The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

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.

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.

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.

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.

One could say that the resemblance between German and Dutch is comparable to that between any pair of mammals. Human and chimpanzee DNA are so similar, they are like English spoken in two slightly different accents. The resemblance between English and Japanese, or between Spanish and Basque, is so slight that no pair of living organisms can be chosen for analogy, not even humans and bacteria.

Humans and bacteria have DNA sequences which are so similar that whole paragraphs are word-for-word identical.

It has been well said that if you wiped out sub-Saharan Africa you would lose the great majority of human genetic diversity, whereas you could wipe out everywhere except Africa and nothing much would change.

We can make an even more stunning deduction. For certain genes, you must be more closely related to some chimpanzees than to some humans. And I am closer to some chimpanzees than to you (or to ‘your’ chimpanzees). Humans as a species, as well as humans as individuals, are temporary vessels containing a mix of genes from different sources. Individuals are temporary meeting points on the crisscrossing routes that genes take through history.

I am bowled over by the forensic power of today’s statistical genetic techniques to unmask signals hitherto buried deep in the past. I forsee yet more stunning revelations, not just about demography and geography but also about the totality of ancestral lives, a digitally chronicled ancestor’s tale.

Modern experimenters have systematically constructed fires of various kinds and then examined them afterwards for their trace effects. It emerges that deliberately built campfires magnetise the soil in a way that distinguishes them from bushfires and from burnt-out tree stumps—I don’t know why.

A good theory is one that needs to postulate little, in order to explain lots. (By this criterion, as I have often remarked elsewhere, Darwin’s theory of natural selection may be the best theory of all time.)

All eyes on our planet are set up in such a way as to exploit the wavelengths of electromagnetic radiation in which our local star shines brightest, and which pass through the window of our atmosphere.

Can the Seal’s Tale tell us something about our own natural breeding system, before civilisation and custom obliterated the traces? Our sexual dimorphism is moderate but undeniable. Lots of women are taller than lots of men, but the tallest men are taller than the tallest women. Lots of women can run faster, lift heavier weights, throw javelins further, play better tennis, than lots of men. But for humans, unlike for racehorses, the underlying sexual dimorphism precludes sex-blind open competition at the top level in almost any sport you care to name. In most physical sports, every single one of the world’s top hundred men would beat every single one of the world’s top hundred women.

To Alfred Russel Wallace and Charles Darwin, co-discoverers of natural selection, the geographical aspect of natural history gave away the fact of evolution. If species were created independently, why should a creator choose to put 50 species of lemur on Madagascar, but nowhere else? Why should the Galapagos host a set of finches so different from species on other oceanic islands, but strikingly similar both to each other and to birds on the nearest mainland? Why indeed did creation on islands involve flying species like birds and bats, but rarely frogs and land mammals?

The australosphenidans were those early mammals that evolved in the great southern continent of Gondwana. And the boreosphenidans evolved in the northern continent of Laurasia, in a sort of earlier incarnation long before the evolution of the laurasiatheres we know today. The monotremes are the only surviving representatives of the australosphenidans. All the rest of the mammals, the therians, including the marsupials that we now associate with Australia, are descended from the northern boreosphenidans.

The key difference between amphibians and amniotes is that amniote skins and eggshells are waterproof. Amphibian skin typically lets water evaporate through it, at the same rate you’d expect from a body of standing water of the same area.

Members of different species cannot, by definition, interbreed with one another, yet surely a child would not be so different from its parents as to be incapable of interbreeding with their kind. Doesn’t this, he wound up, wagging his metaphorical finger in the special way that lawyers, at least in courtroom dramas, have perfected as their own, undermine the whole idea of evolution? That is like saying, ‘When you heat a kettle of cold water, there is no particular moment when the water ceases to be cold and becomes hot, therefore it is impossible to make a cup of tea.’ Since I always try to turn questions in a constructive direction, I told my lawyer about the herring gulls, and I think he was interested. He had insisted on placing individuals firmly in this species or that. He didn’t allow for the possibility that an individual might lie half way between two species, or a tenth of the way from species A to species B. Exactly the same limitation of thought hamstrings the endless debates about exactly when in the development of an embryo it becomes human (and when, by implication, abortion should be regarded as tantamount to murder). It is no use saying to these people that, depending upon the human characteristic that interests you, a foetus can be ‘half human’ or ‘a hundredth human’. ‘Human’, to the qualitative, absolutist mind, is like ‘diamond’. There are no halfway houses. Absolutist minds can be a menace. They cause real misery, human misery. This is what I call the t...

