean that there are fewer such explosions than supernovae.It is undeniable that three supernovae have been observed in our galaxy, and that is because supernovae are so powerful that they are easy to see from very long distances.Our explosion of life would not have been observed, even on other planets very close to us, if radio waves emitted by humans had not started flying from our planet into space a few decades ago.Until recently, perhaps the only obvious concrete manifestation of our life-replication explosion was the Great Barrier Reef.

A supernova is a huge, sudden explosion.Any explosion starts with a triggering event: some amount exceeds a critical value, and things spiral out of control, leading to results far beyond the triggering event.The triggering event of the replication explosion is the spontaneous generation of self-replicating and mutable independent bodies.Self-replication is a potentially explosive phenomenon for the same reason as any explosive event: exponential growth.The more money you have, the more profit you can make.Once you had one self-replicating item last time, you'll soon have two.Then make another copy of each, and you have four.Then there are eight, sixteen, thirty-two, sixty-four copies, and in just thirty generations, you'll have more than a billion copies.After fifty generations, there will be one hundred trillion pieces.After two hundred generations of reproduction, there will be an astronomical number (one thousand and sixty) of reproductions.This is a theoretical calculation.In reality, it could never reach such a large number, because this number is greater than the total number of atoms in the universe.This self-copying explosion process was restrained long before their free multiplication reached two hundred generations.

We have yet to find direct evidence of a replication event that kicked off this process on Earth.We can only deduce that this event must have happened, since we ourselves are part of the converging explosion.We don't know exactly what the original key event that initiated self-replication was, but we can deduce what kind of event it must have been.It starts with a chemical event. Chemistry is the script that plays out in the interiors of all stars and on all planets.The actors who play roles in chemistry are atoms and molecules.Even the rarest atoms are enormously numerous by the counting standards we are used to.Isaac.Asimov (Isaac Asimov) once calculated that the number of 215 atoms in the 16-kilometer-deep formation in North America and South America is only one trillion.The basic units of chemistry are always changing partners, forming constantly changing numbers of larger unit molecules.Yet no matter how great the number, the molecules of the same kind are always the same; unlike individual animals of the same species, or every violin made by a famous craftsman, there is always something different between them.The chemical laws of atomic motion are such that certain molecules are more and more abundant in the world; others are becoming rarer and rarer.Naturally, biologists call those molecules that are more and more present in the population successful.It would be unhelpful, however, to hastily accept this claim.Success, in its literal sense, is a quality attained only later in our story.

So what was the big key event that led to the replication explosion?I have already said that it is the emergence of self-replicating entities; we may also call self-replicating entities the creation of hereditary phenomena.We can call this process the same kind begets the same kind, that is, the dragon begets the dragon, and the phoenix begets the phoenix.This is not a phenomenon usually shown by molecules.Water molecules, though produced in vast populations, do not exhibit any properties approaching true inheritance.On the surface, you might think they have this property.Hydrogen (H) and oxygen (O) are combusted to form water, and the population of water molecules (H2O) increases.Water molecules are decomposed into hydrogen and oxygen by electrolysis, and the groups of water molecules become smaller again.However, despite the dynamics of the population of water molecules involved, it has no genetic properties.True inheritance requires a minimum condition, that is, there must be two distinct H2O molecules that can produce identical copies of themselves.

Molecules sometimes appear as two mirror images.There are two kinds of glucose molecules, both of which are assembled from the same atoms in the same way, the only difference is that they are mirror images of each other.The same goes for other sugar molecules, and many others, including the very important amino acid molecules.Perhaps, for chemical genetics, there is an opportunity for dragons to beget dragons and phoenixes to phoenixes.Can right-handed molecules produce the next generation of right-handed molecules, and can left-handed molecules produce the next generation of left-handed molecules?First, let's get some background on mirror images.This phenomenon was first discovered in the nineteenth century by the great French scientist Louis Pasteur.Pasteur made this discovery while observing crystals of tartrate, a very important substance in wine.A crystal is like a solid building, large enough to be seen with the naked eye, and is sometimes worn as a necklace.The same atoms (or molecules) are piled up next to each other to form a solid, which is a crystal.These atoms (or molecules) are not randomly stacked together, but arranged in a regular geometric array, just like a uniform line of guards of uniform stature.Those molecules that were already part of the crystal became the templates for the newly added molecules; the new molecules were precipitated from the aqueous solution and embedded precisely in the crystal.In this way, the entire crystal grows according to a precise geometric lattice.This is why salt crystals are hexahedral, while diamond crystals are tetrahedral.When any shape becomes a template for building another shape like itself, we have the faintest hint of possible self-replication.

