The Ancestor's Tale Part 1

THE ANCESTOR'S TALE.

A Pilgrimage to the Dawn of Life.

Richard Dawkins.

ACKNOWLEDGMENTS.

I was persuaded to write this book by Anthony Cheetham, founder of Orion Books. The fact that he had moved on before the book was published reflects my unconscionable delay in finis.h.i.+ng it. Michael Dover tolerated that delay with humour and fort.i.tude, and always encouraged me by his swift and intelligent understanding of what I was trying to do. The best of his many good decisions was to engage Latha Menon as a freelance editor. As with A Devil's Chaplain A Devil's Chaplain, Latha's support has been beyond all estimation. Her grasp of the big picture simultaneously with the details, her encyclopaedic knowledge, her love of science and her selfless devotion to promoting it have benefited me, and this book, in more ways than I can count. Others at the publishers helped greatly, but Jennie Condell and the designer, Ken Wilson, went beyond the call of duty.

My research a.s.sistant Yan Wong has been intimately involved at every stage of the planning, researching and writing of the book. His resourcefulness and detailed familiarity with modern biology have been matched only by his green fingers with computers. If, here, I have gratefully a.s.sumed the role of apprentice, it could be said that he was my apprentice before I was his, for I was his tutor at New College. He then did his doctorate under the supervision of Alan Grafen, once my own graduate student, so I suppose Yan could be called my grandstudent as well as my student. Apprentice or master, Yan's contribution has been so great that, for certain tales, I have insisted on adding his name as joint author. When Yan left to cycle across Patagonia, the book in its final stages benefited greatly from Sam Turvey's extraordinary knowledge of zoology and his conscientious care in deploying it.

Advice and help of various kinds were willingly given by Michael Yudkin, Mark Griffith, Steve Simpson, Angela Douglas, George McGavin, Jack Pettigrew, George Barlow, Colin Blakemore, John Mollon, Henry Bennet-Clark, Robin Elisabeth Cornwell, Lindell Bromham, Mark Sutton, Bethia Thomas, Eliza Howlett, Tom Kemp, Malgosia Nowak-Kemp, Richard Fortey, Derek Siveter, Alex Freeman, Nicky Warren, A. V. Grimstone, Alan Cooper, and especially Christine DeBlase-b.a.l.l.stadt. Others are acknowledged in the Notes at the end.

I am deeply grateful to Mark Ridley and Peter Holland, who were engaged by the publishers as critical readers and gave me exactly the right kind of advice. The routine authorial claim of responsibility for the remaining shortcomings is more than usually necessary in my case.

As always, I gratefully acknowledge the imaginative generosity of Charles Simonyi. And my wife, Lalla Ward, has once again been my help and strength.

RICHARD D DAWKINS.

THE CONCEIT OF HINDSIGHT.

History doesn't repeat itself, but it rhymes.

MARK TWAIN.

History repeats itself; that's one of the things that's wrong with history.

CLARENCE DARROW.

History has been described as one d.a.m.n thing after another. The remark can be seen as a warning against a pair of temptations but, duly warned, I shall cautiously flirt with both. First, the historian is tempted to scour the past for patterns that repeat themselves; or at least, following Mark Twain, to seek reason and rhyme for everything. This appet.i.te for pattern affronts those who insist that, as Mark Twain will also be found to have said, 'History is usually a random, messy affair', going nowhere and following no rules. The second connected temptation is the vanity of the present: of seeing the past as aimed at our own time, as though the characters in history's play had nothing better to do with their lives than fore-shadow us.

Under names that need not trouble us, these are live issues in human history and they arise with greater force, and no greater agreement, on the longer timescale of evolution. Evolutionary history can be represented as one d.a.m.n species after another. But many biologists will join me in finding this an impoverished view. Look at evolution that way and you miss most of what matters. Evolution rhymes, patterns recur. And this doesn't just happen to be so. It is so for well-understood reasons: Darwinian reasons mostly, for biology, unlike human history or even physics, already has its grand unifying theory, accepted by all informed pract.i.tioners, though in varying versions and interpretations. In writing evolutionary history I do not shrink from seeking patterns and principles, but I try to be careful about it.

What of the second temptation, the conceit of hindsight, the idea that the past works to deliver our particular present? The late Stephen Jay Gould rightly pointed out that a dominant icon of evolution in popular mythology, a caricature almost as ubiquitous as lemmings jumping over cliffs (and that myth is false too), is a shambling file of simian ancestors, rising progressively in the wake of the erect, striding, majestic figure of h.o.m.o sapiens sapiens: h.o.m.o sapiens sapiens: man as evolution's last word (and in this context it always is man rather than woman); man as what the whole enterprise is pointing towards; man as a magnet, drawing evolution from the past towards his eminence. man as evolution's last word (and in this context it always is man rather than woman); man as what the whole enterprise is pointing towards; man as a magnet, drawing evolution from the past towards his eminence.

There is a physicist's version which is less obviously vainglorious and which I should mention in pa.s.sing. This is the 'anthropic' notion that the very laws of physics themselves, or the fundamental constants of the universe, are a carefully tuned put-up job, calculated to bring humanity eventually into existence. It is not necessarily founded on vanity. It doesn't have to mean that the universe was deliberately made in order that we should exist. It need mean only that we are here, and we could not be in a universe that lacked the capability of producing us. As physicists have pointed out, it is no accident that we see stars in our sky, for stars are a necessary part of any universe capable of generating us. Again, this does not imply that stars exist in order to make us. It is just that without stars there would be no atoms heavier than lithium in the periodic table, and a chemistry of only three elements is too impoverished to support life. Seeing is the kind of activity that can go on only in the kind of universe where what you see is stars.

But there is a little more that needs to be said. Granted the trivial fact that our presence requires physical laws and constants capable of producing us, the existence of such potent ground rules may still seem tantalisingly improbable. Depending upon their a.s.sumptions, physicists may reckon that the set of possible universes vastly outnumbers that subset whose laws and constants allowed physics to mature, via stars into chemistry and via planets into biology. To some, this means that the laws and constants must have been deliberately premeditated from the start (although it baffles me why anybody regards this as an explanation for anything, given that the problem so swiftly regresses to the larger one of explaining the existence of the equally fine-tuned and improbable Premeditator).

