Chapter 2 Chapter 1 The River of Numbers

Any nation has heroic legends about their tribal ancestors.These legends often form their religious worship.People revere and even worship their ancestors, because it is their ancestors, not supernatural gods, who hold the key to reveal the mysteries of life.After the birth of life, most of them die before they grow into adults; only a few can reach adulthood and reproduce offspring; even fewer can reproduce for thousands of generations.The very few of these few are the essence of the ancestors, and they are the true ancestors of future generations.Although the ancestors are few, the descendants are many.

All living organisms today, including every animal and plant, all bacteria and fungi, reptiles of all kinds, and the readers of this book, can look back on their ancestors and declare with pride: Among our ancestors, there was no One died in infancy.They all survived to adulthood, each found at least one partner of the opposite sex and mated successfully*.None of our ancestors died of a rival, or of a virus, or of falling off a cliff, before having at least one child.Many of our ancestors' thousands of contemporaries died unfortunately because of these reasons.However, each of our ancestors escaped this fate.The above statement is self-evident, but there is much more to it than that.

There are also many things that are both incomprehensible and unexpected, or clear and surprising. All of these are what this book will discuss. All organisms inherit their genes from their ancestors, not from their failed contemporaries.Therefore, all living things have a tendency to possess genes for success.They have those things that make them ancestral, viability and reproduction.This is why living things naturally inherit genes that tend to build a well-designed machine, a body that is actively working, as if trying to make itself an ancestor.This is why birds are so good at flying, fish are so good at swimming, monkeys are so good at climbing, and viruses are so good at spreading.That's why we love life, we love sex, we love children.This is because all of us, without exception, inherit all of our genes from a long line of successful ancestors.The world is full of creatures, and these creatures have those things that make them ancestors.In a word, this is Darwin's theory of evolution.Of course, Darwin's theory of evolution is much more than that; moreover, we can say much more now, which is why the book we have read does not stop there.

Here, a misunderstanding of the above passage naturally arises, which is extremely harmful.It would lead one to think that as a result of ancestors doing something successful, the genes they passed on to their children were an improvement over the genes they inherited from the previous generation.Something about their success is left in their genes, which is why their offspring are good at flying, swimming and courtship.Wrong, big mistake!Genes are not improved through use, except for the very rare occasional error, they are simply passed on as is.It's not success that makes good genes, it's good genes that create success.Nothing any individual does in its lifetime has any effect on its genes.Individuals who are born with good genes are the most likely to grow up to have successful ancestors, so good genes are more likely to be passed on to later generations than bad genes.Each generation is a screening program, a sieve: good genes can pass through the sieve to the next generation; bad genes are terminated because individuals die young or have no offspring.Perhaps, bad genes are lucky enough to be in the same body as good genes, and they may also pass a generation or two of selection.However, it takes more than luck to pass through a thousand sieves one after the other.After a thousand generations of successive screening, those genes that allow organisms to undergo screening are likely to be good genes.

I have said that the genes that survive from one generation to the next are supposed to be the ones that successfully produced the ancestors.This is true, but there are notable exceptions which need to be stated here to avoid misunderstanding.Some individuals are instinctively sterile, but they seem to have been born to help pass their genes on to the next generation.Worker ants, worker bees, worker termites among termites, and worker wasps among wasps are sterile.They toil not to become ancestors, but to make ancestors to their fertile relatives (usually their sisters and brothers).There are two things to understand here.

First, in any animal, the probability that sisters and brothers share the same gene is very high; second, it is the external environment, not genes, that determines whether a termite becomes a fertile termite or a sterile worker termite.All termites carry genes that make termites sterile worker termites under certain environmental conditions and fertile termites under other conditions.Reproductive termites pass down genes that allow worker termites to help them reproduce.Worker termites toil away from genes whose copies are present in reproductive termites.These gene copies in worker termites go to great lengths to keep the reproductive copy through generations of selection.The worker termites may be both male and female; but in ants, bees, and wasps, it is the females who do the work.Other than that, the principle is the same.In a broad sense, this is also the case in several species of birds, mammals, and other animals, in which an older brother or sister nurtures a younger brother or sister to some extent.In sum, genes get a pass through the sieve not only with the help of individuals who would themselves be ancestors, but also with the help of individuals with relatives who would become ancestors.

The river mentioned in the title of this book refers to a river of DNA, which flows in time, not in space.It is a river of information, not a river of flesh and blood.In this river flow the abstract instructions for building the body, not the actual body itself.The information passes through the bodies and affects them; however, the information passes through the bodies without being affected by the bodies.The river flows through a succession of bodies, unaffected not only by the experiences and accomplishments of those bodies, but also by a potentially more potent source of pollution: sex.

