Chapter 3 Chapter Two The African and Her Descendants

It is often said to be smart to think of science as merely a modern myth. The Jews had their Adam and Eve, the Sumerians had their Marduk and Gilgamesh, the Greeks had their Zeus and the Olympians ), the ancient Norse had their martyrs.Evolution, some moderns say, is but a modern expression of ancient gods and epic heroes, neither better nor worse, neither truer nor false.There is a popular salon philosophy called cultural relativism, which insists in its extreme form that science is no closer to the truth than tribal myths: that science is nothing more than our modern Western tribal favorite myth.I was once irritated by a fellow anthropologist by expressing the following candidly: Suppose there was a tribe who believed that the moon was an old gourd that was thrown into the sky and hung higher than the tops of the trees Not much, do you really think that our scientific truth that the moon is about 380,000 kilometers away and its diameter is one-fourth that of the earth is no more correct than the tribal myth?Yes, the anthropologist said, we were cultured to see the world in a scientific way, and they were cultured to see it in another way.Neither of the two ways is right or wrong.If you take a cultural relativist a thousand meters into the air, you can see a hypocrite.Airplanes are built on scientific principles.Airplanes can fly into the blue sky and take you from the sky to where you want to go.Aircraft built according to myths and legends will not work.For example, a spaceship made in imitation of the cargo worshippers, or a pair of wings glued with wax like Icarus, cannot fly to the sky*.Suppose you fly to an international conference of anthropologists or literary critics. The reason you arrive at your destination and don't crash is because of the correct design of many scientifically educated engineers; According to the fact that the moon orbits the earth about 400,000 kilometers away, scientists have successfully sent people to the surface of the moon using computers and rockets designed by the West.Tribal scientists believe that the moon is only above the treetops, but they can only touch it forever in their sleep.

When I collaborated on presentations in public, there was always something in the audience that explicitly raised the same line of thought as my anthropologist colleague, and often some people nodded in agreement, causing a murmur of discussion.There is no doubt that nodding people feel good, have a big heart, and are not racist.A more believable and more agreeable statement is this: Basically, your belief in evolution becomes belief, so you are no better than someone who believes in the Garden of Eden.Every tribe has myths and legends about its own origin, which is their story about the universe, life and human beings.In our modern society there is an opinion, at least among the educated classes, that science does offer something corresponding to this.Science might even be described as a religion.I have briefly published a case where science was included as an appropriate subject in a religious education class (unlike in the United Kingdom, where religious education is compulsory on the school syllabus, to avoid offending various any of the incompatible beliefs, schools are prohibited from making religious education compulsory).Both science and religion claim to answer profound questions about the origin of man, the nature of life, and the universe.But that's where their similarities end.Scientific beliefs have evidence and yield answers.Myths and legends and religious beliefs have no evidence and no answer.

Of all the myths and legends about human origins, the Jewish story of the Garden of Eden has had the greatest impact on our culture, so much so that its name has been used to give an important scientific theory about our ancestors the African Eve Theory.I dedicate this chapter to African Eve, partly because it allows me to develop the metaphor of the river of DNA further, but also because I want her to contrast, as a scientific hypothesis, with the fabled matriarch of the Garden of Eden.If I succeed, you will find facts to be more interesting than myths, perhaps even more poetic and moving.We start the walkthrough with pure theory, and the relationships will become clear shortly.

You have two parents, four grandparents, eight great-grandparents, and so on.Every generation you go back, you double the number of ancestors.Going back to g generations, then the number of ancestors is two to the gth power.Only, before we get out of our seats, we'll soon discover that's not possible.In order to convince ourselves of this, we have to go a little way back, say, back to the time of Jesus, which was about two thousand years ago.If we make a conservative assumption: four generations are produced every 100 years (that is to say, people give birth to the next generation at the age of 25 on average), then there are 80 generations in 2000 years.The actual number could be higher (since many women today start giving birth at a very young age), but this is only a hypothetical extrapolation and need not be considered in such detail.Two multiplied by itself eighty times is a gigantic number: one followed by twenty-four zeros, or a billion billion billion billion.That is to say, you have billions and billions of ancestors who were contemporaries of Jesus, and I have so many ancestors!However, at that time, the total population of the world was only a negligible fraction of the number of ancestors we have just calculated.

Clearly, we've gone wrong somewhere.But what's wrong?Our calculations are correct.The mistake is to assume that the number of ancestors doubles every generation.In fact, we forget about intermarriage between cousins.I originally assumed that each of us had eight great-grandfathers.However, there are only six great-grandfathers among the children born from the marriage of first cousins.This is because, in the case of first cousins, the grandparents of one person are the maternal grandparents of the other, and the common grandparents (maternal grandparents) are the great-grandparents of their children.