Other physicists are less confident that the laws and constants were free to vary in the first place. When I was little it was not obvious to me why five times eight had to give the same result as eight times five. I accepted it as one of those facts that grownups a.s.sert. Only later did I understand, perhaps through visualising rectangles, why such pairs of multiplications are not free to vary independently of one another. We understand that the circ.u.mference and the diameter of a circle are not independent, otherwise we might feel tempted to postulate a plethora of possible universes, each with a different value of . Perhaps, argue some physicists such as the n.o.bel Prize-winning theorist Steven Weinberg, the fundamental constants of the universe, which at present we treat as independent of one another, will in some Grand Unified fullness of time be understood to have fewer degrees of freedom than we now imagine. Maybe there is only one way for a universe to be. That would undermine the appearance of anthropic coincidence.

Other physicists, including Sir Martin Rees, the present Astronomer Royal, accept that there is a real coincidence in need of explanation, and explain it by postulating many actual universes existing in parallel, mutually incommunicado, each with its own set of laws and constants.1 Obviously we, who find ourselves reflecting upon such things, must be in one of those universes, however rare, whose laws and constants are capable of evolving us. Obviously we, who find ourselves reflecting upon such things, must be in one of those universes, however rare, whose laws and constants are capable of evolving us.

The theoretical physicist Lee Smolin added an ingenious Darwinian spin which reduces the apparent statistical improbability of our existence. In Smolin's model, universes give birth to daughter universes, which vary in their laws and constants. Daughter universes are born in black holes produced by a parent universe, and they inherit its laws and constants but with some possibility of small random change 'mutation'. Those daughter universes that have what it takes to reproduce (last long enough to make black holes, for instance) are, of course, the universes that pa.s.s on their laws and constants to their daughters. Stars are precursors to black holes which, in the Smolin model, are the birth events. So universes that have what it takes to make stars are favoured in this cosmic Darwinism. The properties of a universe that furnish this gift to the future are the self-same properties that incidentally lead to the manufacture of large atoms, including vital carbon atoms. Not only do we live in a universe that is capable of producing life. Successive generations of universes progressively evolve to become increasingly the sort of universe that, as a by-product, is capable of producing life.

The logic of the Smolin theory is bound to appeal to a Darwinian, indeed to anyone of imagination, but as for the physics I am not qualified to judge. I cannot find a physicist to condemn the theory as definitely wrong the most negative thing they will say is that it is superfluous. Some, as we saw, dream of a final theory in whose light the alleged fine-tuning of the universe will turn out to be a delusion anyway. Nothing we know rules out Smolin's theory, and he claims for it the merit which scientists rate more highly than many laymen appreciate of testability. His book is The Life of the Cosmos The Life of the Cosmos and I recommend it. and I recommend it.

But that was a digression about the physicist's version of the conceit of hindsight. The biologist's version is easier to dismiss since Darwin, though harder before him, and it is our concern here. Biological evolution has no privileged line of descent and no designated end. Evolution has reached many millions of interim ends (the number of surviving species at the time of observation), and there is no reason other than vanity human vanity as it happens, since we are doing the talking to designate any one as more privileged or climactic than any other.

This doesn't mean, as I shall continue to argue, that there is a total dearth of reasons or rhymes in evolutionary history. I believe there are recurring patterns. I also believe, though this is more controversial today than it once was, that there are senses in which evolution may be said to be directional, progressive and even predictable. But progress is emphatically not the same thing as progress towards humanity, and we must live with a weak and unflattering sense of the predictable. The historian must beware of stringing together a narrative that seems, even to the smallest degree, to be homing in on a human climax.

A book in my possession (in the main a good book, so I shall not name and shame it) provides an example. It is comparing h.o.m.o habilis h.o.m.o habilis (a human species, probably ancestral to us) with its predecessors the australopithecines. (a human species, probably ancestral to us) with its predecessors the australopithecines.2 What the book says is that What the book says is that h.o.m.o habilis h.o.m.o habilis was 'considerably more evolved than the Australopithecines'. More evolved? What can this mean but that evolution is moving in some pre-specified direction? The book leaves us in no doubt of what the presumed direction is. 'The first signs of a chin are apparent.' 'First' encourages us to expect second and third signs, towards a 'complete' human chin. 'The teeth start to resemble ours ...' As if those teeth were the way they were, not because it suited the habiline diet but because they were embarking upon the road towards becoming our teeth. The pa.s.sage ends with a telltale remark about a later species of extinct human, was 'considerably more evolved than the Australopithecines'. More evolved? What can this mean but that evolution is moving in some pre-specified direction? The book leaves us in no doubt of what the presumed direction is. 'The first signs of a chin are apparent.' 'First' encourages us to expect second and third signs, towards a 'complete' human chin. 'The teeth start to resemble ours ...' As if those teeth were the way they were, not because it suited the habiline diet but because they were embarking upon the road towards becoming our teeth. The pa.s.sage ends with a telltale remark about a later species of extinct human, h.o.m.o erectus: h.o.m.o erectus: Although their faces are still different from ours, they have a much more human look in their eyes. They are like sculptures in the making, 'unfinished' works.

In the making? Unfinished? Only with the unwisdom of hindsight. In excuse of that book it is probably true that, were we to meet a h.o.m.o erectus h.o.m.o erectus face to face, it might well look to our eyes like an unfinished sculpture in the making. But that is only because we are looking with human hindsight. A living creature is always in the business of surviving in its own environment. It is never unfinished or, in another sense, it is always unfinished. So, presumably, are we. face to face, it might well look to our eyes like an unfinished sculpture in the making. But that is only because we are looking with human hindsight. A living creature is always in the business of surviving in its own environment. It is never unfinished or, in another sense, it is always unfinished. So, presumably, are we.

The conceit of hindsight tempts us at other stages in our history. From our human point of view, the emergence of our remote fish ancestors from water to land was a momentous step, an evolutionary rite of pa.s.sage. It was undertaken in the Devonian Period by lobe-finned fish a bit like modern lungfish. We look at fossils of the period with a pardonable yearning to gaze upon our forebears, and are seduced by a knowledge of what came later: drawn into seeing these Devonian fish as 'half way' towards becoming land animals; everything about them earnestly transitional, bound into an epic quest to invade the land and initiate the next big phase of evolution. That is not the way it was at the time. Those Devonian fish had a living to earn. They were not on a mission to evolve, not on a quest towards the distant future. An otherwise excellent book about vertebrate evolution contains the following sentence about fish which ventured out of the water on to the land at the end of the Devonian Period and jumped the gap, so to speak, from one vertebrate cla.s.s to another to become the first amphibians ...

The 'gap' comes from hindsight. There was nothing resembling a gap at the time, and the 'cla.s.ses' that we now recognise were no more separate, in those days, than two species. As we shall see again, jumping gaps is not what evolution does.