In each of your cells, half of your genes come from your mother and the other half from your father, and the two halves of your genes live side by side.The genes from your mother are most closely combined with the genes from your father, creating you an inseparable, delicate blend of their genes.But the genes from both parents don't fuse themselves, they just play their respective roles, and the genes themselves have a solid integrity. The time has come to move into the next generation, and a gene either gets into the child, or it doesn't take part in such an action.The father's genes do not mix with the mother's genes, they recombine independently.A gene in your body either comes from your mother or your father.This gene came from one of your four grandparents, and only one of them; further inference, this gene came from one of your eight great-grandparents, and only one of them.According to this rule, it can be traced back to further generations.

We spoke of a river of genes, but equally we could say that they are a band of companions traveling through geologic time.In the long run, all genes in the breeding population are partners of each other.In the short term, these genes live within the individual and become temporary, closer companions with other genes that share a body.Only genes that are good at building individuals who can survive and reproduce well in the particular lifestyle chosen by the species can be passed on from generation to generation.To survive well, a gene must work well with other genes of the same species (in the same river).To survive in the long run, a gene must be a good partner.The gene must do well as a companion to other genes in the same river, or in the context of other genes.The genes of other species are in other rivers.Genes from different species do not coexist in the same individual, so, in some sense, they don't have to get along well.

It is characteristic of being a species that all members of any one species flow through the same river of genes, and all genes of the same species must prepare themselves to be good companions to each other.When an existing species splits in two, a new species is born.The river of genes always diverges in due course.Genetically speaking, speciation (the origin of new species) is a farewell.After a short period of incomplete separation, the tributary and the mainstream either part ways forever, or one or both tend to dry up and disappear.The two rivers are safe within their respective channels, and through sexual reorganization, the waters of the rivers mix and remix with each other.However, the water of one river will never overflow its own bank to pollute another river.After a species splits into two species; the two sets of genes are no longer partners.They will never meet again in the same body, so there is no way to ask them to live in harmony.There is no longer any communication between them.The word intercourse here refers to the sexual intercourse between the temporary carriers of the two sets of genes.

Why do species splits occur?What factors lead to the farewell of the two sets of genes?And what causes a river to divide into two branches, so far away from each other that they never join?Although the details are open to debate, no one doubts that the main cause was due to accidental geographical separation.Although the river of genes flows in time, the re-pairing of genes takes place in real individuals, and these individuals occupy certain positions in space.If North American gray squirrels meet British gray squirrels, it is possible for them to mate and reproduce.However, it is unlikely that they will meet.The river of North American gray squirrel genes is practically separated from the river of British gray squirrel genes by an ocean 4,800 kilometers wide.Despite the belief that the two genetic teams would still be good mates given the chance, the reality is that they are no longer mates.They have cherished each other, and of course this is not an irreversible farewell.But if kept separated for thousands of years, the two rivers would drift so far apart that if the two gray squirrels met, they would no longer be able to exchange genes.The word "drifting away" here does not refer to the separation in space, but means that they are no longer compatible. Something like this must have happened after the separation of gray and red squirrels before.These two species of squirrels can no longer interbreed.Although in some parts of Europe