You may ask: so what?People occasionally marry cousins, Darwin's wife Emma.Wedgwood (Emma Wedgwood), is Darwin's first cousin, but surely this doesn't make a lot of difference, does it?No it will.Because what we call intermarriage between cousins ​​also includes intermarriage between third, fifth, sixteenth and even further distant relatives.If you count cousins ​​that far apart, every marriage is a marriage between cousins.Sometimes, you'll hear someone boast that they're a distant relative of the Queen, which is bragging themselves.For, after all, we are all distantly related to the Queen, and to everyone else, in ways too numerous to trace.The only thing that distinguishes royalty and aristocrats is that their lineage can be clearly traced back.As the fourteenth Earl of Home said in response to a taunt about his title by his political opponents: I guess, Mr. Wilson, that you happen to think of yourself as the Fourteenth Wilson.Taken together, we are much more closely related to each other than we usually realize, and the actual number of our ancestors is much smaller than we can figure out by simple calculations.Once, in order to make a female student reason along this line of thought, I asked her to guess how long ago did the most recent common ancestor of her and me live?Staring at my face, she replied in a slow earthy voice without hesitation: In the age of apes.A quick, intuitive reaction that can be forgiven, but 100% wrong.It means millions of years of separation.In fact, her and my most recent common ancestor probably lived within two or three hundred years, long after William I (1066-1087).And, there is definitely multiple kinship between us.

The lineage pattern that causes us to mistakenly overcount our ancestors is a tree that is constantly branching and branching from the branch.Turn it around, and it's an equally erroneous subfamily pattern tree.A typical person has two children, four grandchildren, and eight great-grandchildren, and over a few centuries the number of descendants is unimaginably large.A far more real pattern of lineage is the flowing river of genes, which we introduced in Chapter 1.Between the two banks of the river, genes flow in time like an eternal torrent of water.Genes are criss-crossed, flowing down the river of time, and the current swirls when it divides and when it rejoins.

Take buckets of water at intervals along the river.The pairs of molecules in the barrel mate intermittently as they travel down the river, and mate again later.They have also been isolated far away in the past, and will be isolated far away again in the future.It is extremely difficult to trace where they touch, but we can be absolutely certain that there is contact. If two genes do not touch at a certain point, then you don't have to go very far down the river of genes, whether upstream or downstream. , they will meet again. You may not know it, but you are your husband's cousin.In fact, from a statistical point of view, you don't have to go back too far to your ancestors to find a bloodline connection with your husband.In another direction, let us face the future.Obviously, you have a good chance of having offspring with your husband (or wife).However, here's an even more startling idea.The next time you are with a large group of people, say, at a concert, or at a football game, and you look around at the audience or audience, consider the following hypothesis: Suppose you have descendants living in the distant future, the music There may be people at the meeting who share your ancestors with these future generations, and you may even shake hands with them.Generally speaking, the grandparents of children know that they are the common ancestor of these children.Regardless of whether they hit it off or not, this relationship will definitely bring them kinship.They might look at each other and say: Well, I don't like him very much, but his DNA is mixed with mine, and it's in our shared grandchildren.It can be expected that we will have many common descendants long after we die.It certainly created a bond between us.I mean, if you're lucky enough to have descendants in the distant future, there's a good chance someone among the strangers in the concert hall is a common ancestor of your descendants.You can look around the auditorium and guess which one is destined to share your offspring with you and which one is not.No matter who you are, no matter what color you are, what sex you are, you may be the ancestor of someone in the future like me.Your DNA may be destined to mix with mine.salute!Now, suppose we take a time machine and travel back in time, perhaps to the crowds in the Roman amphitheatre, or to the earlier markets of Ur, or even older places.Look around the crowd as you would look at the audience in a modern concert hall.You can divide these long-gone people into two classes, and only two classes: those who were your ancestors and those who were not.This is all too obvious, but from it we arrive at a remarkable truth.If the time machine could take you far enough back in time, you could divide the people you meet into two categories: the ancestors of everyone who lived in 1995, or the ancestors of people who didn't live in 1995.There is absolutely no one in between.Step out of your time machine, and everyone you see is either the ancestor of all humanity, or the ancestor of no one.

It's a very appealing idea, and one that's easy to demonstrate.What you need to do is to turn the imaginary time machine back to even absurdly long time ago, for example, three.Half a billion years ago, our ancestors were lung-finned fish that had just emerged from the water to become amphibians.It would be inconceivable that a particular fish was my ancestor but not yours.If so, it would mean that your lineage and mine were independent and not cross-referenced in the evolution from fish to amphibians, reptiles, mammals, primates, apes, hominids, and We turned out to be so similar that we could talk and, if we were a man and a woman, be spouses.