It makes no more sense (and no less) to aim our historical narrative towards h.o.m.o sapiens h.o.m.o sapiens than towards any other modern species than towards any other modern species Octopus vulgaris Octopus vulgaris, say, or Panthera leo Panthera leo or or Sequoia sempervirens Sequoia sempervirens. A historically minded swift, understandably proud of flight as self-evidently the premier accomplishment of life, will regard swift-kind those spectacular flying machines with their swept-back wings, who stay aloft for a year at a time and even copulate in free flight as the acme of evolutionary progress. To build on a fancy of Steven Pinker, if elephants could write history they might portray tapirs, elephant shrews, elephant seals and proboscis monkeys as tentative beginners along the main trunk road of evolution, taking the first fumbling steps but each for some reason never quite making it: so near yet so far. Elephant astronomers might wonder whether, on some other world, there exist alien life forms that have crossed the nasal rubicon and taken the final leap to full proboscitude.

We are not swifts nor elephants, we are people. As we wander in imagination through some long-dead epoch, it is humanly natural to reserve a special warmth and curiosity for whichever otherwise ordinary species in that ancient landscape is our ancestor (it is an intriguingly unfamiliar thought that there is always one such species). It is hard to deny our human temptation to see this one species as 'on the main line' of evolution, the others as supporting cast, walk-on parts, sidelined cameos. Without succ.u.mbing to that error, there is one way to indulge a legitimate human-centrism while respecting historical propriety. That way is to do our history backwards, and it is the way of this book.

Backward chronology in search of ancestors really can sensibly aim towards a single distant target. The distant target is the grand ancestor of all life, and we can't help converging upon it no matter where we start elephant or eagle, swift or salmonella, wellingtonia or woman. Backward chronology and forward chronology are each good for different purposes. Go backwards and, no matter where you start, you end up celebrating the unity of life. Go forwards and you extol diversity. It works on small timescales as well as large. The forward chronology of the mammals, within their large but still limited timescale, is a story of branching diversification, uncovering the richness of that group of hairy warmbloods. Backward chronology, taking any modern mammal as our starting point, will always converge upon the same unique ur-mammal: shadowy, insectivorous, nocturnal contemporary of the dinosaurs. This is a local convergence. A yet more local one converges on the most recent ancestor of all rodents, who lived somewhere around the time the dinosaurs went extinct. More local still is the backward convergence of all apes (including humans) on their shared ancestor, who lived about 18 million years ago. On a larger scale, there is a comparable convergence to be found if we work backwards from any vertebrate, an even larger convergence working backwards from any animal to the ancestor of all animals. The largest convergence of all takes us from any modern creature animal, plant, fungus or bacterium back to the universal progenitor of all surviving organisms, probably resembling some kind of bacterium.

I used 'convergence' in the last paragraph, but I really want to reserve that word for a completely different meaning in forward chronology. So for the present purpose I shall subst.i.tute 'confluence' or, for reasons that will make sense in a moment, 'rendezvous'. I could have used 'coalescence', except that, as we shall see, geneticists have already adopted it in a more precise sense, similar to my 'confluence' but concentrating on genes rather than species. In a backward chronology, the ancestors of any set of species must eventually meet at a particular geological moment. Their point of rendezvous is the last common ancestor that they all share, what I shall call their 'Concestor':3 the focal rodent or the focal mammal or the focal vertebrate, say. The oldest concestor is the grand ancestor of all surviving life. the focal rodent or the focal mammal or the focal vertebrate, say. The oldest concestor is the grand ancestor of all surviving life.

We can be very sure there really is a single concestor of all surviving life forms on this planet. The evidence is that all that have ever been examined share (exactly in most cases, almost exactly in the rest) the same genetic code; and the genetic code is too detailed, in arbitrary aspects of its complexity, to have been invented twice. Although not every species has been examined, we already have enough coverage to be pretty certain that no surprises alas await us. If we now were to discover a life form sufficiently alien to have a completely different genetic code, it would be the most exciting biological discovery in my adult lifetime, whether it lives on this planet or another. As things stand, it appears that all known life forms can be traced to a single ancestor which lived more than 3 billion years ago. If there were other, independent origins of life, they have left no descendants that we have discovered. And if new ones arose now they would swiftly be eaten, probably by bacteria.

The grand confluence of all surviving life is not the same thing as the origin of life itself. This is because all surviving species presumably share a concestor who lived after the origin of life: anything else would be an unlikely coincidence, for it would suggest that the original life form immediately immediately branched and more than one of its branches survive to this day. Current textbook orthodoxy dates the oldest bacterial fossils at about 3.5 billion years ago, so the origin of life must at least be earlier than that. If we accept a recent disputation branched and more than one of its branches survive to this day. Current textbook orthodoxy dates the oldest bacterial fossils at about 3.5 billion years ago, so the origin of life must at least be earlier than that. If we accept a recent disputation4 of these apparently ancient fossils, our dating of the origin of life might be a bit more recent. The grand confluence the last common ancestor of all surviving creatures could pre-date the oldest fossils (it didn't fossilise) or it could have lived a billion years later (all but one of the other lineages went extinct). of these apparently ancient fossils, our dating of the origin of life might be a bit more recent. The grand confluence the last common ancestor of all surviving creatures could pre-date the oldest fossils (it didn't fossilise) or it could have lived a billion years later (all but one of the other lineages went extinct).

Given that all backward chronologies, no matter where they start, culminate in the one grand confluence, we can legitimately indulge our human preoccupation and concentrate upon the single line of our own ancestors. Instead of treating evolution as aimed towards us, we choose choose modern modern h.o.m.o sapiens h.o.m.o sapiens as our arbitrary, but forgivably preferred, starting point for a reverse chronology. We choose this route, out of all possible routes to the past, because we are curious about our own great grancestors. At the same time, although we need not follow them in detail, we shall not forget that there are other historians, animals and plants belonging to other species, who are independently walking backwards from their separate starting points, on separate pilgrimages to visit their own ancestors, including eventually the ones they share with us. If we retrace our own ancestral steps, we shall inevitably meet these other pilgrims and join forces with them in a definite order, the order in which their lineages rendezvous with ours, the order of ever more inclusive cousins.h.i.+p. as our arbitrary, but forgivably preferred, starting point for a reverse chronology. We choose this route, out of all possible routes to the past, because we are curious about our own great grancestors. At the same time, although we need not follow them in detail, we shall not forget that there are other historians, animals and plants belonging to other species, who are independently walking backwards from their separate starting points, on separate pilgrimages to visit their own ancestors, including eventually the ones they share with us. If we retrace our own ancestral steps, we shall inevitably meet these other pilgrims and join forces with them in a definite order, the order in which their lineages rendezvous with ours, the order of ever more inclusive cousins.h.i.+p.