People and chimpanzees are certainly linked via a continuous chain of intermediates and a shared ancestor, but the intermediates are extinct: what remains is a discontinuous distribution. The same is true of people and monkeys, and of people and kangaroos, except that the necklace of extinct intermediates is longer, and most links lived longer ago. Because the intermediates are nearly always extinct, we can usually get away with assuming that there is a sharp discontinuity between every species and every other.

When we are talking about all the animals that have ever lived, not just those that are living now, evolution tells us there are lines of gradual continuity linking literally every species to every other.

Ernst Mayr, distinguished elder statesman of twentieth-century evolution, has blamed the delusion of discontinuity—under its philosophical name of Essentialism—as the main reason why evolutionary understanding came so late in human history. Plato, whose philosophy can be seen as the inspiration for Essentialism, believed that actual things are imperfect versions of an ideal archetype of their kind. Hanging somewhere in ideal space is an essential, perfect rabbit, which bears the same relation to a real rabbit as a mathematician’s perfect circle bears to a circle drawn in the dust. To this day many people are deeply imbued with the idea that sheep are sheep and goats are goats, and no species can ever give rise to another because to do so they’d have to change their ‘essence’. There is no such thing as essence.

No evolutionist thinks that modern species change into other modern species. Cats don’t turn into dogs or vice versa. Rather, cats and dogs have evolved from a common ancestor, who lived tens of millions of years ago. If only all the intermediates were still alive, attempting to separate cats from dogs would be a doomed enterprise, as it is with the salamanders and the gulls. Far from being a question of ideal essences, separating cats from dogs turns out to be possible only because of the lucky (from the point of view of the essentialist) fact that the intermediates happen to be dead. Plato might see the irony in the fact that it is actually an imperfection—the sporadic ill-fortune of death—that makes the separation of any one species from another possible. This of course applies to the separation of human beings from our nearest relatives—and, indeed, from our more distant relatives too. In a world of perfect and complete information, fossil information as well as recent, discrete names for animals would become impossible. Instead of discrete names we would need sliding scales, just as the words hot, warm, cool and cold are better replaced by a sliding scale such as Celsius or Fahrenheit.

By the interbreeding criterion every individual is a member of the same species as its parents. This is an unsurprising, not to say platitudinously obvious conclusion, until you realise that it raises an intolerable paradox in the essentialist mind. Most of our ancestors throughout evolutionary history have belonged to different species from us by any criterion, and we certainly couldn’t have interbred with them. In the Devonian Period our direct ancestors were fish. Yet, although we couldn’t interbreed with them, we are linked by an unbroken chain of ancestral generations, every one of which could have interbred with their immediate predecessors and immediate successors in the chain.

There would never be a generation in which it made sense to say of an individual that he is Homo sapiens but his parents are Homo ergaster. You can think of it as a paradox if you like, but there is no reason to think that any child was ever a member of a different species from its parents, even though the daisy chain of parents and children stretches back from humans to fish and beyond. Actually it isn’t paradoxical to anybody but a dyed-in-the-wool essentialist. It is no more paradoxical than the statement that there is never a moment when a growing child ceases to be short and becomes tall. Or a kettle ceases to be cold and becomes hot. The legal mind may find it necessary to impose a barrier between childhood and majority—the stroke of midnight on the eighteenth birthday, or whenever it is. But anyone can see that it is a (necessary for some purposes) fiction. If only more people could see that the same applies to when, say, a developing embryo becomes ‘human’.

Creationists love ‘gaps’ in the fossil record. Little do they know, biologists have good reason to love them too. Without gaps in the fossil record, our whole system for naming species would break down.