Now, let us return to Pasteur's tartrate.Pasteur noticed that two different crystals appeared in the same aqueous solution of tartrate.The two crystals are identical except that they are mirror images of each other.With great difficulty he separated the two crystals into two piles.Dissolving them separately again, he obtained two tartrate solutions.Although the two solutions shared most of their properties, Pasteur discovered that they rotated polarized light in opposite directions.Of these two solutions, one rotates polarized light counterclockwise and the other rotates polarized light clockwise, so these two types of molecules are often referred to as left-handed and right-handed molecules, respectively.You would imagine that if the two solutions were crystallized again, the pure crystals produced by each liquid would be the mirror image of the other.

The mirror image molecules are indeed different from each other, just like the left shoe is different from the right shoe, no matter how hard you try, it is impossible to replace them with each other.Pasteur's original solution was a mixture of two types of molecules which, when crystallized, each insisted on aligning itself with its like.The existence of two (or more) distinct types in an individual is a necessary condition for true inheritance to exist, but it is not sufficient.To achieve true inheritance among crystals, when the left-handed and right-handed crystals reach a certain critical size, they should be divided into two, and each half becomes a template for growth, and then grows to a critical size.Under these conditions, we do see two growing opposing populations of crystals.We should perhaps really say success in crystal populations, since both types of crystals are vying for the same atomic composition, and one type of crystal may become much more numerous by being good at self-replication at the expense of the opponent.Unfortunately, the vast majority of known molecules do not possess such outstanding genetic properties.

Why do I say sorry?For medical purposes, chemists want to make molecules that are all left-handed, and they would like to be able to breed them.However, as far as molecules play the role of templates for forming other molecules, these molecules usually become templates for their mirror image molecules, instead of left-handed molecules developing left-handed molecules and right-handed molecules developing right-handed molecules. This complicates matters, because if you start with the left-handed form, you end up with a 50/50 mixture of left-handed and right-handed molecules.Chemists in this field are trying to trick molecules into giving birth to daughter molecules that spin in the same direction.However, this is an extremely difficult trick to pull off. In fact, some form of this trick (although it may not involve such a thing as left-handed and right-handed) was performed naturally and spontaneously four billion years ago.At that time, the world was just born, and the big bang that transformed into life and information had just begun.However, something more than simple genetics was needed before the Big Bang could proceed smoothly.Even if both left-handed and right-handed molecules displayed genuine genetic traits, competition between them would not yield very interesting results, since there are only two of them.For example, once the Leftists win the competition, it's over.There will be no more progress. Larger molecules can exhibit handedness at different parts of the molecule.For example, the antibiotic monensin has seventeen asymmetric centers.Each of these seventeen centers has a left-handed configuration and a right-handed configuration.Two to the seventeenth power is equal to 131072, therefore, this molecule has 131072 different molecular isomers.If the 131,072 isoforms all possessed a true genetic identity, and each configuration produced only its own type of molecule, then intricate competition would ensue, as the most successful member of the 131,072 configurations would be the most successful member in