Pilgrimages? Join forces with pilgrims? Yes, why not? Pilgrimage is an apt way to think about our journey to the past. This book will be cast in the form of an epic pilgrimage from the present to the past. All roads lead to the origin of life. But because we are human, the path we shall follow will be that of our own ancestors. It will be a human pilgrimage to discover human ancestors. As we go, we shall greet other pilgrims who will join us in strict order, as we reach the common ancestors we share with them.

The first fellow pilgrims we shall greet, some 5 million years ago, deep in Africa where Stanley memorably shook hands with Livingstone, are the chimpanzees. The chimpanzee and bon.o.bo pilgrims will already have joined forces with each other 'before' we greet them. And here we have a little linguistic trickiness which I must face at the outset, before it dogs us any further. I placed 'before' in inverted commas because it could confuse. I used it to mean before in the backwards sense 'before, in the course of the pilgrimage to the past'. But that of course means after after in the chronological sense, the exact opposite meaning! My guess is that no reader was confused in this particular case, but there will be other instances where the reader's patience may be tested. While writing this book I tried the experiment of coining a new preposition, tailored to the peculiar needs of a backward historian. But it didn't fly. Instead, I shall adopt the convention of 'before' in inverted commas. When you see 'before', remember that it really means after! When you see before, it really means before. And the same for 'after' and after, in the chronological sense, the exact opposite meaning! My guess is that no reader was confused in this particular case, but there will be other instances where the reader's patience may be tested. While writing this book I tried the experiment of coining a new preposition, tailored to the peculiar needs of a backward historian. But it didn't fly. Instead, I shall adopt the convention of 'before' in inverted commas. When you see 'before', remember that it really means after! When you see before, it really means before. And the same for 'after' and after, mutatis mutandis mutatis mutandis.

The next pilgrims with whom we shall rendezvous as we push back along our journey are gorillas, then orang utans (quite a lot deeper into the past, and probably no longer in Africa). Next we shall greet gibbons, then Old World monkeys, then New World monkeys, then various other groups of mammals ... and so on until eventually all the pilgrims of life are marching together in one single backward quest for the origin of life itself. As we push on back, there will come a time when it is no longer meaningful to name the continent in which a rendezvous takes place: the map of the world was so different, because of the remarkable phenomenon of plate tectonics. And further back still, all rendezvous take place in the sea.

It is a rather surprising fact that we human pilgrims pa.s.s only about 40 rendezvous points in all, before we hit the origin of life itself. At each of the 40 steps we shall find one particular shared ancestor, the Concestor, which will bear the same labelling number as the Rendezvous. For example, Concestor 2, whom we meet at Rendezvous 2 Rendezvous 2, is the most recent common ancestor of gorillas on the one hand and {humans + {chimpanzees + bon.o.bos}} on the other. Concestor 3 is the most recent common ancestor of orang utans and {{humans + {chimpanzees + bon.o.bos}} + gorillas}. Concestor 39 is the grand ancestor of all surviving life forms. Concestor 0 is a special case, the most recent ancestor of all surviving humans.

We shall be pilgrims, then, sharing fellows.h.i.+p ever more inclusively with other pilgrim bands, which also have been swelling on their own way to their rendezvous with us. After each meeting, we continue together on the high road back to our shared Archaean goal, our 'Canterbury'. There are other literary allusions, of course, and I almost made Bunyan my model and Pilgrim's Regress Pilgrim's Regress my t.i.tle. But it was to Chaucer's my t.i.tle. But it was to Chaucer's Canterbury Tales Canterbury Tales that I and my research a.s.sistant Yan Wong kept returning in our discussions, and it seemed increasingly natural to think of Chaucer throughout this book. that I and my research a.s.sistant Yan Wong kept returning in our discussions, and it seemed increasingly natural to think of Chaucer throughout this book.

Unlike (most of) Chaucer's pilgrims, mine do not all set out together, although they do set off at the same time, the present. These other pilgrims aim towards their ancient Canterbury from different starting points, joining our human pilgrimage at various rendezvous along the road. In this respect, my pilgrims are unlike those who gathered in London's Tabard Inn. Mine are more like the sinister canon and his understandably disloyal yeoman, who joined Chaucer's pilgrims at Boughton-under-Blee, five miles short of Canterbury. Following Chaucer's lead, my pilgrims, which are all the different species of living creature, will have the opportunity to tell tales along the way to their Canterbury which is the origin of life. It is these tales that form the main substance of this book.

Dead men tell no tales, and extinct creatures such as trilobites are deemed not to be pilgrims capable of telling them, but I shall make exceptions of two special cla.s.ses. Animals such as the dodo, which survived into historical times and whose DNA is still available to us, are treated as honorary members of the modern fauna setting off on pilgrimage at the same time as us, and joining us at some particular rendezvous. Since we are responsible for their so recent extinction, it seems the least we can do. The other honorary pilgrims, exceptions to the rule that dead men tell no tales, really are men (or women). Since we human pilgrims are directly seeking our own ancestors, fossils that might plausibly be considered candidates for being being our ancestors are deemed members of our human pilgrimage and we shall hear tales from some of these 'shadow pilgrims', for example the Handyman, our ancestors are deemed members of our human pilgrimage and we shall hear tales from some of these 'shadow pilgrims', for example the Handyman, h.o.m.o habilis h.o.m.o habilis.

I decided it would be twee to let my animal and plant tale-tellers speak in the first person singular, and I shall not do so. Save for occasional asides and prefatory remarks, Chaucer's pilgrims don't either. Many of Chaucer's Tales have their own Prologue, and some have an Epilogue too, all written in Chaucer's own voice as narrator of the pilgrimage. I shall occasionally follow his example. As with Chaucer, an epilogue may serve as a bridge from one tale to the next.

Before his Tales begin, Chaucer has a long General Prologue in which he sets out his cast list: the professions and in some cases the names of the pilgrims who are about to set off from the tavern. Instead, I shall introduce new pilgrims as they join us. Chaucer's jovial host offers to guide the pilgrims, and encourages them to tell their tales to while away the journey. In my role as host I shall use the General Prologue for some preparatory remarks about methods and problems of reconstructing evolutionary history, which must be faced and solved whether we do our history backwards or forwards.