they co-exist and often meet and compete for nuts together, they can no longer mate to produce fertile offspring.The genetic rivers of the two squirrels are so far apart that their genes can no longer cooperate with each other in the same body.Many, many generations ago, the ancestors of gray squirrels were originally the same individuals as the ancestors of red squirrels.But then they were separated geographically, perhaps by mountains, perhaps by water, and ultimately by the Atlantic Ocean.So the gray squirrel's set of genes diverged from the red squirrel's.Geographical isolation leads to their lack of adaptability in reproduction.Good mates become bad mates.Or, if you do a test to mate them, they will appear to be not good companions.Bad mates get worse and worse, until now they are no longer mates at all.The parting between them was farewell, two rivers of genes parted, destined to grow farther and farther apart.The same thing happened a long, long time ago between our human ancestors and our elephant ancestors.Or it happened between the ancestor of the ostrich (which is also one of the ancestors of man) and the ancestor of the scorpion. Today, the river of DNA may have nearly 30 million tributaries.So I say this because it is estimated that there are about so many species on the earth now.Moreover, there is another estimate that the total number of species in existence is only one percent of the total number of species that once existed on the earth.From this, it can be concluded that there have been three billion tributaries of DNA rivers on the earth.There has been an irreversible separation between today's thirty million tributaries. Most of them will eventually disappear without a trace, because most species will eventually die out.If you follow the origins of these thirty million rivers (for the sake of brevity, I will use the word river instead of the tributaries of the river), you will find that they all joined other rivers one by one.About seven million years ago, the river of human genes merged with the river of chimpanzee genes, and almost at the same time, the river of gorilla genes merged with the river of chimpanzee genes.Going back millions of years, the river of Ape genes we joined merged with the river of Orangutan genes.In earlier times than that, the river of our ancestors' genes flowed in parallel with that of the gibbons (and the river of gibbons flowed downstream, splitting off into several different species of gibbons and siamangs). If we push back in time, some of the rivers that once flowed into our genetic river diverged into many streams as they flowed forward: European monkeys, American monkeys, and Madagascar lemurs, to name a few.In the more distant past, our human gene river has merged with other gene rivers, and some major mammalian populations, such as rodents, cats, bats, elephants, etc., have since split off from those gene rivers .Further on, we see other rivers of genes from which various reptiles, birds, amphibians, fish, and invertebrates split off. In the metaphor of the river of genes, we must pay attention to an important aspect.When we think of the headwaters of all mammals (rather than the subdivisions down to the gray squirrels), the grandiose picture of the Mississippi and its tributary, the Missouri, comes to mind.In the final analysis, the mammalian branch was constantly branching and branching until it gave rise to all mammals from bonobo shrews to elephants, from burrowing moles to tree-topping monkeys, and so on.One would think: the headstream of mammals must be a great torrent rolling onward, for how else could it fill its thousands of vital tributaries?However, this association is dead wrong.The divergence of the ancestors of all modern mammals from non-mammals does not appear to be a monumental event compared with the formation of any other species.Had a naturalist happened to be present at the time, the process would not have even attracted his attention.The new fork of the Gene River is like a brook, inhabited by little nocturnal creatures.The differences between these tiny creatures and their non-mammalian relatives are as minimal as red squirrels are to gray squirrels.It was only later that we saw the ancestors of mammals as mammals.At first, they were just another mammal-like reptile, not significantly different from a dozen other small animals with snouts, all of which were morsels of dinosaur food. All large groups of animals, such as