These facts are true between you and me, and they are true between any two people. We have shown that, going back far enough in the past, everyone we meet is either an ancestor of all of us or none of us.But how long is long enough?Clearly, we don't need to go back to the time of the common finfish, that's a reductio absurdum, but how far back in the past do we have to go to see the common ancestor of people alive in 1995?This problem is much harder and is one of the problems I will turn to next.This question can't be answered by just patting your head.We need real information, and measurements from certain factual aspects.

Honorable British geneticist and mathematician Ronald.Sir Ronald Fisher is considered to be Darwin's most outstanding successor in the 20th century and the father of modern statistics.He once said in 1930 that it was only due to geographical and other barriers to marriage between different races that all human beings have been prevented from gradually sharing the same ancestors in the last thousand years.For members of the same ethnic group, they were almost one family five hundred years ago.The only differences that can be preserved for two thousand years are the differences between different races, and some of these may indeed be extremely old, but this only occurs when there is little blood mixing between individual groups for a long time. Using our river of genes analogy, Fisher is actually taking advantage of the fact that the genes of all members of a geographically unified race flow down the same river.However, when it comes to his specific figures of 500 years, 2000 years, and the age of separation of different races, Fisher has to make some guesses.In his day, no relevant facts were available.Today, with the revolution of molecular biology, the abundance of data has made people at a loss.It was molecular biology that gave us the extraordinary powers of African Eve. The river of data is not the only metaphor that has been used so far.It's tempting to liken the DNA inside each of us to a home Bible. DNA is a very long scripture, as we said in Chapter 1, it is written in four letters.These letters were meticulously copied from, and only from, our ancestors.Uncannily accurate, even from a very distant ancestor to the present.It should be possible to compare scriptures preserved by different peoples and to reconstruct their kinship and trace back to their common ancestors.There will be many word differences between scriptures between distant relatives.Their DNA has been alienated for a longer period of time, such as Norwegian aborigines and Australian aborigines.Scholars use various versions of the text to do this.Unfortunately, in the DNA file, there is an unexpected obstacle, and that is sex. Sex is an archivist's nightmare.The ancestral scriptures were not preserved intact, save for the occasional inevitable error; but sex was lavishly intervened, and the evidence destroyed.A bull breaking into a porcelain store is nothing compared to the trampling of DNA files by sex.There is no such thing in Bible studies.True, a scholar who traces the origins of, say, the Song of Songs realizes that the original Song of Songs was not the same as the present.Now there are some odd and incoherent passages in the Song of Solomon that suggest that it is actually fragments of several poems (and only the erotic parts of them) put together.It contains error mutations, especially translation errors. Take us the foxes, the little foxes, that spoil the vines. (Catch the foxes, those little foxes, they ruin the vines) is a mistranslation.Despite the haunting appeal of a lifetime of recitation, it is still no match for the correct translation: Catch for us the fruit bats, the little fruit bats. (Get the flying foxes, those little foxes). Behold, the winter is gone, and the rain is gone.Flowers are blooming on the earth; the moment comes when the birds sing, and the sound of the tortoise is heard in the field. How intoxicating is this poem!So I really loathe to point out that there is an undoubted variation here too.Like the current translation, adding dove after turtle (turtle) and changing it to