Then we shall embark on our backwards history itself. Although we shall concentrate on our own ancestors, noting other creatures usually only when they join us, we shall from time to time look up from our road and remind ourselves that there are other pilgrims on their own more or less independent routes to our ultimate destination. The numbered rendezvous milestones, plus a few intermediate markers necessary to consolidate the chronology, will provide the scaffolding for our narrative. Each will mark a new chapter, where we halt to take stock of our pilgrimage, and maybe listen to a tale or two. On rare occasions, something important happens in the world around us, and then our pilgrims may pause briefly to reflect on it. But, for the most part, we shall mark our progress to the dawn of life by the measure of those 40 natural milestones, the trysts that enrich our pilgrimage.

1 This 'many universes' idea is not to be confused (though it often is) with Hugh Everett's 'many worlds' interpretation of quantum theory, brilliantly advocated by David Deutsch in This 'many universes' idea is not to be confused (though it often is) with Hugh Everett's 'many worlds' interpretation of quantum theory, brilliantly advocated by David Deutsch in The Fabric of Reality The Fabric of Reality. The resemblance between the two theories is superficial and meaningless. Both theories could be true, or neither, or one, or the other. They were proposed to answer completely different problems. In the Everett theory, the different universes don't differ in their fundamental constants. But it is the entire point of the theory we are here considering that the different universes have different fundamental constants.

2 The laws of zoological nomenclature follow strict precedence, and I fear there is no hope of changing the name The laws of zoological nomenclature follow strict precedence, and I fear there is no hope of changing the name Australopithecus Australopithecus to something less confusing to the contemporary majority who lack a cla.s.sical education. It has nothing to do with Australia. No member of the genus has ever been found outside Africa. to something less confusing to the contemporary majority who lack a cla.s.sical education. It has nothing to do with Australia. No member of the genus has ever been found outside Africa. Australo Australo simply means southern. Australia is the great southern continent, the Aurora australis is the southern equivalent of the Aurora borealis (boreal means northern), and simply means southern. Australia is the great southern continent, the Aurora australis is the southern equivalent of the Aurora borealis (boreal means northern), and Australopithecus Australopithecus was first found in was first found in south south Africa, in the person of the Taung child. Africa, in the person of the Taung child.

3 I am grateful to Nicky Warren for suggesting this word. I am grateful to Nicky Warren for suggesting this word.

4 J. W. Schopf's much-cited evidence for 3.5 billion-years-old bacteria has been sharply criticised by my Oxford colleague Martin Brasier. Brasier may be right about Schopf's evidence, but new evidence, published when this book was in proof, may reinstate 3.5 billion years as the date of the oldest fossils. The Norwegian scientist Harald Furnes and his coworkers found tiny holes in volcanic gla.s.s of that age in South Africa, which they believe were etched by micro-organisms. These 'burrows' contain carbon, which the discoverers claim is of biological origin. No trace of the micro-organisms themselves remains. J. W. Schopf's much-cited evidence for 3.5 billion-years-old bacteria has been sharply criticised by my Oxford colleague Martin Brasier. Brasier may be right about Schopf's evidence, but new evidence, published when this book was in proof, may reinstate 3.5 billion years as the date of the oldest fossils. The Norwegian scientist Harald Furnes and his coworkers found tiny holes in volcanic gla.s.s of that age in South Africa, which they believe were etched by micro-organisms. These 'burrows' contain carbon, which the discoverers claim is of biological origin. No trace of the micro-organisms themselves remains.

THE GENERAL PROLOGUE.

How shall we know the past, and how date it? What aids to our vision will help us peer into theatres of ancient life and reconstruct the scenes and the players, their exits and their entrances, of long ago? Conventional human history has three main methods, and we shall find their counterparts on the larger timescale of evolution. First there is archaeology, the study of bones, arrowheads, fragments of pots, oystersh.e.l.l middens, figurines and other relics that survive as hard evidence from the past. In evolutionary history, the most obvious hard relics are bones and teeth, and the fossils fossils that they eventually become. Second, there are that they eventually become. Second, there are renewed relics renewed relics, records that are not themselves old but which contain or embody a copy or representation of what is old. In human history these are written or spoken accounts, handed down, repeated, reprinted or otherwise duplicated from the past to the present. In evolution, I shall propose DNA as the main renewed relic, equivalent to a written and recopied record. Third, there is triangulation triangulation. This name comes from a method of judging distances by measuring angles. Take a bearing on a target. Now walk a measured distance sideways and take another. From the intercept of the two angles, calculate the distance of the target. Some camera rangefinders use the principle, and map surveyors traditionally relied upon it. Evolutionists can be said to 'triangulate' an ancestor by comparing two (or more) of its surviving descendants. I shall take the three kinds of evidence in order, beginning with hard relics and, in particular, fossils.

Fossils Bodies or bones may survive for our attention, having somehow escaped that of hyenas, burying beetles and bacteria. The 'Ice Man' of the Italian Tyrol was preserved in his glacier for 5,000 years. Insects have become embalmed in amber (petrified gum from trees) for 100 million years. Without benefit of ice or amber, hard parts like teeth, bones and sh.e.l.ls stand the best chance of being preserved. Teeth last longest of all because, to do their job in life, they had to be harder than anything their owner was likely to eat. Bones and sh.e.l.ls need to be hard for different reasons, and they too can last a long time. Such hard parts and, under exceptionally lucky circ.u.mstances, soft parts too, occasionally become petrified as stone fossils that last for hundreds of millions of years.

In spite of the fascination of fossils, it is surprising how much we would still know about our evolutionary past without them. If every fossil were magicked away, the comparative study of modern organisms, of how their patterns of resemblances, especially of their genetic sequences, are distributed among species, and of how species are distributed among continents and islands, would still demonstrate, beyond all sane doubt, that our history is evolutionary, and that all living creatures are cousins. Fossils are a bonus. A welcome bonus, to be sure, but not an essential one. It is worth remembering this when creationists go on (as they tediously do) about 'gaps' in the fossil record. The fossil record could be one big gap, and the evidence for evolution would still be overwhelmingly strong. At the same time, if we had only only fossils and no other evidence, the fact of evolution would again be overwhelmingly supported. As things stand, we are blessed with both. fossils and no other evidence, the fact of evolution would again be overwhelmingly supported. As things stand, we are blessed with both.