The concestor that we meet here, approximately our 185-million-greats-grandparent, was a sarcopterygian, a lobefin fish (see plate 22). In the same way that we accept whales as mammals, descended from a mammalian common ancestor, we land-lubbing tetrapods should strictly classify ourselves as lobe-finned fish, albeit highly modified by our full-bodied commitment to life in air. It is a modification that happened piecemeal, some time between Rendezvous 18 and 17, as our ancestors meandered their way from water to land. The story demands a tale, recounted in a moment by the trickle of pilgrims who join us here, six species who never really left the water. They are our closest fishy relatives, the lungfish.

To a fish, a lake is an island.

In order for speciation to happen, there must be populations that are sufficiently isolated for gene flow between them to be rare; but not so isolated that no founding individuals arrive there at all. The recipe for speciation is ‘Genes flow but not much’.

Gastrulation is something that all animals do early in their life. Typically, before gastrulation, an animal embryo consists of a hollow ball of cells, the blastula, whose wall is one cell thick. During gastrulation the ball indents to form a cup with two layers. The opening of the cup closes in to form a small hole called the blastopore. Almost all animal embryos go through this stage, which presumably means it is a very ancient feature indeed. You might expect that so fundamental an opening would become one of the two deep holes in the body, and you’d be right. But now comes the big divide in the animal kingdom, between the Deuterostomia (every pilgrim who arrived before Rendezvous 26, including us) and the Protostomia (the huge throng who are now joining at Rendezvous 26). In deuterostome embryology, the eventual fate of the blastopore is to become the anus (or at least the anus develops close to the blastopore). The mouth appears later as a separate perforation at the other end of the gut. The protostomes do it differently: in some, the blastopore becomes the mouth, and the anus appears later; in others, the blastopore is a slit that subsequently zippers up in the middle, with the mouth at one end and the anus at the other. Protostome means ‘mouth first’. Deuterostome means ‘mouth second’.

The insects alone constitute at least three-quarters of all animal species, and probably more. As Robert May, distinguished former President of the Royal Society, has said, to a first approximation all species are insects.

With the exception of the eurypterids, those Palaeozoic sea scorpions* which, we conjectured, terrorised the Palaeozoic fishes, arthropods have not achieved the enormous body size of some extreme vertebrates. This is often attributed to limits set by their method of encasing themselves in an armour-plated exoskeleton, with their limbs in hard jointed tubes. It means they can grow only by ecdysis: casting their outer casing aside at regular intervals and hardening a new, larger one. How the eurypterids managed to exempt themselves from this alleged size limitation is not entirely clear to me.

There is a growing consensus that insects are a derived sort of crustacean.

The other major phylum in the superphylum Ecdysozoa is that of the nematode worms. They too are extremely numerous, a fact made memorable long ago by the American zoologist Ralph Buchsbaum: If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognisable . . . we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes . . . Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.

Any animal that moves, in the sense of covering the ground from A to B rather than just sitting in one place and waving its arms or pumping water through itself, is likely to need a specialised front end. It might as well have a name, so let’s call it the head. The head hits novelty first. It makes sense to take in food at the end that encounters it first, and to concentrate the sense organs there too—eyes perhaps, some kind of feelers, organs of taste and smell. Then the main concentration of nervous tissue—the brain—had best be near the sense organs, and near the action at the front end, where the food-catching apparatus is. So we can define the head end as the leading end, the one with the mouth, the main sense organs and the brain, if there is one. Another good idea is to void wastes somewhere near the back end, far from the mouth, to avoid re-imbibing what has just been passed out. By the way, although all this makes sense if we think worm, I should remind you that the argument evidently does not apply to radially symmetrical animals such as starfish. I am genuinely puzzled why starfish and their kind opt out of this argument, which is one reason why I referred to them as ‘Martians’. To return to our primeval worm, having dealt with its fore and aft asymmetry, how about up-down asymmetry? Why is there a dorsal side and a ventral side? The argument is similar, and this one applies to starfish just as much as to worms. Gravity being what it is, there are lots of inevita...

This is a lovely argument.

Let me instead divide the animal kingdom into the dorsocords and the ventricords. The dorsocords are all deuterostomes. The ventricords are mostly protostomes, plus some early deuterostomes perhaps including the acorn worms. The echinoderms, with their remarkable reversion to radial symmetry, don’t fit into this classification at all.