vertebrates, molluscs, crustaceans, insects, annelids, flatworms, and jellyfish, among others, split up early on with little drama from their ancestors.When the ancestral river of molluscs (and other animals) was just diverging from the river of genes ancestral to vertebrates (and other animals), the two groups of organisms (probably worm-like) were so alike that they were not separated. Can mate.They failed to meet and mate only by chance being separated by geographical barriers.For example, maybe it was a drought, and the land separated the original unified body of water.No one could have guessed that one group was destined to give birth to molluscs and the other to vertebrates.The two rivers of DNA are just streams that have just barely parted, and the two groups of animals are still indistinguishable. Zoologists are well aware of this, but whenever they look seriously at large groups of animals, such as molluscs and vertebrates, they tend to forget them and are lured into thinking that the major groups Separation is what matters.Zoologists have gone astray because they have been taught an almost religious belief that every great division of the animal kingdom is marked by some highly distinctive sign, which the Germans call the Bauplan (biological blueprint). ).Even though the word means nothing more than a blueprint, it has become an accepted term.Although it's not yet in the new OED (which I'm slightly shocked by), I've always had to find an equivalent for it. In a technical sense, Bauplan is often translated as the basic design of the body.The use of the word fundamental is dangerous, and it leads zoologists to make serious mistakes. For example, a zoologist once suggested that during the Cambrian period (about 600 million to 500 million years ago), the evolutionary process must have been completely different from the later evolutionary process.His reasoning went something like this: new species arose now, and new classes (such as molluscs and crustaceans) arose during the Cambrian period.The fallacy is obvious!Even organisms as radically different as molluscs and crustaceans were originally the same species, separated only by geography.During the initial period of separation, if they could meet, they could have mated with each other and reproduced offspring, but they did not meet.After millions of years of evolution, they each acquired their existing characteristics.According to these characteristics, modern zoologists call them molluscs and crustaceans respectively.These characteristics are called biological blueprints.But the main biological blueprints of the animal kingdom evolved gradually after diverging from a common source. There are obviously small differences in views on exactly how a gradual change or how a sudden change occurs.Although there are many opinions expressed by everyone, the differences are not particularly large.No one (and I mean no one) thinks that evolution will go one step too fast to produce a whole new biological blueprint.The article by the author I am talking about was published in 1958.Few zoologists today accept his claims explicitly, but they sometimes do so implicitly, as if major groups of animals did not split off from an ancestral group by accidental geographical isolation, but emerged on their own. Came, and all at once fully formed, like Athena emerging from the head of Zeus. Studies in molecular biology have shown that, in any case, the closeness between groups of large animals is much greater than we usually imagine.You can think of the genetic code as a dictionary.In this dictionary, the sixty-four words (three out of four letters, sixty-four possible triplets) of one language are mapped to the twenty-one words (twenty amino acid plus a punctuation mark).Two times of sixty-four: the possibility that the arrangement of twenty-one is exactly the same is extremely small, only one in a hundred trillion billion billion.However, in fact, all the animals, plants and bacteria we have seen so far have the same genetic code.Everything that is alive on earth must have descended from one ancestor.No one disputes this.Taking insects and vertebrates as examples, people have found in research that not only their genetic codes themselves, but even the ordering of genetic information are surprisingly similar.A rather complex genetic mechanism determines the differentiation of somites in insects, and a surprisingly similar genetic mechanism has been found in mammals.Molecularly speaking, all animals are