turtledove is correct, but it is very dull, and it sounds like the rhythm is broken.However, when articles are neither printed in thousands of copies nor etched on high-fidelity computer discs, but are copied and re-circulated by ancient scribes from rare and fragile papyrus manuscripts, then, These are the smallest errors we can hope for, and are inevitable slight degradations. But now that sex is introduced (I don't mean that sex is introduced into Song of Solomon), what I mean by sex is to tear a file in half, and tear it into pieces at random, and then share it with another file. Half of the documents were mixed up in a mess.It sounds incredible, even savage destruction, but it's what happens when germ cells form.For example, when a man makes a sperm cell, the chromosomes he inherited from his father pair up with the chromosomes he inherited from his mother, and many of them change places. A child's chromosomes are actually a mishmash of irretrievably mixed and pieced together chromosomes from his ancestors, all the way back to distant ancestors.The intended ancient text, its letters, and possibly its words, might be able to be passed down from generation to generation intact.Chapters, pages, and even paragraphs, however, are ruthlessly and efficiently shredded and put back together; the destruction is so ruthless that it is almost useless from the point of view of tracing historical traces.Sex is always a formidable obstacle when it comes to lineage. As long as sex is really not involved, we can use DNA archives to reconstruct history.Two very important examples come to mind.One is African Eve, whom I will talk about below.Another example is the reconstruction of more distant ancestors that looks at relatedness between species rather than within species.We have seen in the previous chapter that sexual reproduction occurs only within species.When a parent species establishes a daughter species, the river of genes splits into two streams.By the time they diverge long enough to move away from its barriers as archivists, sexual reproduction in each tributary actually helps to reconstruct lineage and kinship among species.Sex only messes up the evidence when it comes to relatedness within species.For interspecific kinship, sex can only be helpful: it tends to automatically ensure that each individual is a good genetic sample of the species as a whole. In the well-stirred river water, it doesn't matter where you get a bucket of water from, and which bucket of water is representative of the water in that river. In order to draw the genealogy of species, literal comparisons of DNA texts taken from typical samples of different species have in fact been carried out with great success.According to one influential school of thought, it was even possible to date the bifurcation.This chance arises from the controversial notion of a molecular clock, which posits a constant rate of mutations per million years in any region of genetic text.We will soon return to the hypothesis about the molecular clock. In our genes, the paragraph describing a protein called cytochrome c has three hundred and thirty-nine letters.Horses are fairly distantly related to us, with a twelve-letter change in human cytochrome C from horse cytochrome C.Monkeys are our fairly close relatives, and human cytochrome C differs from monkey cytochrome C by only one letter.Donkeys are close relatives of horses, and there is only one letter difference in cytochrome C between horses and donkeys.In addition, pigs are slightly more distantly related to horses, with a three-letter difference between their cytochrome c.There is a forty-five-letter difference between human and yeast cytochrome C, and a similarly forty-five-letter difference between pig and yeast.It is not surprising that these two sets of numbers are the same.If you follow the human gene river upstream, you will find that the time when the human and pig gene rivers converge is much closer than the time when they merged with the yeast gene river.However, there is also a small stain in these numbers.The difference between horse and yeast cytochrome c is forty-six letters rather than forty-five