The word fossil is conventionally used to mean any relic dating back more than 10,000 years: not a helpful convention, for there is nothing special about a round number like 10,000. If we had fewer or more than ten fingers, we'd recognise a different set of numbers as round.1 When we speak of a fossil, we normally mean that the original material has been subst.i.tuted or infiltrated by a mineral of a different chemical composition and therefore given, as one might say, a new lease of death. An imprint of the original form may be preserved in stone for a very long time indeed, perhaps mixed with some of the original material. There are various ways in which this can happen. I leave the details what is technically called taphonomy for the Ergast's Tale. When we speak of a fossil, we normally mean that the original material has been subst.i.tuted or infiltrated by a mineral of a different chemical composition and therefore given, as one might say, a new lease of death. An imprint of the original form may be preserved in stone for a very long time indeed, perhaps mixed with some of the original material. There are various ways in which this can happen. I leave the details what is technically called taphonomy for the Ergast's Tale.

When fossils were first discovered and mapped, their ages were unknown. The most we could hope for was a rank ordering of oldness. Age ranking depends upon the a.s.sumption known as the Law of Superposition. For obvious reasons, younger strata lie atop older ones, unless the circ.u.mstances are exceptional. Such exceptions, though they sometimes cause temporary puzzlement, are usually pretty obvious. A lump of old rock, complete with fossils, may be thrown on top of a younger stratum, say by a glacier. Or a series of strata may be turned over wholesale, and its vertical ordering exactly reversed. These anomalies can be taken care of by comparing equivalent rocks in other parts of the world. Once this is done, the palaeontologist can piece together the true sequence of the whole fossil record, in a jigsaw of overlapping sequences from different parts of the world. The logic is complicated in practice, though not in principle, by the fact (see the Elephant Bird's Tale) that the map of the world itself changes as the ages go by.

Why is the jigsaw necessary? Why can't we just dig down as far as we like, and treat this as equivalent to digging steadily backwards through time? Well, time itself may flow smoothly, but this doesn't mean that anywhere in the world there is a single sequence of sediment deposited smoothly and continuously from start to finish through geological time. Fossil beds are laid down in fits and starts, when the conditions are right.

In any one location, at any one time, it is rather likely that no sedimentary rocks, and no fossils, are being laid down. But it is quite likely that, in some some part of the world, fossils are being deposited at any given time. By hopping around the world, from site to site where different strata happen to be accessibly near the surface, the palaeontologist can aspire to piece together something approaching a continuous record. Of course individual palaeontologists don't hop from site to site. They hop from museum to museum looking at specimens in drawers, or from journal to journal in university libraries looking at written descriptions of fossils whose site of discovery has been carefully labelled, and they use these descriptions to piece together the fragments of the puzzle from different parts of the world. part of the world, fossils are being deposited at any given time. By hopping around the world, from site to site where different strata happen to be accessibly near the surface, the palaeontologist can aspire to piece together something approaching a continuous record. Of course individual palaeontologists don't hop from site to site. They hop from museum to museum looking at specimens in drawers, or from journal to journal in university libraries looking at written descriptions of fossils whose site of discovery has been carefully labelled, and they use these descriptions to piece together the fragments of the puzzle from different parts of the world.

The task is eased by the fact that particular strata, with recognisably characteristic rock properties, and consistently housing the same kinds of fossils, keep turning up in different regions. Devonian rock, so-called because it was first recognised as the 'Old Red Sandstone' of the beautiful county of Devon, crops up in various other parts of the British Isles, in Germany, Greenland, North America and elsewhere. Devonian rocks are recognisable as Devonian wherever they may be found, partly because of the quality of the rock but also because of the internal evidence of the fossils that they contain. This sounds like a circular argument but it really isn't: no more so than when a scholar recognises a Dead Sea Scroll, from internal evidence, as a fragment of the First Book of Samuel. Devonian rocks are reliably labelled by the presence of certain characteristic fossils.

The same goes for rocks from other geological periods, right back to the time of the earliest hard-bodied fossils. From the ancient Cambrian through to the present Holocene, the geological periods listed in the chart on plate 1 were mostly separated on the basis of changes in the fossil record. And as a result, the end of one period and the start of another is often delimited by extinctions that conspicuously interrupt the continuity of the fossils. As Stephen Jay Gould has put it, no palaeontologist has any trouble identifying whether a lump of rock lies before or after the great end-Permian ma.s.s extinction. There is almost no overlap in animal types. Indeed, fossils (especially microfossils) are so useful in labelling and dating rocks that the oil and mining industries are among their princ.i.p.al users.

Such 'relative dating', then, has long been possible by vertical piecing together of the jigsaw of rocks. The geological periods were named for purposes of relative dating, before absolute dating became possible. And they are still useful. But relative dating is more difficult for rocks with scarce fossils and that includes all rocks older than the Cambrian: the first eight-ninths of Earth's history (see plate 1) (see plate 1).

Absolute dating had to wait for recent developments in physics, especially the physics of radioactivity. This needs some explaining, and the details must wait for the Redwood's Tale. For now, it is enough to know that we have a range of reliable methods for putting an absolute age on fossils, or the rocks that contain or surround them. Moreover, different methods in this range provide sensitivity across the whole spectrum of ages from hundreds of years (tree rings), through thousands of years (carbon 14), millions, hundreds of millions (uranium-thorium-lead) to billions of years (pota.s.sium-argon).

Renewed Relics Fossils, like archaeological specimens, are more-or-less direct relics of the past. We turn now to our second category of historical evidence, renewed renewed relics, copied successively down the generations. For historians of human affairs this might mean eyewitness accounts, handed down by oral tradition or in written doc.u.ments. We cannot ask any living witnesses what it was like to live in fourteenth-century England, but we know about it thanks to written doc.u.ments, including Chaucer's. They contain information that has been copied, printed, stored in libraries, reprinted and distributed for us to read today. Once a story gets into print or, nowadays, a computer medium of some kind, copies of it have a fair chance of being perpetuated into the distant future. relics, copied successively down the generations. For historians of human affairs this might mean eyewitness accounts, handed down by oral tradition or in written doc.u.ments. We cannot ask any living witnesses what it was like to live in fourteenth-century England, but we know about it thanks to written doc.u.ments, including Chaucer's. They contain information that has been copied, printed, stored in libraries, reprinted and distributed for us to read today. Once a story gets into print or, nowadays, a computer medium of some kind, copies of it have a fair chance of being perpetuated into the distant future.