fairly closely related to each other, even plants.You have to go to bacteria to find our distant relatives.Even so, the genetic code of bacteria is the same as ours.Why is it possible to make such precise calculations on the genetic code, while the same meticulous study of anatomical maps is impossible?The reason is that the genetic code is strictly digital, and the number of digits can be precisely calculated.The river of genes is a river of numbers, and I must explain this engineering term in detail below. Engineers draw a major distinction between digital and analog signals.Phonographs, answering machines, and until recently most telephones used analog signals. CD players, computers, and most modern telephone systems use digital.In an analog telephone system, continuous fluctuations in air pressure caused by sound are converted into corresponding voltage fluctuations in the telephone wires.A turntable works on a similar principle: As the record turns, its undulating grooves cause the stylus to vibrate, which in turn is converted into corresponding voltage fluctuations.At the other end of the telephone line, the metal film in the phone earphone of the receiver vibrates under the action of the changing current, so the receiver can hear the sound.Similarly, the changing current passes through the speaker of the record player, and the music recorded on the record is played out.This cipher is simple and straightforward: the fluctuations in the current in the wires are proportional to the fluctuations in air pressure. All possible voltages, within certain limits, can be transmitted over wires, and the differences between voltages are important. In a digital telephone, there are only two possible voltages across the wires, or a number of possible discrete voltage values, such as eight or two hundred and fifty-six.The information is not contained in the voltage itself, but in a graph of discrete levels.This is called pulse code modulation.At any one time, the actual voltage is rarely exactly equal to an integer multiple of eight (called the nominal value), but the receiving device will erase the zero to bring it to the nearest specified voltage, so that even if the transmission is poor, the At the other end of the wire, the signal is still nearly perfect.All you have to do is make these discrete levels differ enough that the receiving device doesn't interpret random fluctuations as wrong levels.This is a major feature of digital coding, which is why audiovisual systems, as well as other information technologies, are increasingly going digital.Of course, computers have long since embraced digital in everything they do.For convenience, we use binary code, which has only two levels instead of eight or two hundred and fifty-six levels. Even in digital phones, the sound going into the microphone and coming out of the earpiece is still an analog fluctuation in air pressure.All information is digital from switch to switch.One would create some kind of code to translate the analog value, microsecond by microsecond, into a series of discrete pulses, the numerical value represented digitally.When you're begging your lover on the phone, every nuance of tone, every choke, sigh of emotion, timbre of longing, is carried over the wire by the mere digital form.You will be digitally moved to tears, but there is a prerequisite that those moving sounds are encoded and decoded fast enough.Modern telephone exchanges operate so fast that they can divide up line time in the same way that a chess master can compete with twenty players at the same time.Using this method, thousands of conversations can be carried on at the same time on a single telephone line; these conversations appear to be happening simultaneously, but are electronically separated and independent, without interfering with each other.A data trunk is a digital river.Today, many data lines are not wires but beams of radio waves that can be sent directly from mountaintop to mountaintop or relayed by satellites.It is a river of thousands of numbers that only superficially share the same course, made possible by clever time-allocation techniques; like red and gray squirrels who live in the same tree but whose genes Never mixed. Let's go back to the world of engineers.As long as the analog signal is not copied over and over again, its flaws are not a big deal.Tapes may have hiss, but it's very small and you probably won't notice it unless you turn it up really loud.However, as you amplify