letters.This does not mean that pigs are any more closely related to yeast than horses.They are both vertebrates, and, in fact, beasts, in exactly the same degree of affinity with yeast.Perhaps, after the most recent common ancestor with the pig, an additional change inadvertently arose in the lineage that evolved into the horse.It doesn't matter.Overall, the number of cytochrome C letter differences between pairs of organisms is in good agreement with what was inferred from the aforementioned idea of ​​branching evolutionary trees. The molecular clock theory mentioned above holds that the rate of change of a genetic text every million years is roughly fixed.Regarding the forty-six cytochrome C letter variations that distinguish horses from yeast, it is generally believed that about half of them occurred during the evolution from their common ancestor to modern horses, and about the other half occurred from their common ancestor to modern yeast During evolution (obviously, the two evolutionary pathways go through exactly the same amount of time).At first, this assumption may seem surprising.After all, their common ancestor was likely more yeast-like and less horse-like.An adjustment to the assumption that large parts of the genetic text can be freely changed without affecting the meaning of the genetic text has gradually gained acceptance since the eminent Japanese geneticist Motoo Kimura first expressed support. Using different fonts in a sentence is a good analogy. A horse is a mammal.Yeast is a type of fungus.Although each word is in a different font in the two sentences, their meaning is clear.Over millions of years, molecular clocks ticked away what amounted to meaningless font changes.The changes determined by natural selection and the changes describing the difference between horses and yeast, that is, changes in the meaning of sentences, are the tip of the iceberg. Some molecules have a faster clock rate than others.But the evolution of cytochrome c is rather slow: about one letter change every 25 million years.This may be because cytochrome c is extremely important to the survival of organisms, and its role mainly depends on its specific shape. Most of the changes that occur in molecules where shape is crucial are not tolerated by natural selection.Some other proteins, such as fibrin, are also important but come in multiple forms that do the same job.Fibrin is what makes blood clot, and you can change most of their fine structure without compromising their ability to clot.The mutation rate of these proteins is about one change every 600,000 years, more than forty times faster than cytochrome c.Thus, fibrins can be used to reproduce relatively recent lineages, such as in the mammalian range, but they cannot be used to reproduce ancient lineages.There are hundreds of different proteins, each with its own characteristic rate of mutation, so each can be used independently to reconstruct a family tree.They all produced almost identical genealogies.By the way, this is pretty good proof (if evidence is needed) that evolution is correct. We have begun our discussions by recognizing that sexual promiscuity confounds the historical record.We have identified two ways in which sexual interference can be avoided.One of these was discussed in detail earlier.Our discussion is based on the fact that sex does not mix genes from different species, which opens up the possibility of using DNA sequences to reconstruct the genealogy of our distant ancestors who lived before we became human. A long time ago.But we have agreed that if we go back far enough, we must all be descended from the same individual, after all. We also tried to find out how recently we could still claim the same blood as everyone else.To find out, we have to turn to another class of DNA evidence.Thus, African Eve came into our story. African Eve is also sometimes called Mitochondrial Eve.Mitochondria are tiny diamond-shaped bodies that fill each of our cells by the thousands.They are basically hollow inside but have a complex membrane structure.These membranes are much larger in area than you might imagine from the outside of the mitochondria, and