The more general moral of the Brine Shrimp’s Tale is this. Major transitions in evolution may have begun as changes in behavioural habit, perhaps even non-genetic learned changes of habit, which only later were followed by genetic evolution. I fancy that a comparable tale could be told for the first bird ancestor to fly, the first fish to come out on the land, and the first whale ancestor to return to the water (as Darwin speculated with his fly-catching bear).

I must at this point reiterate my strong objection to being asked to fill in forms in which I have to tick a box labelling my ‘race’ or ‘ethnicity’, and voice my strong support for Lewontin’s statement that racial classification can be actively destructive of social and human relations—especially when people use racial classification as a way of treating people differently, whether through negative or positive discrimination. To tie a racial label to somebody is informative in the sense that it tells you more than one thing about them. It might reduce your uncertainty about the colour of their hair, the colour of their skin, the straightness of their hair, the shape of their eye, the shape of their nose and how tall they are. But there is no reason to suppose that it tells you anything about how well-qualified they are for a job. And even in the unlikely event that it did reduce your statistical uncertainty about their likely suitability for some particular job, it would still be wicked to use racial labels as a basis for discrimination when hiring somebody. Choose on the basis of ability, and if, having done so, you end up with an all-black sprinting team, so be it. You have not practised racial discrimination in arriving at this conclusion.

Discriminating against individuals purely on the basis of a group to which they belong is, I am inclined to think, always evil. There is near-universal agreement today that the apartheid laws of South Africa were evil. Positive discrimination in favour of ‘minority’ students on American campuses can fairly, in my opinion, be attacked on the same grounds as apartheid. Both treat people as representative of groups rather than as individuals in their own right. Positive discrimination is sometimes justified as redressing centuries of injustice. But how can it be just to pay back a single individual today for the wrongs done by long-dead members of a plural group to which he belongs?

The origami analogy fits early embryology better than late. The main organisation of the body is initially laid down by a series of foldings and invaginations of layers of cells. Once the main body plan is safely in place, later stages in development consist largely of growth, as if the embryo were being inflated, in all its parts, like a balloon. It is a very special kind of balloon, however, because different parts of the body inflate at different rates, the rates being carefully controlled. This is the important phenomenon known as allometry. The Fruit Fly’s Tale is concerned mostly with the earlier, origami phase of development, not the later, inflationary one.

In the very early embryo, a cell needs to ‘know’ where it lies along two main dimensions: fore and aft (anterior/posterior) and up-down (dorsal/ventral). What does ‘know’ mean? It initially means that a cell’s behaviour is determined by its position along chemical gradients in each of the two axes. Such gradients necessarily start in the egg itself, and are therefore under the control of the mother’s genes, not the egg’s own nuclear genes. For example, there is a gene called bicoid in the Drosophila mother’s genotype, which expresses itself in the ‘nurse’ cells that make her eggs. The protein made by the bicoid gene is shipped into the egg, where it is concentrated at one end, whence it fades towards the other end. The resulting concentration gradient (and others like it) labels the anterior/posterior axis. Comparable mechanisms at right-angles label the dorsal/ventral axis.

A Hox gene, then, is a gene whose mission in life is to know whereabouts in the body it is, and so inform other genes in the same cell. We are now armed to understand homeotic mutations. When things go wrong with a Hox gene, the cells in a segment are misinformed about which segment they are in, and they fashion the segment they ‘think’ they are in. So, for instance, we see a leg growing in the segment that would normally grow an antenna. This makes perfect sense. The cells in any segment are perfectly capable of assembling the anatomy of any other segment. Why should they not? The instructions for making any segment lurk in the cells of every segment. It is the Hox genes, under normal conditions, that call forth the ‘correct’ instructions for making the anatomy appropriate to each segment. As William Bateson rightly suspected, homeotic abnormality opens a revealing window on how the system normally works. Recall that flies, unusually among insects, normally have only one pair of wings, plus a pair of gyroscopic halteres. The homeotic mutation Ultrabithorax misleads cells in the third thoracic segment into ‘thinking’ they are in the second thoracic segment. They therefore collaborate to make an extra pair of wings, instead of a pair of halteres (see plate 36). There is a mutant flour beetle (Tribolium) in which all 15 segments develop antennae, presumably because the cells all ‘think’ they are in segment 2.