Written records are more reliable than oral tradition, by a disconcerting margin. You might think that each generation of children, knowing their parents as well as most children do, would listen to their detailed reminiscences and relay them to the next generation. Five generations on, a voluminous oral tradition should, one might think, have survived. I remember my four grandparents clearly, but of my eight great-grandparents I know a handful of fragmentary anecdotes. One great-grandfather habitually sang a certain nonsense rhyme (which I can sing), but only while lacing his boots. Another was greedy for cream, and would knock the chess board over when losing. A third was a country doctor. That is about my limit. How have eight entire lives been so reduced? How, when the chain of informants connecting us back to the eyewitness seems so short, and human conversation so rich, could all those thousands of personal details that made up the lifetimes of eight human individuals be so fast forgotten?

Frustratingly, oral tradition peters out almost immediately, unless hallowed in bardic recitations like those that were eventually written down by Homer, and even then the history is far from accurate. It decays into nonsense and falsehood after amazingly few generations. Historical facts about real heroes, villains, animals and volcanoes rapidly degenerate (or blossom, depending upon your taste) into myths about demiG.o.ds, devils, centaurs and fire-breathing dragons.2 But oral traditions and their imperfections needn't detain us because, in any case, they have no equivalent in evolutionary history. But oral traditions and their imperfections needn't detain us because, in any case, they have no equivalent in evolutionary history.

Writing is a huge improvement. Paper, papyrus and even stone tablets may wear out or decay, but written records have the potential to be copied accurately for an indefinite number of generations, although in practice the accuracy is not total. I should explain the special sense in which I mean accuracy and, indeed, the special sense in which I mean generations. If you handwrite me a message and I copy it and pa.s.s it on to a third person (the next copying 'generation'), it will not be an exact replica, for my handwriting is different from yours. But if you write with care, and if I painstakingly match each of your squiggles with exactly one from our shared alphabet, your message has a good chance of being copied by me with total accuracy. In theory this accuracy could be preserved through an indefinite number of 'generations' of scribes. Given that there is a discrete alphabet agreed by writer and reader, copying lets a message survive the destruction of the original. This property of writing can be called 'self-normalising'. It works because letters of a true alphabet are discontinuous. The point, reminiscent of the distinction between a.n.a.logue and digital codes, needs a little more explanation.

There exists a consonant sound which is intermediate between the English hard c and g (it is the French hard c in comme comme). But n.o.body would think of trying to represent this sound by writing a character which looked intermediate between c and g. We all understand that a written character in English must be one, and only one, member of our 26-letter alphabet. We understand that French uses the same 26 letters for sounds that are not exactly the same as ours and which may be intermediate between ours. Each language, indeed each local accent or dialect, separately uses the alphabet for self-normalising on different sounds.

Self-normalisation fights against the 'Chinese Whispers'3 degrading of messages over generations. The same protection is not available to a drawing, copied and recopied along a line of imitative artists, unless the drawing style incorporates ritual conventions as its own version of 'self-normalisation'. An eyewitness record of some event, which is written down, as opposed to drawn as a picture, has a good chance of still being accurately reproduced in history books centuries later. We have what is probably an accurate account of the destruction of Pompeii in 79 degrading of messages over generations. The same protection is not available to a drawing, copied and recopied along a line of imitative artists, unless the drawing style incorporates ritual conventions as its own version of 'self-normalisation'. An eyewitness record of some event, which is written down, as opposed to drawn as a picture, has a good chance of still being accurately reproduced in history books centuries later. We have what is probably an accurate account of the destruction of Pompeii in 79 AD AD because a witness, Pliny the Younger, wrote down what he saw, in two epistles to the historian Tacitus, and some of Tacitus's writings survived, by successive copying and eventually printing, for us to read them today. Even in pre-Gutenberg days when doc.u.ments were duplicated by scribes, writing represented a great advance in accuracy compared with memory and oral tradition. because a witness, Pliny the Younger, wrote down what he saw, in two epistles to the historian Tacitus, and some of Tacitus's writings survived, by successive copying and eventually printing, for us to read them today. Even in pre-Gutenberg days when doc.u.ments were duplicated by scribes, writing represented a great advance in accuracy compared with memory and oral tradition.

It is only a theoretical ideal that repet.i.tive copying retains perfect accuracy. In practice scribes are fallible, and not above ma.s.saging their copy to make it say things that they think (no doubt sincerely) the original doc.u.ment ought to have said. The most famous example of this, painstakingly doc.u.mented by nineteenth-century German theologians, is the doctoring of New Testament history to make it conform to Old Testament prophecies. The scribes concerned were probably not wilfully mendacious. Like the gospel-makers, who themselves lived long after Jesus's death, they genuinely believed he had been the incarnation of Old Testament messianic prophecies. He 'must', therefore, have been born in Bethlehem, and descended from David. If the doc.u.ments unaccountably failed to say so, it was the scribe's conscientious duty to rectify the deficiency. A sufficiently devout scribe would, I suppose, no more have regarded this as falsification than we do when we automatically correct a spelling mistake or a grammatical infelicity.

Quite apart from positive ma.s.saging, all repeated copying is subject to straightforward errors like skipping a line, or a word in a list. But in any case writing cannot take us back beyond its invention, which was only about 5,000 years ago. Identification symbols, counting-marks and pictures go back a bit further, perhaps some tens of thousands of years, but all such periods are chickenfeed compared with evolutionary time.

Fortunately, when we turn to evolution there is another kind of duplicated information which goes back an almost unimaginably large number of copying generations and which, with a little poetic licence, we can regard as the equivalent of a written text: a historical record that renews itself with astounding accuracy for hundreds of millions of generations precisely because, like our writing system, it has a self-normalising alphabet. The DNA information in all living creatures has been handed down from remote ancestors with prodigious fidelity. The individual atoms in DNA are turning over continually, but the information that they encode in the pattern of their arrangement is copied for millions, sometimes hundreds of millions, of years. We can read this record directly, using the arts of modern molecular biology to spell out the actual DNA letter sequences or, slightly more indirectly, the amino acid sequences of protein into which they are translated. Or, much more indirectly as through a gla.s.s darkly, we can read it by studying the embryological products of the DNA: the shapes of bodies and their organs and chemistries. We don't need fossils to peer back into history. Because DNA changes very slowly through the generations, history is woven into the fabric of modern animals and plants, and inscribed in its coded characters.