they are useful.These membranes are the production line of a chemical factory (more precisely, a power plant).A precisely controlled chain reaction proceeds along these membranes, with more steps in this chain reaction than there are in any human chemical factory.As a result, the energy derived from the food molecules is released step by step in a controlled manner and stored in a reusable form for later combustion wherever it is needed in the body.Without mitochondria, we would be dead in a second. This is the function of mitochondria, but we are more concerned here with where mitochondria come from.It turned out that in the long course of evolution, they were once bacteria.This excellent theory was developed by the venerable Lynn of the University of Massachusetts, Amherst.Margulis (Lynn Margulis) first proposed.The theory, which at first was regarded as heresy and only a few people were interested in it at that time, has now been widely accepted and can be said to have won a big victory.Two billion years ago, the distant ancestors of mitochondria were free-living bacteria.Later, they colonize larger cells with other types of bacteria.The resulting (prokaryotic) bacterial community in turn becomes a large (eukaryotic) cell.Each of us is a community of trillions of interdependent eukaryotic cells.Each of these cells, in turn, is a colony of thousands of domesticated bacteria that are completely enclosed in the cell.They multiply inside cells like bacteria.According to calculations, if all the mitochondria in a person's body were connected end to end and lined up in a row, it would not circle the earth once, but could circle it 2,000 times.An animal or a plant is a huge community composed of interacting small communities of different levels, just like a rainforest.Take the rainforest as an example, it itself is a community of tens of millions of species, and each member of each species is a community itself, and it has many communities composed of domesticated bacteria.Dr. Margulis' theory of the origin of the cell as a closed bacterial paradise is far more exciting, exciting, and exhilarating than the story of the Garden of Eden.It also has the added advantage that it is almost certainly real. Like most biologists, I now assume that the Margulis theory is correct, and I mention it in this chapter only to continue to explore a special implication: Mitochondria have their own DNA, unlike other bacteria. Same, only exists in a single ring chromosome.Now we discuss this point.Mitochondrial DNA does not participate in any sexual mixing, neither with the main nuclear DNA of the body nor with other mitochondrial DNA.Like many bacteria, mitochondria multiply only by dividing.Whenever a mitochondria splits into two daughter mitochondria, each daughter mitochondria gets an exact copy of the original chromosome.From our genealogist's point of view, it's time for you to see the beauty of it.We found that where we fear that the ordinary DNA text is being sexually cluttered with evidence each generation, confusing paternal and maternal contributions, mitochondrial DNA is fortunately left alone. We only get mitochondria from our mother.Sperm are so small that they can only hold a few mitochondria; they can only produce enough energy to power the sperm tail when the sperm swims to the egg; during fertilization, the head of the sperm is absorbed by the egg, and these mitochondria are identical The last one was abandoned.In comparison, the individual eggs are huge.Inside the egg's bulky, fluid-filled interior is a rich collection of mitochondrial germplasm.These mitochondrial germplasm enter the bodies of the offspring.So whether you're female or male, your mitochondria all come from the mitochondria of your mother who was originally vaccinated.Whether you're male or female, all of your mitochondria come from your grandmother's mitochondria.Not one of your mitochondria came from your father, not one from your grandfather, not one from your grandmother.Mitochondria constitute an independent record of the past, uncontaminated by the DNA of the main cell nucleus, which came equally from each of your four grandparents, each