the music, the hiss is also amplified and introduces new noise.But if you dubbed the tape, and dubbed and dubbed, and dubbed again and again, after a hundred duplications, all that's left on the tape is the horrible hiss.In the age of all-analog telephones, this phenomenon became a problem.Any telephone signal will attenuate after passing through a long section of telephone line, so they must be boosted, that is, amplified, such as once every 100 kilometers.In the age of analog signals, this was a headache because each level of amplification added background hiss.Digital signals also need to be amplified, but for reasons we already know, this amplifying process will not introduce any error: no matter how many times of amplifying, we can use certain devices to improve the signal and move perfectly to the next stage .Even if the digital signal travels thousands of kilometers, the hiss will not get louder. When I was a child, my mother told me that nerve cells are the telephone wires in the human body.However, are they analog or digital?The answer is: they are an interesting hybrid of the two.A nerve cell is not like a wire.It was a long thin tube through which a wave of chemical change passed, like the fuse of a dynamite pack, sizzling and burning on the ground.However, unlike leads, nerve cells recover quickly; after a short rest, the nerves are sibilant again.The amplitude of the wave (like the temperature of the lead) changes as the wave travels along the nerve, but this has no effect.The code doesn't care about fluctuations in magnitude.With chemical pulses and without chemical pulses, it's like two separate levels in a digital phone.In this sense, the nervous system is digital, but nerve impulses are not encoded as bits: they cannot form discrete code values.Instead, the intensity of the information (such as the loudness of the sound, the brightness of the light, or even the outburst of emotion, etc.) is expressed by the frequency of the pulse.Engineers call this approach pulse frequency modulation.Before the emergence of pulse code modulation technology, pulse frequency modulation technology was widely used. The pulse frequency is an analog quantity, but the pulse itself is digital: the pulse is either present or absent, and has no other value.Like other digital systems, the nervous system benefits greatly from this feature.In the nervous system, there is also something equivalent to an amplifier, but there is not one for every hundred kilometers, but one for every millimeter. There are 800 such amplifiers between the spinal cord and your fingertips.If the sheer magnitude of nerve impulses were at play, the length of a human arm would be enough to distort the information transmitted, let alone the length of a giraffe's neck.Each amplification introduces more random errors, like doing eight hundred tape duplications.Or, like copying with a photocopier, copying the copy again, and after eight hundred copies, the final result will be a blur of gray.The digital code provided the only solution to the problem of the nerve cell, and natural selection has fully embraced it, as is the case with genes. Francis.Crick (Francis Crick) and James.Watson (James Watson) is the person who unraveled the mystery of the molecular structure of genes.I think they should be revered for generations like Aristotle and Plato.They won the Nobel Prize in Biology and Medicine, and they deserve it.However, this reward is trivial compared to their contribution.It's almost an oxymoron to speak of ongoing revolutions, yet not just medicine, but our entire understanding of life is revolutionized again and again.These revolutions were the result of a change in ideas initiated by these two young men in 1953.Genes themselves, and inherited diseases, are just the tip of the iceberg.The real revolutionary change in molecular biology after Watson and the Crick was that it became digital. After Watson and Crick, we know that the gene itself is a long string of pure digital information in terms of internal microstructure.In particular, the genetic structure is truly digital, fully digital like computers and CD players, not partially digital like the nervous system.The genetic code, unlike computer code, is not binary; the genetic code is not octal, like some telephone systems; and the genetic code is a quaternary code, which has four symbols.The genetic code is exactly like the machine code of a computer. Except for the different technical terms, every