successive generations of molecules Group statistics will gradually show their own advantages.But even so, this is only a finite inheritance, because 131072, although a large number, is still a finite number.Inheritance is necessary for a veritable reproduction explosion; so is infinite number, that is, endless variety. Regarding the issue of mirror image inheritance, we have already discussed monensin as an example. However, the difference between left-handed and right-handed is not the only difference that might be used for genetic replication. Chemist Julius of the Massachusetts Institute of Technology.The problem of generating self-replicating molecules was seriously studied by Julius Rebek and his colleagues.What they are using is not a mirror image variant.Rebeck et al. used two small molecules. It doesn't matter what they are called, let's call them A and B.When A and B are mixed in solution, they combine to form a third compound C.The role of each C molecule is like a template (or mold). Molecules A and B, floating freely in the solution, found themselves crammed into the mould.An A and a B jostle and fall into a certain position in the mold, so that they line up exactly and join together to form a new C molecule, exactly like the previous C molecule.These C molecules are not tightly packed together to form crystals, but separated from each other.Both C molecules now serve as templates for generating new C molecules, and the population of C molecules grows exponentially. So far, the system has not shown true inheritance, but it has registered results.All forms of B molecules combine with A molecules to form their own class of allomorphs of C molecules.We would then have various types of C molecules: c1, c2, c3, and so on.Each class of C molecule variants is a template for forming its own C molecule.Therefore, the population of C molecules is heterogeneous.In addition, different types of C molecules differ in their efficiency at making daughter molecules.Therefore, there is competition among the various C molecule alloforms in the C molecule population.Even better, however, UV radiation can induce spontaneous mutations in the C molecule.The new mutant is truly bred, and it produces progeny molecules that resemble it. Satisfyingly, the new mutant outcompetes its parent and soon fills the test tube where the parent molecules live. The A/B/B complex is not the only group of molecules that behaves this way.And D, E, and F, which is a comparable triad.Rebeck's group was even able to cross components of the A/B/C complex and the D/E/F complex to form self-replicating hybrids. DNA and RNA, the truly self-replicating molecules we know of in nature, are generally more potential for variation.If Rebek's replica is a short chain with only two segments, then the DNA molecule is an infinitely long chain: this long chain has thousands of segments, and each segment has four types; and, when a specific DNA As a template for the formation of new DNA molecules, each link becomes a template for another specific type of link.These four units are called alkali groups, and they are four compounds: adenine, thymine, cytosine and guanine, abbreviated as A, T, C and G, respectively. A is always a model of T, and vice versa, T is always a model of A; G is always a model of C, and vice versa, C is always a model of G.Any conceivable permutation of A, T, C, and G is possible and faithfully reproduced.In addition, since the length of the DNA chain is uncertain, the range of possible variations is practically unlimited.This is a potential recipe for an information explosion whose reverberations can reach from this planetary home to every star. The replication explosion of our solar system, for most of the four billion years since it began, has had its effects confined to its own planet.It is only within the last million years that a nervous system has emerged capable of inventing radio technology.And, only in the last few decades has this nervous system actually developed radio technology.Now, radio waves carrying a wealth