of your eight great-grandparents, and so on, And so on. Although mitochondrial DNA is immune to contamination, it is not immune to mutations, random errors in replication.Moreover, it has a higher mutation rate than our own DNA.That's because, like all bacteria, it doesn't have the precise proofreading mechanisms that our cells have developed over millions of years.There will be several differences in our mitochondrial DNA. According to the number of differences, we can deduce how long ago our ancestors diverged. Here, it is not all ancestors, but we are female︱female︱ Ancestors on the female front line.If your mother was a pure Aboriginal Australian, or a pure Chinese, or a Kalahari Aboriginal, there are indeed many differences between your mitochondrial DNA and mine.It doesn't matter who your father is: he could be an English marquis, or a Sioux chief, and it doesn't matter to your mitochondria.The same goes for all your male ancestors. Thus, there is a separate mitochondrial Apocrypha, which is handed down from generation to generation along with the main family Bible, and has the great advantage of being passed down the maternal line only.This view is not sexist, nor is it bad if it is passed down the patriline.Its advantage lies in its integrity, which is neither fragmented nor fused in each generation.As DNA genealogists, what we need is a purely single-sex lineage, not a lineage of both sexes. Like surnames, the Y chromosome is only passed down the paternal line, which is also theoretically applicable, but it contains too little information to be useful.It would be ideal to use the mitochondrial Apocrypha for dating common ancestors within a species. A group of researchers had worked with the late Alan A. of the University of California, Berkeley.Wilson (ALlan WiLson) collaborative study of mitochondrial DNA.In the 1980s, Wilson and his colleagues sampled 135 women from all over the world for mitochondrial DNA sequence investigation.Among these women were Aboriginal Australians, New Guinea Highlanders, Native Americans, Europeans. Chinese, and representatives of various ethnic groups in Africa.On a pair-by-pair basis, they looked at the number of mitochondrial DNA letters that each woman differed from every other woman.They fed these numbers into a computer and asked the computer to construct the simplest family tree it could find. The most convenient here is to exclude as far as possible the hypothetical agreement.This requires some explanation. Recall our earlier discussion of horses, pigs, yeast, etc., and our analysis of the cytochrome C letter sequence, and you will recall that there are only three letters that differ between horses and pigs, and between pigs and yeast. Forty-five letters differ, and between the horse and the yeast there are forty-six letters of this difference.We have pointed out that, theoretically, horses and pigs should be at exactly the same distance from yeast because of a relatively recent common ancestor that related them to each other.The difference between forty-five and forty-six is ​​an aberration that should not exist in an ideal world.It may be due to an additional mutation during the evolution of horses; perhaps it is due to an opposite mutation during the evolution of pigs. There is indeed an absurd point of view here.It is theoretically conceivable that pigs are more closely related to yeast than to horses.In theory, pigs and horses came to resemble each other (their cytochrome C text differs by only three letters, and their bodies are essentially the same mammalian pattern) through various coincidences. We do not believe this view because pigs are much more like horses than pigs are like yeast.It is undeniable that pigs are closer to yeast in one DNA letter, but this is overwhelmed by the myriad other similarities between pigs and horses. The reason is the easiest.如果我們假設豬與馬的關係近，我們只需接受一個因巧合形成的相似之處；如果我們假設豬與酵母的關係近，我們就必須假定一連串的相似之處都是互不相干地因巧合而形成的，其數量多到不現實的程度。 在馬、豬和酵母的相互關係問題上，上述的簡便理由是顯而易見，無可置疑的。但是，人類不同種族的線粒體ＤＮＡ相似程度極高，這是沒有什麼可懷疑的。簡便的理由仍然存在，但只表現為微不足道的數量上的論據，而不是有力的、決定性的論據。從理論上說，電腦要做的工作是：列出全部有關一百三十五名婦女的可能的家族樹，然後檢驗這一系列家族樹，從中選出最簡單化的一種，即因巧合而致的相似之處最少的一種。