DNA messages are written in a true alphabet. Like the Roman, Greek and Cyrillic writing systems, the DNA alphabet is a strictly limited repertoire of symbols with no self-evident meaning. Arbitrary symbols are chosen and combined to make meaningful messages of unlimited complexity and size. Where the English alphabet has 26 letters and the Greek one 24, the DNA alphabet is a four-letter alphabet. Most useful DNA spells out three-letter words from a dictionary limited to 64 words, each word called a 'codon'. Some of the codons in the dictionary are synonymous with others, which is to say that the genetic code is technically 'degenerate'.4 The dictionary maps 64 code words onto 21 meanings the 20 biological amino acids, plus one all-purpose punctuation mark. Human languages are numerous and changing, and their dictionaries contain tens of thousands of distinct words, but the 64-word DNA dictionary is universal and unchanging (with very minor variations in a few rare cases). The 20 amino acids are strung into sequences of typically a few hundred, each sequence a particular protein molecule. Whereas the number of letters is limited to four and the number of codons to 64, there is no theoretical limit to the number of proteins that can be spelled out by different sequences of codons. It is beyond all counting. A 'sentence' of codons specifying one protein molecule is an identifiable unit often called a gene. The genes are not separated from their neighbours (whether other genes or repet.i.tive nonsense) by any delimiters apart from what can be read from their sequence. In this respect they resemble TELEGRAMS THAT LACK PUNCTUATION MARKS COMMA AND HAVE TO SPELL THEM OUT AS WORDS COMMA ALTHOUGH EVEN TELEGRAMS HAVE THE ADVANTAGE OF s.p.a.cES BETWEEN WORDS COMMA WHICH DNA LACKS STOP TELEGRAMS THAT LACK PUNCTUATION MARKS COMMA AND HAVE TO SPELL THEM OUT AS WORDS COMMA ALTHOUGH EVEN TELEGRAMS HAVE THE ADVANTAGE OF s.p.a.cES BETWEEN WORDS COMMA WHICH DNA LACKS STOP DNA differs from written language in that islands of sense are separated by a sea of nonsense, never transcribed. 'Whole' genes are a.s.sembled, during transcription, from meaningful 'exons' separated by meaningless 'introns' whose texts are simply skipped by the reading apparatus. And even meaningful stretches of DNA are in many cases never read presumably they are superseded copies of once useful genes that hang around like early drafts of a chapter on a cluttered hard disk. Indeed, the image of the genome as an old hard disk, badly in need of a spring clean, is one that will serve us from time to time during the book.

It bears repeating that the DNA molecules of long-dead animals are not themselves preserved. The information information in DNA can be preserved for ever, but only by dint of frequent re-copying. The plot of in DNA can be preserved for ever, but only by dint of frequent re-copying. The plot of Jura.s.sic Park Jura.s.sic Park, though not silly, falls foul of practical facts. Conceivably, for a short while after becoming embalmed in amber, a bloodsucking insect could have contained the instructions needed to reconstruct a dinosaur. But unfortunately, after an organism is dead, the DNA in its body, and in blood that it has sucked, doesn't survive intact longer than a few years only days in the case of some soft tissues. Fossilisation doesn't preserve DNA either.

Even deep freezing doesn't preserve it for very long. As I write this, scientists are excavating a frozen mammoth from the Siberian permafrost in the hope of extracting enough DNA to grow a new mammoth, cloned in the womb of a modern elephant. I fear this is a vain hope, though the mammoth is only a few thousand years dead. Among the oldest corpses from which readable DNA has been extracted is a Neanderthal man. Imagine the kerfuffle if somebody managed to clone him. But alas, only disjointed fragments of his 30,000-year-old DNA can be recovered. For plants in permafrost, the record is about 400,000 years.

The important point about DNA is that, as long as the chain of reproducing life is not broken, its coded information information is copied to a new molecule before the old molecule is destroyed. In this form, DNA information far outlives its molecules. It is renewable copied and since the copies are literally perfect for most of its letters on any one occasion, it can potentially last an indefinitely long time. Large quant.i.ties of our ancestors' DNA information survives completely unchanged, some even from hundreds of millions of years ago, preserved in successive generations of living bodies. is copied to a new molecule before the old molecule is destroyed. In this form, DNA information far outlives its molecules. It is renewable copied and since the copies are literally perfect for most of its letters on any one occasion, it can potentially last an indefinitely long time. Large quant.i.ties of our ancestors' DNA information survives completely unchanged, some even from hundreds of millions of years ago, preserved in successive generations of living bodies.

Understood in this way, the DNA record is an almost unbelievably rich gift to the historian. What historian could have dared hope for a world in which every single individual of every species carries, within its body, a long and detailed text: a written doc.u.ment handed down through time? Moreover, it has minor random changes, which occur seldom enough not to mess up the record yet often enough to furnish distinct labels. It is even better than that. The text is not just arbitrary. In Unweaving the Rainbow Unweaving the Rainbow, I made a Darwinian case for regarding an animal's DNA as a 'Genetic Book of the Dead': a descriptive record of ancestral worlds. It follows from the fact of Darwinian evolution that everything about an animal or plant, including its bodily form, its inherited behaviour and the chemistry of its cells, is a coded message about the worlds in which its ancestors survived: the food they sought; the predators they escaped; the climates they endured; the mates they beguiled. The message is ultimately scripted in the DNA that fell through the succession of sieves that is natural selection. When we learn to read it properly, the DNA of a dolphin may one day confirm what we already know from the telltale giveaways in its anatomy and physiology: that its ancestors once lived on dry land. Three hundred million years earlier, the ancestors of all land-dwelling vertebrates, including the land-dwelling ancestors of dolphins, came out of the sea where they had lived since the origin of life. Doubtless our DNA records this fact if we could read it. Everything about a modern animal, especially its DNA, but its limbs and its heart, its brain and its breeding cycle too, can be regarded as an archive, a chronicle of its past, even if that chronicle is a palimpsest, many times overwritten.

The DNA chronicle may be a gift to the historian, but it is a hard one to read, demanding deeply informed interpretation. It is made more powerful if combined with our third method of historical reconstruction, triangulation. It is to this that we now turn, and again we start with the a.n.a.logous case of human history, specifically the history of languages.

Triangulation.

Linguists often wish to trace languages back through history. Where written records survive it is rather easy. The historical linguist can use the second of our two methods of reconstruction, tracing back renewed relics, in this case words. Modern English goes back via Middle English to Anglo-Saxon using the continuous literary tradition, through Shakespeare, Chaucer and Beowulf Beowulf. But speech obviously goes back long before the invention of writing, and many languages have no written form anyway. For the earlier history of dead languages, linguists resort to a version of what I am calling triangulation. They compare modern languages and group them hierarchically into families within families. Romance, Germanic, Slavic, Celtic and other European language families are in turn grouped with some Indian language fam