of information are traveling from this planet to distant space at the speed of light. I say rich in information because there are already many radio waves traveling through the universe.Stars emit radiation at visible light frequencies, which we already know; stars also emit radiation at radio frequencies.The background noise left over from the primordial big bang remains, its baptism of space-time.But these radio waves don't form a meaningful pattern, it's not quite informative yet.Radio astronomers on a planet in the constellation Proxima Centauri would detect this background noise as well as radio astronomers on Earth, but they would also notice that there was a pattern coming from the direction of the star, the Sun. More complex radio waves.They may not see this pattern as a mix of signals from four-year-old TV shows, but they will see the radio waves coming from the direction of the sun as more complex and informative than the usual background noise .Radio astronomers in Centaurus would report with excitement and excitement that an informational supernova explosion had occurred on the sun (they would guess but not be sure that it was actually from a planet orbiting the sun). As we know, life-replicating explosions are much slower in terms of time course than supernova explosions.Our own life-replication explosion took billions of years to reach the gates of radio: at this point, a portion of our human world began to flow out into the universe and began to bathe the star systems adjacent to our solar system in meaningful messages from us. in the pulse.If our information explosion is typical, we can imagine that the information explosion will cross a series of progressive thresholds.In the course of the explosion of life duplication, the radio-threshold and the previous threshold, the language-threshold, appeared relatively late.At least on our planet, these two thresholds are preceded by neuronal thresholds, and before that are multicellular thresholds.The first threshold, the ancestor of all these thresholds, is called the replicator threshold, and it is the triggering event that leads to the whole Big Bang process. Why are replicators so important?How is it possible that by accident a molecule with an innocuous property that can serve as a template for the synthesis of another molecule identical to itself is the trigger of a Big Bang whose eventual reverberations may extend beyond the planets?As we have seen, part of the replicator's power lies in its exponential growth.A simple example is the so-called chain letter.You received a postcard in the mail, which said: Copy the contents of this postcard on six postcards, and distribute them to your six friends within a week.If you don't, you will be under the spell and die within a month of horrific anguish.If you were sensible, you would throw this postcard aside.However, there are quite a few people who are not so wise. They either participate blindly, or they are afraid of intimidation and send six copies of the same thing to others.Of the six, maybe two were persuaded to send postcards to the other six.Assuming that, on average, 1/3 of the recipients complied with the request, the circulation of the postcards would double every week.Theoretically speaking, this means that the number of postcards mailed in a year will be as many as two to the fifty-second power, which is equal to four quadrillion.So many postcards, enough to suffocate men, women and children all over the world. Exponential growth, if it is not stopped by resource scarcity, can have astonishingly large results in an astonishingly short amount of time.In reality, resources are finite, and there are other factors that can limit exponential growth.In the example we envisioned, people would probably avoid the chain letter the second time they received it.In the race for resources, certain variants of replicators arise because they effectively multiply.These more efficient replicators gradually weed out less efficient competitors.It is important to understand that none of these replicas