我們必須面對這樣的事實：即使是最好的家族樹，也可能迫使我們去接受一些小的巧合，就像我們被迫接受如下事實：從一個ＤＮＡ字母的角度看，酵母距豬更近些，離馬更遠些。但是，至少在理論上，電腦應該能夠輕易地完成這一任務，並向我們展示，在眾多可能的家族樹中，哪一棵是最簡便的，也就是巧合最少的。 這只是在理論上。但在實際上存在著一個障礙。可能的家族樹的數量，比你、我，或任何數學家所能想像到的要大得多。在馬、豬和酵母一例中只有三種可能的家族樹。顯然正確的一種是（豬、馬）酵母，豬和馬一起，被放置在內括弧裡，而酵母則是無關聯的局外者。另外兩種理論上的家族樹，是（豬、酵母）馬和（馬、酵母）豬。如果我們再加上第四種生物，比如就魚，家族樹的數目就會上升到十五個。我不想一一列出所有這十五種家族樹，但是願舉出正確（最簡便）的一個，即（豬、馬）魷魚酵母。豬和馬作為近親又一次舒適地被放置在最內層的括弧裡。下一個參加該俱樂部的是魷魚。跟酵母相比，它與豬／馬世系擁有更為晚近的共同祖先。其他十四個家族樹中的任何一個，比如（豬、魷魚）、（馬、酵母），肯定都不夠簡便。如果豬真是魷魚的近親，馬確實是酵母的近親，那麼，豬和馬極不可能各自獨立地發展了如此多的相似之處。 如果說三種生物有三種可能的家族樹，四種生物有十五種可能的家族樹，那麼一百三十五名婦女會構成多少種可能的家族樹呢？答案將是一個大得可笑的數字，甚至沒法把它寫出來。假如用世界上最大、最快的電腦來列出所有的可能的家族樹，直到世界末日，也還不能取得明顯進展。 然而，這個問題還是有希望解決的。我們習慣於利用審慎的抽樣方法來對付無法想像的巨大數字問題。雖然我們數不清亞馬遜平原有多少昆蟲，但是我們可以在森林中隨機選取一些小地塊，假設這些地塊具有代表性，便可根據小地塊抽樣的數字推算出亞馬遜平原昆蟲總數。我們的電腦不可能檢驗有關一百三十五名婦女的所有可能的家族樹，但是電腦可以從所有可能的家族樹中進行隨機抽樣。如果你什麼時候從一百億億個可能的家族樹中抽取一個樣本，注意到若干最簡便的樣本都具有某些共同特性，你可以由此斷定，在所有家族樹中，大概最簡便的家族樹都有同樣的特性。 這是人們迄今已經做過的事情。然而這並不一定表明它是解決問題的最佳方法。正如昆蟲學家可能不同意用最典型的方法給巴西雨林抽樣，ＤＮＡ系譜學者也採用了各種不同的抽樣方法。而且，遺憾的是，結果並非總是一致的。然而，由於他們的工作極有價值，我將把伯克利研究小組對人類線粒體ＤＮＡ所做的最初分析的結論介紹給讀者。他們的結論非常有趣，並且頗有爭議。根據他們的分析，最簡便的樹繫牢牢地紮根於非洲。這話的意思就是，一些非洲人與其他非洲人的關係比與全世界其他地方任何人的關係更遠。世界上所有其他地方的人歐洲人，美洲土著，澳大利亞土著，中國人，新幾內亞人，因紐特人，以及所有其他人構成了一群相對較為親近的表親。部分非洲人屬於這一組近親，但其他非洲人則不屬這一組。根據這一分析，最簡便的家族樹看來是這樣的：某些非洲人其他非洲人（再其他一些非洲人（再再其他一些非洲人和其他所有人））。他們因此斷言，我們所有人的母系遠祖住在非洲，叫做非洲夏娃。我曾提到，這個論斷是有爭議的。其他一些科學家聲稱，可以找到同樣簡便的家族樹，這些樹系最外面的分支出現在非洲以外的地方。他們還認為，伯克利小組之所以得到這個結果，部分原因是出在查找可能家族樹的電腦的指令上。 顯然，查找指令無關緊要。也許大多數專家仍會把錢花在線粒體夏娃是非洲人上面，然而他們卻不會有很大信心了。 對伯克利小組的第二個結論爭議不大。不管線粒體夏娃居住在什麼地方，他們都能估計出她生活的時間。由於已經知道了線粒體ＤＮＡ的進化速度，因而你能給線粒體ＤＮＡ趨異之樹的每個分岔點標上一個大概的時間。聯結所有女性的分岔點，即線粒體夏娃的誕生日是在十五至二十五萬年前。 不論線粒體夏娃是不是一個非洲人，我們的祖先來自非洲都無疑是正確的，要避免在這個意思上可能發生的混亂，這是很重要的。線粒體夏娃是所有現代人類最近的祖先。她是智人（Homo sapiens）中的一員。在非洲和非洲以外的地方都曾經發現過早得多的人科直立人（Homo erectus）化石。而比直立人更早的能人（Homo habilis）和各種南方古猿屬（Australopithecus）化石（包括新近發現的四百多萬年前的南方古猿化石）則僅在非洲發現過。因此，如果我們是在過去二十五萬年內遷徙世界各地的非洲人的後裔，那麼這是非洲人的第二次大遷徙。有一批非洲人離開得更早，大約是在一百五十萬年以前，那時直立人迂迴曲折地離開非洲，移居到中東和亞洲一些地方。非洲夏娃理論認為，並非不存在早期亞洲人，而是那些早期亞洲人沒有留下後代。不論你怎樣看待這個問題，如果回到二百萬年以前，我們都是非洲人。非洲夏娃理論還認為，即使回到僅僅幾十萬年前，現今存活的人類也都是非洲人。如果有新的證據支持，有可能從在非洲所有現代線粒體ＤＮＡ追溯到一個非洲之外的母系祖先（比如說亞洲夏娃），同時又能贊同我們更古老的祖先僅發現於非洲。 此刻，我們假設伯克利研究小組是正確的，讓我們研究一下他們的結論意味著什麼，以及不意味什麼。夏娃這個綽號已經產生了不幸的影響。一些熱心人輕率地認為：她肯定是一個孤獨寂寞的女人，是地球上唯一的女人，是最初的遺傳瓶頸口，甚至是《創世紀》的一個證明！這完全是誤解。她是那個時代地球上唯一的女人，在她那個時代，人口比較少，這兩種說法都不正確。她的男女同伴也許既數量眾多，而且生殖力旺盛。他們也許至今還有大量後代活在世上。但是因為他們與我們之間的聯繫在某一點上通過了一位男性，他們的線粒體的所有後代已經消失。同樣，一個貴族姓氏（姓氏是與Y染色體聯繫在一起的，並且與線粒體的情形恰好相反，只由父子相傳）可能消失，但這並不是說擁有這個姓氏的人就沒有後代。他們或許會有數不清的後代，只不過不是通過父子相傳，而是通過其他途徑傳下來。正確的說法僅能是：線粒體夏娃是離我們最晚近的一位純粹母系的共同祖先，所有的現代人都是她的後代。必定有一名婦女能夠符合這個結論。唯一的爭論是關於她住在這裡還是那裡，是在此時還是彼時。她確實曾生活於某時、某地，這個事實是確定的。 這裡還有第二種誤解。這種誤解更為普遍，甚至在從事線粒體ＤＮＡ領域研究工作的領頭科學家那裡，我都曾經聽到過。那就是相信線粒體夏娃是我們最晚近的共同祖先。這種誤解的根源是對兩個概念的混淆：一個是最晚近的共同祖先，另一個是最晚近的純粹母系的共同祖先。線粒體夏娃是我們最晚近的純粹母系的共同祖先，然而還有很多其他途徑可以延續後代，不僅僅是純粹母系這一條線。其他途徑有成百萬。讓我們回到關於祖先數目的計算中來（讓我們忘卻表親聯姻的複雜情況，它曾是前面爭論的焦點）。你雖然有八位曾祖父母、外曾祖父母，但其中只有一位是純粹母系的。你雖然有十六位高祖，但其中也只有一位是純粹母系的。 即使允許表親結婚減少了在某一代中祖先的數目，但是，與純粹母系相比，確實還有更多、更多、更多成為祖先的途徑。當我們沿著遺傳之河經過遙遠的古代溯流而上，可能會有許許多多夏娃，許許多多亞當。這是些焦點人物。生活在一九九五年的所有人可能都是她或他的後代。線粒體夏娃只是其中之一。沒有什麼特別理由可以認為線粒體夏娃是所有這些夏娃和亞當中離我們最近的一位。情況與此相反。她被規定了一個特別的途徑：在遺傳之河中，我們是由這個特定的途徑成為她的後代。與純粹母系途徑相並行的可能途徑，數目非常巨大，因此從數學上線粒體夏娃極其不可能是所有夏娃和亞當中距我們最近的一位。它是一條特殊的單行道（只經母系遺傳）。如果又是最近的途徑之一，那將是一個了不起的巧合。 再有一點也多少使人感到興趣，那就是我們最晚近的共同祖先更有可能是一個亞當，而不是夏娃。一夫多妻比數男共一妻可能性更大，這是因為，僅從生理上說，男性能生育幾百、甚至幾千個孩子。在《吉尼斯世界紀錄大全》中，這一紀錄已超過一千，創造這紀錄的是以實瑪利。女性即使在理想條件下，最多也只能生育二三十個孩子。女性的子女數較男性更平均。少數男人的孩子可能多得荒唐可笑，但這就意味著另一些男人連一個孩子都沒有。如果有一個人完全不能生育，這個人更可能是個男人而不是女人；而如果有一個人擁有大多的後裔，這個人也更可能是個男人。因此，人類最為晚近的共同祖先，很可能是一個亞當，而不是一個夏娃。舉個極端的例子，誰更可能是今日全體摩洛哥人的祖先？是以實瑪利，還是他的某個不幸的妻妾？，我們可以得出以下結論：第一，必定存在著一位我們可稱為線粒體夏娃的女性，她是所有現代人最晚近的純粹母系共同祖先。第二，必定還有一個稱為焦點祖先的人，其性別尚不為人知，卻是所有現代人通過任何遺傳途徑而來的最近的共同祖先。第三，雖然線粒體夏娃和焦點祖先有可能是同一個人，但很不可能。第四，焦點祖先更有可能是男性，而不是女性。第五，線粒體夏娃很可能生活在二十五萬年前。第六，對於線粒體夏娃生活於何地，尚存爭議，但是從已報導的觀點看，人們仍傾向於非洲。只有第五條和第六條要靠科學證據來檢驗，前四條只需借助常理進行推斷即可得出。 但是，我曾說過，祖先們掌握著解開生命之謎的鑰匙。非洲夏娃的故事只是一個更壯麗、更古老的史詩限於人類這一局部的縮微。我們要再次求助於基因之河我們的伊甸園之河這一比喻。 但是，我們會溯流而上穿越更為久遠的時間跨度，比神話夏娃的幾千年和非洲夏娃的幾十萬年，久遠得不可同日而語。ＤＮＡ之河沿著一條不間斷的時間線路，途經我們的歷代祖先，至少流了三十億年。 原注：我已不是第一次使用這個必勝的論據。我必須強調指出，這僅僅針對（見下頁）與我那位舊葫蘆同事持同樣想法的人。還有一些人，儘管他們也令人不解地自稱文化相對論者，但他們的觀點完全不同，而且十分明智。對於他們，文化相對論僅意味著，如果你試圖用自己的文化術語去詮釋另一種文化的信念，你是不可能理解它的。你必須將某一文化的每個信念放在該種文化的其他各種信念背景下觀察。我猜，這種理解才是文化相對論的本意。而我所批評的那種，是對文化相對主義的一種極端主義的曲解，雖然它頗能蠱惑人心。明智的文化相對論者應該努力與這種荒唐的文化相對論劃清界限。