page in the Journal of Molecular Biology can be replaced by the content of the Journal of Computer Technology.The digital revolution at the heart of life has had many consequences, the most important of which is that it deals the final blow to vitalists' idea that living matter is vastly different from inanimate matter.Until 1953 it was probably believed that there was some fundamental, indivisible, mysterious substance in living protoplasm.No one has thought that way since then.Even those philosophers who were originally inclined to a mechanical view of life dared not expect that their wildest dreams would be fulfilled. The plot of the following science fiction novel still makes sense, but the technology is different from today's, and it is slightly ahead of today's technology.Jim.Professor Crickson (Jim Crickson) was kidnapped by foreign evil forces, and they forced Crickson to work in a biological warfare laboratory.In order to save world civilization, the most important thing he should do is to pass some top-secret information to the outside world.However, all normal channels of communication were severed.With one exception, the DNA code has sixty-four triplet codons, enough to form a complete English alphabet consisting of twenty-six uppercase letters and twenty-six lowercase letters, plus ten numbers, a blank character number, and a full stop.Professor Crickson took a vicious influenza virus from the sample rack in the laboratory, and designed the complete information he wanted to convey to the outside world in impeccable English sentences in the genetic map of the virus.He repeated his information over and over again on the designed genetic map, and added a highly recognizable marker sequence, that is to say, led by ten leading digits.He infected himself with the virus, then went into a room full of people and sneezed incessantly.A flu wave sweeps the globe.Medical labs around the world are beginning to analyze the sequencing of the virus's genetic profile in hopes of designing a vaccine against it.It was soon discovered that there was a strange repeating pattern in the genetic map of the virus.The guiding numbers raise the alarm that these numbers are unlikely to arise spontaneously. So someone inadvertently thought of using cryptanalysis techniques to solve the problem.In this way, without much effort, the English information delivered by Professor Crickson was understood by people, and the news spread all over the world. Our genetic system (and the universal genetic system of all life on our planet) is completely digital.You could write the New Testament verbatim into the part of the human genetic map occupied by idle DNA that is not yet being used, at least not by the body in the usual way.Every cell in your body contains the equivalent of forty-six gigantic data tapes, digitally marked by countless read heads working simultaneously.These tape chromosomes contain the same information in every cell, but the read heads in different types of cells pick out different parts of the database for their own specific purposes.This is why muscle cells differ from liver cells.There is no life force driven by the heart, no heartbeat, moaning, growth, nor the original protoplasm, the mysterious colloid.Life is just countless bits of digital information. Genes are pure information, information that can be encoded, re-encoded, and decoded without degenerating or changing its content.純粹的資訊是可以複製的，而且由於它是數位資訊，所以複製的保真度可以是極高的。 ＤＮＡ符號的複製，其精確度可與現代工程師們所做的任何事情相媲美。它們一代代被複製，僅有的極偶然的差錯只足以引起變異。 在這些變種中，那些在這個世界上數量增多的編碼組合，當它們在個體內解碼和執行時，顯然能自動地使個體採取積極步驟去保持和傳播同樣的ＤＮＡ資訊。我們一切有生命的物質都是存活下來的機器，這些機器按照程式的指令，傳播了設計這個程式的資料庫。現在看來，達爾文主義就是在純粹數碼水準上的眾多幸存者中倖存下來的。 現在看來，不可能有其他的情況。我們可以想像一下類比式的基因系統。我們已經知道，類比資訊經過連續若干代的複製之後會產生什麼樣的後果：變成了一片雜訊。在設有許多放大器的電話系統中，在多次轉錄磁帶的過程中，在複印再複印的過程中總之，在累積退化過程中，類比信號極易受到損害。因此，複製只能進行有限的幾代。然而，基因則不然，它可以自我複製千萬代而幾乎沒有任何退化。達爾文的進化論之所以成立，僅僅是因為複製過程是完美無缺的除去一些分立的變異，自然選擇法則決定了這些變異或者被淘汰，或者被保留下來。只有數字式的基因系統能夠使達爾文的進化論在地質時代中永放光華。一九五三年是雙螺旋年，它將被看作是神秘論生命觀和愚昧主義生命觀的末日，而達爾文主義者則把一九五三年視為他們的學科最終走向數位化之年。 純粹的數位資訊之河，莊嚴地流過地質年代，並分解成三十億條支流。它是一個強有力的形象。但是，它在什麼地方留下了熟悉的生命特徵？它在什麼地方留下了軀體、手腳、眼睛、大腦和鬍鬚，樹葉、樹幹和樹根？它又在什麼地方留下了我們和我們的各個部分？我們動物、植物、原生動物、菌類和細菌難道僅僅是供數碼式資料從中流過的小河河道嗎？從某種意義上來說，是這樣。 但是，正如我在前面所說，還不止這些。基因並不僅僅是複製它們自己，一代一代往下傳。它們實際上把時間消磨在軀體內，它們存在於軀體內，它們影響一代接一代的軀體。軀體，從外觀到行為都受到它們的影響。軀體也是重要的。 就拿北極熊來說。北極熊的軀體不僅僅是數位小溪的河道，它還是一部像熊那麼大而複雜的機器。整個北極熊種群的所有基因是一個集體它們是好的夥伴，肩並肩地走過時間旅途。但是，它們並不把時間都消耗在陪伴這個集體中所有其他成員上：他們在集體的一群成員內更換夥伴。集體這個詞的定義是，它是一套基因，這些基因有可能與這集體中的任何其他基因（但不是世界上其他三千萬個集體之一的成員）相遇。實際的相遇總是發生在某一北極熊軀體的某一個細胞之中。