are consciously multiplying themselves.But what will happen is just this: the world will be filled with more efficient replicators. In the chain letter example, it may be more effective to write more nice words on the postcard.Remove the unbelievable language that says if you don't do what the postcard says, you will die in horrific pain within a month and replace it with I implore you, to save your soul and mine, please Take no chances; even if you are a little skeptical about this letter, please follow the instructions on the letter and send this letter to six other people.Mutations like this will happen over and over again, and the end result will be that the letters in the mail will be a hodgepodge of many different pieces of information, all from the same original, but with specific wording variations, each letter The degree and method of flattery are also different.More successful mutants gain increased success at the expense of their less successful counterparts.Success is a simple synonym for frequency of circulation.The St. Jude's letter is a famous example of this success; the letter has circulated several times around the world, probably increasing in number in the process.As I write this book, Oliver Olivier of the University of VermontDr. Oliver Goodenough sent me that letter, and we co-wrote an article for Nature on it, "Thought Viruses."The letter of St. Zude is like this: With love, everything can be done. This article is sent to you to send you good luck.This stuff originated in New England.It has been mailed nine times around the world so far.This time it's time to deliver the goodies to you.Good luck if you follow up on this letter within four days of receiving it.This is no joke.You will receive good luck in the mail.Do not send money.Please send a copy of this article to anyone you think needs good luck.Never send money, because faith is priceless.Do not keep this letter.This letter must leave you within ninety-six hours.Joe A.R.P., an officer of the Aspect Research Project (A.R.P).Eliade received forty million dollars.Jie.Welch's wife died five days after he received the letter because he hadn't mailed it.However, before his wife died, Welch received seven.fifty thousand dollars.Please send a copy of this letter and see what happens in four days.This chain letter comes from Venezuela, is Saul.Anthony.Degenes, a South American missionary.Copies of the letter must travel the world.You must make twenty copies of this letter, send it to your friends and contacts, and in a few days, you will have a surprise.Even if you're not superstitious, it's love.Please note the following facts: Cantona.Dias received the letter in 1903, and he asked his secretary to copy it and post it.A few days later, he won $2 million in the lottery.Carl.Dobbit was an office worker who, after receiving the letter, forgot that he had to mail it within ninety-six hours.He lost his job.Later, he found the letter, copied twenty copies and sent it.A few days later, he got a better job.Dolan.Fairchild didn't believe the letter, threw it aside, and died nine days later.When a young woman in California received this letter in 1987, it was very worn and illegible.She thought to herself, to retype it and send it.So put the letter aside and do it later.She suffered from various problems, including spending large sums of money to fix her car, because the letter sat in her hands for ninety-six hours without delivery.Later, she finally retyped the letter, fulfilling the promise in her heart, and she got a new car.Remember, don't send money.Don't ignore it, it works. St. Jude's comical document.There are traces of several mutations everywhere.The text is full of holes, misnomers, and other versions are known to be in circulation.After our article was published in Nature, I received several distinctly different versions from around the world.For example, in one of the letters A. R． P．