The sublime beauty of nature and evolution is reflected in the Hox gene.

This brings us to the most wonderful part of the Fruit Fly’s Tale. After they had been discovered in Drosophila, Hox genes started turning up all over the place: not only in other insects such as beetles, but in almost all other animals that have been looked at, including ourselves. And—this really is almost too good to be true—they very often turn out to be doing the same kind of thing, even down to informing cells which segment they are in and (better still) being arrayed in the same order along chromosomes. Let’s now turn to the mammal story, which has been most thoroughly worked out in the laboratory mouse—that Drosophila of the mammal world.

human alpha globin is truly a closer cousin to, say, lizard alpha globin than it is to human beta globin—which is in turn a closer cousin to lizard beta globin.

Hitherto, animals were defined as opposed to plants, in a rather unsatisfactorily negative way. Slack, Holland and Graham suggested a positive, specific criterion that has the effect of uniting all animals and excluding all non-animals, such as plants and protozoa. The Hox story shows that animals are not a highly varied, unconnected miscellany of phyla, each with its own fundamental body plan acquired and maintained in lonely isolation. If you forget morphology and look only at the genes, it emerges that all animals are minor variations on a very particular theme. What delight to be a zoologist at such a time.

What stuck in Maynard Smith’s craw is that the bdelloids as a whole reproduce only asexually—every last one of them, evidently descended from a bdelloid common ancestor that must have lived long enough ago to beget 18 genera and 360 species. Remains in amber suggest that this male-spurning matriarch lived at least 40 million years ago, very probably more. The bdelloids are a highly successful group of animals, astonishingly numerous and a dominant part of the freshwater faunas of the world. Not a single male has ever been found.*

I think they are right to conclude that the bdelloid rotifers really are anciently, continuously, universally and successfully asexual. They really are an evolutionary scandal. For perhaps 80 million years they have flourished by doing something that no other group of animals can get away with, except for very short periods before going extinct.

Contrary to all appearances, barnacles are crustaceans. The ordinary acorn barnacles, which encrust the rocks like miniature limpets, helping your shoes not to slip if you have them and hurting your feet if you don’t, are completely unlike limpets internally. Inside the shell, they are distorted shrimps lying on their backs, kicking their legs in the air.

Most deceptive of all—indeed probably holding the record for animals not looking remotely like the thing that zoologists know them to be—are the parasitic barnacles, such as Sacculina. Sacculina is not what it seems with a vengeance. Zoologists would never have realised that it is in fact a barnacle, but for its larva. The adult is a soft sac that clings to the underside of a crab and sends long, branching, plant-like roots inside to absorb nourishment from the crab’s tissues. The parasite not only doesn’t look like a barnacle, it doesn’t look like a crustacean of any kind. It has completely lost all trace of the armour plating, and all trace of the bodily segmentation that nearly all other arthropods have. It might as well be a parasitic plant or fungus. Yet, in terms of its evolutionary relationships, it is a crustacean, and not just a crustacean but specifically a barnacle. Barnacles are indeed not what they seem.

The gene called Abdominal-A, which normally supervises the development of a typical crustacean abdomen, is not expressed in Sacculina. It looks as though you can turn a swimming, kicking, leggy animal into a shapeless fungoid just by suppressing Hox genes.

Creationists love the Cambrian Explosion because it seems, to their carefully impoverished imaginations, to conjure a sort of palaeontological orphanage inhabited by parentless phyla: animals without antecedents, as if they had suddenly materialised overnight from nothing, complete with holes in their socks.*

Creationists foolishly liken Darwinian natural selection to a hurricane blowing through a junkyard and having the luck to assemble a Boeing 747. They are wrong, of course, for they completely miss the gradual, cumulative nature of natural selection. But the junkyard metaphor is entirely apt to the hypothetical overnight invention of a new phylum. An evolutionary step of the same magnitude as, say, the overnight transition from earthworm to snail, really would have to be as lucky as the hurricane in the junkyard.