因而，那個軀體並不是消極地接受ＤＮＡ的一個容器。 每個細胞裡都有完整的一套基因，想像一下細胞的巨大數目便足以使你震驚：一隻大公熊體內有九億億個細胞。如果將一隻北極熊的所有細胞排成一隊，足以從地球到月球排一個來回。這些細胞分成幾百種截然不同的類型，所有的哺乳動物基本上都有幾類細胞：肌肉細胞、神經細胞、骨細胞、表皮細胞等等。同一類型的細胞聚集在一起形成組織：肌肉組織、骨組織等等。所有不同類型的細胞都具有構成這種類型所需的基因指令。只有與相關組織相適應的基因才能被啟動。這就是為什麼不同組織的細胞形狀和大小均不相同。更有趣的是，特定類型細胞中被啟動的基因導致這些細胞長成特定形狀的組織。骨骼並不是沒有形狀的、硬實而堅固的大塊組織。骨骼有它們特定的形狀：有中空棒狀的，有球狀和凹窩狀的，還有脊椎骨和骨距等等。細胞由它內部啟動了的基因編好了程式，就好像它們很清楚自己的相鄰細胞是哪些，自己的位置在哪裡。它們就這樣形成了自己的組織，長成了耳垂，或者心臟瓣膜、眼球、括約肌等不同的形狀。 像北極熊這樣的複雜有機體，它有很多層次。北極熊的軀體就是許多具有精確形狀的器官（如肝臟、腎臟、骨骼等）的複雜集合體。每一個器官又是由特定組織構成的複雜的大廈，建造這些組織的磚塊就是細胞，它們通常是一層層或一片片的，也常常是塊狀的實體。從更小的尺度上講，每一個細胞都有高度複雜的內部結構，即折疊膜結構。這些折疊起來的膜，以及膜之間的液體，是發生多種不同類型的、錯綜複雜的化學反應的場所。在一家化學工業公司或者碳化物公司裡，可能有數百種性質截然不同的化學反應正在進行。這些化學反應被燒瓶壁、管道壁等分隔開來。在一個活的細胞裡，可能有差不多數量的化學反應同時發生。從某種程度上來說，細胞內部的膜就如同實驗室裡的玻璃器皿。當然，從兩個方面來看，這種比喻不太恰當。其一，儘管有許多化學反應是發生在膜之間，但也有不少化學反應發生在膜上。其二，還有更重要的手段將不同的反應分隔開來。每一種反應都是由它自己特殊的霉來催化的。 霉是一種非常大的分子，它的三維立體結構提供了能促進反應的表面，從而加速特定類型的化學反應。由於對生物分子來說最重要的是它們的三維立體結構，所以我們可以把霉分子看作一台大型機床，它通過仔細篩選，形成一條製造特定形狀分子的生產線。因此，在任何一個細胞裡面，都同時獨立地發生著數以百計的不同化學反應，這些化學反應都是在不同的霉分子表面發生的。在一個特定的細胞中，發生哪些特定的化學反應，取決於存在哪些特定類型的霉分子。每一個霉分子，包括其十分重要的形狀，都是在特定基因的決定性影響下裝配出來的。確切地說，基因中數百個密碼符號的精確順序，根據一套完全已知的法則（遺傳密碼），決定了氨基酸在霉分子中的序列。每一個霉分子都是一條氨基酸長鏈，而每一條氨基酸長鏈都自動盤繞成一個獨一無二的、特定的三維立體結構，就像一個繩結。在繩結中，長鏈的某些部分與另一些部分形成交聯鍵。繩結確切的三維結構是由氨基酸的一維順序決定的，因此也就是由基因中密碼符號的一維順序決定的。這樣，細胞裡發生什麼化學反應，就取決於究竟是哪些基因被啟動了。 然而，在一個特定的細胞中，是什麼決定了應該啟動哪些基因呢？回答是：那些存在於細胞內的化學物質。這裡包含著一個雞和蛋的悖論，然而並非不可超越。實際上，對這一悖論作出解答，雖然在細節上很複雜，但是在原理上卻很簡單。計算機工程師把這種解答叫做引導程式。我第一次開始使用電腦是在六十年代，所有程式都必須通過穿孔紙帶來輸入（那時美國的電腦通常使用穿孔卡片，但原理是一樣的）。在你裝入大型程式紙帶之前，你必須先裝入一個小的程式，稱為引導裝入程式。這引導裝入程式只做一件事：告訴電腦怎樣裝入紙帶。但是，這裡就有了雞和蛋的悖論：這引導裝入程式自己又是怎樣裝入的呢？在現代電腦中，相當於引導裝入程式的功能已經由電腦裡的硬體來完成了。但是在早期，你必須按照規定的順序操作鍵盤才能開始工作。這一系列按鍵告訴計算機怎樣開始閱讀引導裝人程式的第一部分。然後，引導裝人程式紙帶的第一部分又告訴電腦，如何閱讀引導裝人程式的下一部分等等。待引導裝人程式全部輸入計算機後，電腦就知道如何閱讀任何紙帶了，這時它才變成一台有用的電腦。 胚胎是這樣開始形成的：一個單細胞（也就是受精卵）分裂成兩個；這兩個又分別分裂，這就變成了四個細胞；每個細胞再分裂，變成了八個細胞等等。用不了多少代，細胞數就增加到了萬億個，這就是指數分裂的力量。但是，如果僅此而已的話，那麼這數以萬億計的細胞就全是一模一樣的了。如若不是這樣，它們又是怎樣分化（請允許我使用技術術語）成為肝細胞、腎細胞、肌肉細胞等等，並且各有不同的基因被啟動，各有不同的霉在活動呢？通過引導。它是這樣工作的：儘管卵子看上去像是個球體，實際上它內部的化學物質存在極性。它有頂部和底部，很多情況下還有前後之別（因此也有左邊和右邊）。這些極性的表現形式是化學物質的梯度。 某些化學物質的濃度從前向後逐漸升高；另一些化學物質的濃度自上而下逐漸升高。這些早期的梯度雖很簡單，卻足以構成引導運作的第一步。 例如，當一個卵子分裂成為三十二個細胞的時候，也就是第五次分裂之後，這三十二個細胞中的某些細胞會得到多於平均數的頂部化學物質，另一些細胞得到了多於平均數的底部化學物質。細胞首尾之間化學物質也可能出現不平衡。這些差異足以在不同細胞內啟動不同的基因組合。因此，在早期胚胎各不同部分的細胞中，會出現霉的不同組合。這就使得在不同的細胞中有更多的不同基因組合被啟動。因此，細胞不再與胚胎內它們的克隆祖先保持一致，細胞出現了世系趨異。 細胞的世系趨異完全不同於前面談到的物種趨異。這些細胞趨異是依程式進行的，並且可以預見其細節。而物種趨異是由於地理上的突然變化所帶來的偶然性結果，並且是不可預見的。此外，當物種趨異時，基因本身發生趨異，這種情況我曾充滿想像力地稱之為永別。當胚胎中發生細胞世系趨異時，分裂雙方都接受相同的基因全都相同。但是不同的細胞接受化學物質的不同組合，而不同的化學物質組合會啟動與之相配合的不同基因，而且有些基因的使命就是啟動或阻斷其他基因。自我引導就這樣進行下去，直到我們有了不同類型細胞的全部指令系統。 正在發育的胚胎不僅僅是分化為數百個不同類型的細胞。它的外部和內部形態還同時經歷了精緻的動態變化。或許最具戲劇性的是最早期的一個變化原腸胚形成的過程。著名胚胎學家路易士．沃爾波特（Lewis Wolpert）甚至這樣說：雖然不是出生、結婚或死亡，但它真正是你一生中最重要的時刻，這就是原腸胚的形成。在原腸胚形成時到底發生了什麼呢？一個由細胞組成的空球經彎曲變形之後變成了一隻帶襯裡的杯子。一般來說，整個動物界的所有胚胎都要經歷相同的原腸胚形成過程，這是胚胎學多樣性的共同基礎。在此，我只是把原腸胚的形成作為一個例子，一個特別引人注目的例子提出。在胚胎發育中經常見到，整層細胞不停地做著類似折紙的動作，原腸胚形成只是這類動作中的一個。 在折紙能手的表演結束後，經過一層層細胞無數次的折疊、伸展、鼓起和拉平後，在胚胎的一些部分生機勃勃而和諧有序地生長，另一些部分被消耗後；在分化為化學性質和物理性質上各異的數百種細胞後，當細胞的數量達到以萬億計的時候，這最終的產物便是一個嬰兒。不，嬰兒誕生並不是終點，因為個體的整個生長還有，軀體的某些部分比另一些部分生長得快包括經由成年直到老年的過程，應該視為同一個胚胎學過程的延伸，這才是完整的胚胎學。 個體之間的不同之處，就在於它們在整個胚胎學過程中在數量細節上存在差異。一層細胞在折疊起來之前長得有點過頭，這結果是什麼呢？可能是鷹鉤鼻子，而不是朝天鼻子；還可能是扁平足，這也許能使你免於戰死疆場，因為扁平足者不准參軍；也可能肩腫骨的形狀特殊，它使你長於投擲標槍（或者擲手榴彈，或者打板球，這取決於你所處的環境）。有時，細胞層折紙動作的個別變化會帶來悲劇性後果，比如天生胳膊殘疾或無手的嬰兒。由純粹化學因素而非細胞層折紙動作引起的個體差異，其後果也同樣嚴重：不能吸收牛奶、同性戀傾向、對花生過敏，或者吃芒果時感覺像吃松節油一般噁心。 胚胎發育是一種非常複雜的物理化學過程。在它的發展進程中，任何一點細節上的改變，都會給全過程帶來不可估量的影響。 你只要回憶一下這個過程在多麼大的程度上借助於引導，對這一點就不會感到太驚奇了。許多個體發育過程中的差異是由環境不同引起的例如缺氧，或者受到引起胎兒畸形的藥劑的影響。 許多其他個體差異是由基因的不同引起的不僅僅是那孤立的基因，還有那些與其他基因相互作用的基因，還有與環境差異發生相互作用的基因。胚胎發育是個複雜的、千變萬化的過程，又是個錯綜複雜、互相影響的自我引導過程。胚胎發育過程既強有力又很敏感。說它強有力，是因為它排除了許多潛在的變異，對抗著有時看來勢不可擋的種種可能，產生出一個活生生的子代。與此同時它對變化非常敏感，以致不存在兩個所有特徵都完全一樣的個體，甚至沒有一模一樣的雙胞胎。 現在，這一討論已漸漸引到這個論點上。在由於基因所引起的個體問差異（差異的程度可大可小）面前，自然選擇可能傾向於胚胎折紙動作或胚胎化學的後代，而淘汰其他個體。就你的投擲手臂受到基因的影響來說，自然選擇有可能接受它，也可能淘汰它。如果善投擲對個體存活時間是不是夠長，是否能有自己的孩子產生了一定影響（不管這影響是多麼輕微），而投擲能力又受基因的影響，這些基因便有較大的可能性傳到下一代。任何一個個體都可能因為與他的投擲能力沒有關係的原因而死去，但是，導致個體善投擲的基因（有這個基因的人比沒有這個基因的人更善於投擲）將寄居於很多人體內，延續許多世代。從這個特定基因的角度來看，其他的死因將是均等的。以基因的觀點來看，存在的只是流經世世代代的ＤＮＡ之河，只是在某一個軀體內臨時寄居，只是與夥伴基因臨時共用同一個軀體，而這些夥伴基因可能是成功的，也可能是失敗的。 經過很長一段時期之後，基因之河裡充滿了歷經萬難仍存活的基因。成功的原因是多種多樣的：有的基因使人投擲長矛的能力略有提高，有的基因使人辨別毒物的能力略有提高，以及其他等等。作為一種平衡，有些基因對生存不利：可能會使具有它的人眼睛散光因而標槍投不准，或者使具有它的人缺乏吸引力因而找不到配偶。它們最終會從基因之河中消失。在所有這一切中，要牢記我們在前文指出的論點：在基因之河中能夠長期存活下去的基因，將是那些有利於這一物種在一般環境中存活下去的基因；所謂一般環境，或許最主要是指這個物種的其他基因，即指這個基因不得不與之在同一個軀體中共存的那些基因，與之在同一條基因之河中共同遊過地質時代的那些基因。 原注：嚴格他講，例外的情況是存在的。有些動物，比如蚜蟲，是無性繁殖。現在，採用人工授精之類的技術，人類不經交媾就可以有孩子，甚至，鑒於體外繁殖的卵子可以從女性胎兒體內取得，人類不等發育成熟即可生育。但就論題而言，我的觀點的說服力並未減弱。