Officials are written as R. A． F officer.The U.S. Postal Service is very familiar with San Jude letters.The Postal Service report said the letter came back before the Postal Service had official records, and there were signs of another outbreak. There are many examples in the letter, good luck for those who follow them, and disasters for those who refuse to do so; however, please note that it is impossible for those who follow them or those who refuse to write about their own affairs in the letter to go.Those who were lucky did not encounter good things before the letter was sent, and the victim never sent the letter at all. How could they write about their own experience?We can surmise that these stories are fabricated, as one would expect from the incredible content of the letter.This is the main difference between chain letters and those natural replicators that lead to a replication explosion.Chain letters were originally sent by humans, and variations on wording were created by human minds.However, there was no thought, no creativity, no intention at the beginning of the replication explosion, there was only chemistry.However, once the self-replicating chemicals had a chance to grow, there was a spontaneous tendency for the more successful variants to grow frequently at the expense of the less successful ones. As in chain letters, in chemical replicators success is synonymous with turnover rate.But that's just a definition, almost a tautology.Success is achieved by actual ability, and actual ability means something tangible, anything other than a tautology.A successful replication-competent molecule has certain chemical properties that enable it to replicate.Although the nature of the replicators themselves appears to be strikingly uniform, in practice this means that there can be almost endless variation. DNA is so consistent that it consists entirely of variations in the sequence of the four letters A, T, C, and G.DNA sequences, by contrast, use a bewildering variety of methods to replicate themselves, as we have seen in previous chapters.Let's give a few examples: giving hippos more powerful hearts, fleas more elastic legs, swifts more aerodynamically streamlined wings, and fish more buoyant swim bladders.All the organs and limbs of animals, the roots, leaves and flowers of plants, all eyes, brains and thoughts, even fear and hope, are the tools through which successful DNA sequences send themselves into the future.The tools themselves are ever-changing, but, conversely, the methods of making them are incredibly singular: just the arrangement of A, T, C, G mutating again and again. Things may not always be that way.We have no evidence yet that the seed code was written in DNA letters at the beginning of the information explosion. Indeed, the entire information technology based on DNA/protein is so complex, chemist Graham.Graham Cairns-Smith called it high technology, and you can hardly imagine that it arose by accident without some other self-reproducing system as its precursor.The precursor might be RNA; or it might be something like Julius.Rebek's simple self-replicating molecule; or it could be something very different: a challenging possibility I discuss at length in The Blind Watchmaker, which Cairns-Smith himself proposed (see his "Seven Clues to the Origin of Life").He said that the inorganic clay crystals were the original replicators.We may never be sure of this. All we can do is guess at the chronology of a replicating Big Bang on any planet, anywhere in the universe.What exactly happened must depend on local conditions.In the cold world of liquid ammonia, the DNA/protein system wouldn't do much, but perhaps some other genetic and embryonic system would.Anyway, these are exactly the kind of details I want to ignore, since I want to focus on a few principles of a general approach that are not planet-related.I now turn to a more systematic discussion of the series of thresholds that must pass through a life-replication explosion on any planet.Some of these thresholds are likely to be universal, while others are limited to our planet.Determining which ones are general and which ones are only local is not an easy task, and it is a question of great interest in itself. The first threshold, of course, is the replicator threshold, that is to say, some kind of self-copying system has emerged.In such a system, there is at least one imperfect genetic variant that makes occasional random errors in replication.Once the first threshold is crossed, there is a mixed population on the planet, in which various mutants compete with each other for resources.Resources will be in short supply, or will become scarce when competition reaches fever pitch.Replicas of some variants will be quite successful in competing for scarce resources, while replicas of other variants will be less successful.Thus, we now have a basic form of natural selection. In the beginning, success among competing replicators depended purely on the immediate properties of the replicators themselves, for example, how well their shape conformed to the template, but now, after many generations of evolution, we have come to the second Road threshold, that is, phenotype threshold.Replicators survive not by virtue of their own characteristics, but by having a causal effect on something, which we call a phenotype.On our planet, phenotypes are easily identified, such as those parts of plants and animals that are affected by genes.This means all parts of the body.Think of phenotypes as the levers of competence by which successful replicators control the path to the next generation.In general, it might be possible to define a phenotype as something that is produced by a replicator and affects how successful the replicator is, without itself being replicated.Let's take an example. On an island in the Pacific Ocean, there is a snail that has a special gene that determines whether the snail shell is right-helical or left-helical. The DNA molecule itself is neither right-handed nor left-handed, but its phenotype has a helical direction.At the point of providing protection for the snail's body, left-handed shells and right-handed shells may not be as effective as right-handed shells, and they are not all equally successful.Genes in snail shells affect the shape of snail shells.The genes that make successful snail shells outnumber the genes that don't make successful snail shells.As a phenotype, snail shells do not give birth to the next generation of snail shells.Every snail shell is made of DNA, and it is DNA that produces DNA. DNA sequences affect their phenotypes (such as the helical orientation of snail shells) through a series of intermediate events of varying complexity, all of which are grouped under the general heading of embryology.On our planet, the first link in this chain is always the synthesis of protein molecules.Every detail of a protein molecule is precisely determined by the sequence of four letters in DNA through the well-known genetic code.However, these details are likely to have only partial significance.In general, a planet will contain replicators whose phenotype (by whatever means) has a beneficial effect on the replicator's success in replicating.一旦跨越了表現型門檻，複製者就以其代表表現型在這個世界上存活下去。在我們這個行星上，這些表現型通常被限制在基因所存在的軀體內。但是，並非一定如此。有一種學說，叫做擴大的表現型（我曾以此為題寫過一本書）。它認為，複製者用以實現其長期生存的表現型力量杠杆並不一定限制在複製者自己體內。基因能越出特定的軀體，自由地影響世界，包括影響其他軀體。 我不知道表現型門檻會達到怎樣的廣泛程度。我推測，在生命複製的大爆炸已經超越最初階段的所有行星上，這個門檻應該已被跨過去了。並且，我還推測，列在我這個清單上的下一個門檻也是如此。這就是第三個門檻，即複製者門檻。這個門檻，在某些星球上可能先於、或者與表現型門檻同時被跨過。在早期，複製者很可能是獨立的存在體，與裸露的複製者對手一起在基因之河的源頭浮動著。但是，我們地球上的現代ＤＮＡ／蛋白質資訊-技術系統的特徵，是基因不可能孤立地起作用。基因在其中發揮作用的那個化學世界，不是外部環境中的那種孤立無助的化學。誠然，外部環境構成了背景，但它是一種非常遙遠的背景。ＤＮＡ複製者賴以生存的最直接且至關重要的化學世界，是一個小得多的、比較集中的化學物質的口袋細胞。在某種意義上講，把細胞叫做化學物質的口袋會誤導讀者，因為很多細胞內部都有複雜的折疊膜結構，極重要的化學反應就在膜內、膜上，以及膜之間進行。化學微觀世界細胞是由千百個基因聯合組成的，而一些高等細胞是由數十萬基因構成的。每一個基因都對環境有所貢獻，所有基因都在開拓環境，以保證生存。基因是成組發揮作用的，在第一章中我們已從稍稍不同的角度看到了這一點。 在我們的行星上，最簡單的自發ＤＮＡ複製系統是細菌細胞，而為了製造所需的組分，它們至少要有數百個基因。非細菌的細胞稱為真核細胞。我們自己的細胞，以及所有的動物、植物、真菌和原生動物的細胞，都是真核細胞。它們通常有數萬個乃至數十萬個基因，所有的基因都成組地發揮作用。正如本書第二章所說，看來很可能真核細胞是從結對的五個左右的細菌細胞開始的。但是，這是一種更高級的群體工作方式，不屬於我在這裡要講的內容。我要強調的事實是，所有的基因都在化學環境中起作用，而這個環境是由細胞裡的基因聯合構成的。 在我們領會了基因是以組群方式工作之後，顯然立即會設想：現今的達爾文選擇是在基因組競爭對手之間進行的，也就是假設選擇已經上升到更高一級的組織。的確很誘人，但依我的觀點來看，這種說法在更深層次上出現了錯誤。如下的說法要明確得多：達爾文選擇仍然是在基因競爭對手之間進行，得寵的是那些在其他基因面前更為成功的基因；儘管這些其他基因在另一些基因面前是屬於得寵的。這一點，我們已在第一章中討論過了。我們曾提到，共用數字之河的同一條支流的基因逐漸變成了好夥伴。 當生物複製炸彈在一個行星上聚集力量時，必須跨越的下一個重要門檻，或許就是多細胞門檻，我把它稱為第四道門檻。正如我們在前面說過的，在我們的身體中，每個細胞都是一個小小的局部化學物質之海，有一組基因沉浮其中。雖然它容納了整個組，然而它卻是由組內一個分組所製成的。細胞將自己一分為二，然後每一部分又長成整個細胞那麼大，於是細胞數增加了一倍。在發生這種情況的時候，基因組的所有成員都增加了一倍。如果這兩個細胞並未完全分離，而是互相間保持著接觸，就能形成一個大的體系、大的建築。這時，細胞扮演著磚塊的角色。建造多細胞體系的能力是非常重要的，不管在我們的世界裡，還是在其他的世界裡都是如此。跨越多細胞門檻之後，表現型就出現了。表現型的外觀和功能只有在比單個細胞大得多的尺度上才會被領會。鹿角或樹葉，眼睛的晶狀體或蝸牛殼，等等，所有這些形狀都是由細胞組成的，但是細胞的形狀與它們完全不同。換句話說，多細胞器官的生長不同於晶體的生長。可以說，至少在我們的星球上，它們的生長更像一座建築物的建設。建築物的形狀畢竟不像一塊長得過大的磚。手有它獨特的形狀，但它並不是由手形細胞構成的；然而，如果表現型像晶體那樣生長的話，手就會是由手型細胞構成的了。另外，像建築物一樣，多細胞器官之所以具有特定的形狀和大小，是因為一層層的細胞（磚塊）根據某些規則在一定的時間停止生長。從某種意義上說，細胞還必須知道相對於其他細胞它們自己應處在什麼位置。肝細胞的表現，就好像它們知道自己是肝細胞一樣，此外，它們還知道自己是處在肝葉的邊緣，還是在中間部位。至於它們是怎樣做到這一點的，這是個很難解答的問題，需要進一步研究。對這一問題的回答，可能只適合我們這個星球的局部情況，因此我就不在此作進一步的討論了。在第一章裡提到過這些情況。無論它們的細節如何，如同生命中其他方面的改進一樣，肝細胞的生長方法也被完全相同的一般過程所優化了：成功基因的有規律生存是由其作用所決定在這個事例中，作用是指對細胞與相鄰細胞的關係發揮影響。 我要考慮的下一個主要的門檻，是高速資訊處理門檻，我覺得它的意義可能已超越了一個具體的星球。在我們這個星球上，這第五道門檻是由一類特殊的細胞神經原（或稱神經細胞）跨越的，因此，在地球上我們可以把這道門檻叫做神經系統門檻。不論在一顆行星上是怎樣跨越這道門檻的，這個階段都是極重要的。 因為到了這一步，產生一個動作所需要的時間，要比基因靠化學作用直接運作所花時間短得多。捕獵者撲向它們的美餐，而獵物為求活命而躲閃，都是動用肌肉和神經系統採取行動和作出反應，其速度要比基因最初構建器官胚胎的折疊速度不知快多少倍。當然，在其他星球，速度的絕對值和反應時間的長短會與地球的情形大不相同。但是，在任何星球上，只要由複製者構建的裝置的反應時間，開始比複製者自己創立胚胎的時間快上若干數量級時，就意味著跨越了一道重要的門檻。我們尚不能肯定，其他星球上的那種裝置是否一定與我們這個星球上的神經原和肌肉細胞這些東西相似。然而在那些星球，一旦跨越了類似我們星球的神經系統門檻，一系列重要的進展就會接踵而來，而複製爆炸將踏上衝出星球走向宇宙的旅程。 在這些結果中，腦作為資料處理單元的巨大集合體，能夠處理被感覺器官所獲取的複雜資料，並且能夠把資料記錄於記憶體中。跨過神經原門檻之後，便出現更為複雜、更為神秘的結果知覺意識，我把它稱為第六個門檻：知覺門檻。在我們這個星球上，每隔多少時間出現一次這個門檻，我們還不清楚。有一些哲學家認為，知覺門檻與語言有決定性的密切關係；看來這道門檻只曾達到過一次，是被二足的猿類智人越過的。無論知覺是否需要語言，我們都應該認識到語言門檻是一道主要的門檻，即第七道門檻，是在某一顆星球上也許被越過了，也許還沒有被越過的門檻。 至於語言的一些細節，諸如它是通過聲音，還是通過其他物質媒介來傳播，這取決於當地的具體條件。 從這一點來看，語言是一個起聯網作用的系統。借助這個系統，腦（這是我們這個星球上的稱呼）之間可以友好地交換資訊，從而使協作的技術得以發展。協作技術，從製造石器工具開始，經歷了金屬冶煉、有輪交通工具、蒸汽機時代等，直至今日的電子技術時代，其本身就具有大爆炸的許多特徵。因此，協作技術的始創就完全配得上協作技術門檻這個稱呼，這是第八個門檻。的確，人類文化可能已經培育出一枚全新的複製炸彈，它具有一種新的自我複製機制（我在《自私的基因》一文中稱為meme），在一條文化之河中增殖和進化。meme炸彈可能正在發射，它與基因炸彈的軌跡相平行；是後者較早建立了腦／文化條件，使得發射成為可能。然而對我們這一章來說，這又是一個大大的題目。我必須回到行星上的大爆炸這個主題上去，我們還要注意到，一旦達到了協作技術這個階段，發展到一定程度時，就很可能擁有向本行星以外施加影響的能力。於是，就跨過了第九道門檻無線電門檻。那顆行星之外的觀察者就有可能注意到，有一顆恒星系統新近發生爆炸，那是一顆複製炸彈。 正像我們已經看到的，外星上的觀察者們最早收到的很可能是無線電波，那是這顆行星上進行無線電通信時洩漏到外空的電波。此後，複製炸彈的技術繼承人可能自己就會主動地將注意力轉向天外恒星。我們自己業已朝這個方向邁出了小小的一步：向宇宙空間發射專為外星智慧生命特製的資訊。但是，你完全不瞭解外星智慧的特性，又怎能為它特製資訊呢？顯然，這是很困難的，而且我們的努力很有可能已經被錯誤地接受了。 迄今為止，人類主要的精力都放在努力使其他星球上的觀察者們注意到我們的存在，而不是向他們發出有實質內容的資訊。這項任務與第一章中提到的那個假想的克裡克森教授面臨的任務同樣艱鉅。克裡克森把要傳達給外界的資訊導入病毒的基因組中，我們可以採用與克裡克森相同的策略，借助無線電傳送資訊，向地球以外的世界標示我們的存在，這可能是一個很靈敏的方法。音樂或許是宣傳我們這個物種的一個好辦法，即使收到音樂信號者沒有耳朵，他們也會以他們自己的方式來理解它。著名科學家兼作家路易斯．湯瑪斯（Lewis Thomas）曾建議，應該播放巴赫的作品，巴赫的全部作品，除了巴赫以外什麼也不播放。當然，湯瑪斯自己也擔心，他這種意見會被當作是一種自我吹噓。然而，同樣地，完全是外星思想的人可能會錯誤地認為那音樂是脈衝星有節奏地發射出來的脈衝波。脈衝星是一種每隔幾秒鐘（或更短時間）有規律地發出一個無線電波脈衝的恒星。一九六七年，劍橋大學的一個射電天文學小組首次發現脈衝星，一時間引起轟動，因為人們以為這些信號可能是外星人給我們發來的信號。然而人們很快就認識到，比較嚴謹的解釋是：一顆以極高速度旋轉的小恒星，不斷地向外發射無線電掃描波束，就像燈塔工作時那樣。直至今天，我們還沒有接到過從宇宙中傳來的確實無疑的通信信號。 在無線電波之後，關於我們自己的大爆炸的下一步發展，我們所能想像到的就是實際的太空旅行了。這就是第十道門檻：太空旅行門檻。科幻小說家們一直夢想著人類的子代群體在其他行星上繁衍，或者幻想著他們的機器人在行星際增殖。這些子代群體可以被視為自我複製資訊新袋中的種苗；這些資訊袋會以衛星複製炸彈，繼續用爆炸的方式向外再次擴張，既傳播基因，又傳播meme。如果這一幻想有朝一日成為現實，那麼我們作如下想像也許不算太離譜：未來的某個馬婁（ChristopherMarlowe）重新描述數字之河，他說道：看哪，看哪，生命的洪流在大空中流淌！迄今為止，我們幾乎還沒有向外邁出第一步。是的，我們已經登上月球；儘管這是個宏偉的成就，儘管月球不是一隻葫蘆，但是從我們終將與之取得聯繫的外星人的角度來看，月球距地球大近了，幾乎算不上什麼太空旅行。我們已經向太空深處發射了數目不多的幾個無人宇宙飛行器，它們正沿著沒有盡頭的軌道向前飛行。 其中之一，作為富於幻想的美國天文學家卡爾．薩根（CarlSagen）突發靈感的結果，攜帶著專門設計來讓任何偶然遇到它的外星智慧生物破譯的資訊。鐫刻著資訊的金屬牌上還飾有製造者的形象一男一女的裸體像。 這看來是帶我們兜了一圈，回到本書開始時關於祖先的神話傳說中去。然而，這對男女不是亞當和夏娃，他們優雅形體下面鐫刻的資訊，是比《創世紀》中記載的任何東西部更有價值的、關於我們生命複製大爆炸的聖經。人們還設計了能得到普遍理解的圖像語言：那個牌上記載著他們來自某顆恒星的第三顆行星，以及那顆恒星在星系中的精確座標。作為我們人類向外星人遞交的國書，這塊牌上還刻著一些表示化學和數學基本原理的圖像。如果這個密閉的小艙有幸偶然為外星智慧生物所獲，他們會相信，創造出它的那種文明必定具有某種比原始部落的迷信更進步的東西。他們將會知道，在茫茫太空的那一邊，很久以前就發生了另一起生命複製大爆炸，並且已經達到一種文明的高峰，值得與之對話。 哎，無奈這個小小密封艙在一個秒差距（相當於三．二六光年）的距離內飛經另一個複製炸彈的可能性簡直微乎其微。一些評論家認為，這件事的價值，僅僅是地球人在表達他們的激情而已。舉起雙手，做出和平姿態的一對男女裸體浮雕，被人們鄭重其事地送入太空，走上了星際飛行的無盡旅途。其意圖顯然是將它作為我們人類第一次出口有關我們自己的複製大爆炸知識的成果。這件事對我們通常很狹隘的、微不足道的意識可能會有某種有益的作用；而對於眾所公認的，威廉．華茲華斯（William Wordsworth）的偉大意識來說，則可能會是劍橋大學三一學院裡牛頓雕像的富有詩意效果的回聲： 枕上遙望， 窗外一片星月之光。 依稀看見 那教堂前廳中央。 這裡矗立著 牛頓的雕像。 嚴肅而平靜 牛頓的面龐， 華美的標誌， 為這大智慧 在陌生的思想之海 永遠獨自遠航。 (End of the book)