The Ancestor's Tale Part 15

Mark Welch and Meselson used these opposite predictions to test the theory that the bdelloids really have been s.e.xless and male-less for a very long time with stunning success. They looked at modern bdelloids to see whether it was indeed true that paired chromosomes (or chromosomes that had once been paired) were much more unlike each other than they 'should' be, gene for gene, if s.e.xual recombination had been holding them together. They used other rotifers, non-bdelloids who do have s.e.x, as a control for comparison. And the answer is yes. Bdelloid chromosomes are far more different from their pairs than they 'should' be, by an amount compatible with the theory that they gave up s.e.x not just 40 million years ago, which is the age of the oldest amber in which bdelloids have been found, but about 80 million years ago. Mark Welch and Meselson scrupulously bend over backwards to discuss possible alternative interpretations of their results, but these are far-fetched, and I think they are right to conclude that the bdelloid rotifers really are anciently, continuously, universally and successfully as.e.xual. 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.

Why would we normally expect as.e.xual reproduction to lead to extinction? Well, that is a big question because it amounts to the question of what is good about s.e.x itself the question that better scientists than I have spent book after book failing to answer. I shall just point out that the bdelloid rotifers are a paradox within a paradox. In one way, they are like the soldier in the marching platoon whose mother cried out, 'There goes my boy he's the only one in step.' Maynard Smith called them an evolutionary scandal, but he was the one mainly responsible for pointing out that s.e.x itself, on the face of it, is the evolutionary scandal. At least a naive view of Darwinian theory would predict that s.e.x should be heavily disfavoured by natural selection, outcompeted twofold by as.e.xual reproduction. In that sense the Bdelloidea, far from being a scandal, appear to be the only soldier in step. Here's why.

The problem is the one Maynard Smith dubbed the twofold cost of s.e.x. Darwinism, in its modern form, expects that individuals will strive to pa.s.s on as many of their genes as possible. So isn't it just daft to throw half your genes away with every egg or sperm you make, in order to mix the other half with the genes of somebody else? Wouldn't a mutant female who behaves like a bdelloid rotifer, and pa.s.ses on 100 per cent of her genes to every offspring instead of 50 per cent, do twice as well?

Maynard Smith added that the reasoning breaks down if the male partner works hard, or contributes economic goods, in such a way that a couple can rear twice as many offspring as an as.e.xual loner. In that case, the twofold cost of s.e.x is cancelled out by a doubling in the number of offspring. In a species such as an emperor penguin, with male and female parent contributing approximately equally towards the labour and other costs of childrearing, the twofold cost of s.e.x is abolished, or at least mitigated. In species where economic and labour contributions are unequal, it is nearly always the father who s.h.i.+rks, devoting his energies instead to duffing up other males. This magnifies the cost of s.e.x, up to the full twofold penalty of the original reasoning. This is why Maynard Smith's alternative name, the twofold cost of males, is preferable. In this light which Maynard Smith himself was largely responsible for s.h.i.+ning it isn't the bdelloid rotifers who are the evolutionary scandal but everything else. More pertinently, the male s.e.x is an evolutionary scandal. Except that it does exist and, indeed, is almost universal throughout the animal kingdom. What is going on? As Maynard Smith wrote, 'One is left with the feeling that some essential feature of the situation is being overlooked.'

The twofold cost is the starting point for ma.s.ses of theorising by Maynard Smith, Williams, Hamilton and many younger colleagues. The widespread existence of males who don't earn their keep as fathers must mean that there really are very substantial Darwinian benefits to s.e.xual recombination itself. It is not too difficult to think of what they might be in qualitative terms, and lots of possible benefits, some obvious, some esoteric, have been proposed. The problem is to think of a benefit of sufficient quant.i.tative magnitude magnitude to counteract the ma.s.sive twofold cost. to counteract the ma.s.sive twofold cost.

To do justice to all the theories would take a book it has already taken several, including the seminal works I have previously mentioned by Williams and Maynard Smith, and Graham Bell's beautifully written tour de force The Masterpiece of Nature The Masterpiece of Nature. Yet no definitive verdict has emerged. A nice book aimed at a non-specialist audience is Matt Ridley's The Red Queen The Red Queen. Though primarily favouring one of the theories on offer, W. D. Hamilton's theory that s.e.x serves an unceasing arms race against parasites, Ridley does not neglect to explain the problem itself and the other answers to it. As for me, I shall swiftly recommend Ridley's book and the others before going straight to the main purpose of this tale, which is to draw attention to an under-appreciated consequence consequence of the evolutionary invention of s.e.x. s.e.x brought into existence the gene pool, made meaningful the species, and changed the whole ball game of evolution itself. of the evolutionary invention of s.e.x. s.e.x brought into existence the gene pool, made meaningful the species, and changed the whole ball game of evolution itself.

Think what evolution must look like to a bdelloid rotifer. Think how different the evolutionary history of those 360 species must have been from the normal pattern of evolution. We portray s.e.x as raising diversity and so, in a sense, it does: that is the basis of most theories of how s.e.x overcomes its twofold cost. But, paradoxically, it also has a seemingly opposite effect. s.e.x normally acts as a kind of barrier to evolutionary divergence. Indeed, a special case of this was the basis of Mark Welch and Meselson's research. In a population of mice, say, any tendency to strike out in some enterprising new evolutionary direction is held in check by the swamping effect of s.e.xual mixing. The genes of the would-be enterprising diverger are swamped into conformity by the inertial ma.s.s of the rest of the gene pool. That is why geographical isolation is so important to speciation. It takes a mountain range or a difficult sea crossing to allow a newly striking-out lineage to evolve its own way without being dragged back to the inertial norm.

Think how different evolution must be for the bdelloid rotifers. Far from being swamped into normalcy by the gene pool, they don't even have have a gene pool. The very idea of a gene pool has no meaning if there is no s.e.x. a gene pool. The very idea of a gene pool has no meaning if there is no s.e.x.21 'Gene pool' is a persuasive metaphor because the genes of a s.e.xual population are being continually mixed and diffused, as if in a liquid. Bring in the time dimension, and the pool becomes a river, flowing through geological time an image that I developed in 'Gene pool' is a persuasive metaphor because the genes of a s.e.xual population are being continually mixed and diffused, as if in a liquid. Bring in the time dimension, and the pool becomes a river, flowing through geological time an image that I developed in River out of Eden River out of Eden. It is the binding effect of s.e.x that provides the river with its limiting banks, channelling the species into some kind of evolutionary direction. Without s.e.x, there would be no coherently channelled flow, but a shapeless outward diffusion: less like a river than like a smell, wafting out in all directions from some point of origin.

Natural selection presumably takes place among the bdelloids, but it must be a very different kind of natural selection from the one the rest of the animal kingdom is accustomed to. Where there is s.e.xual mixing of genes, the ent.i.ty that is carved into shape by natural selection is the gene pool. Good genes tend statistically to help the individual bodies in which they find themselves to survive. Bad genes tend to make them die. In s.e.xually reproducing animals, it is the deaths and reproductions of individual animals that const.i.tute the immediate selective events, but the long-term consequence is a change in the statistical profile of genes in the gene pool. So, it is the gene pool, as I say, that is the object of the Darwinian sculptor's attention.

Moreover, genes are favoured for their capacity to co-operate with other genes in building bodies. That is why bodies are such harmonious engines of survival. The right way to look at this, given s.e.x, is that genes are continually being tried out against different genetic backgrounds. In every generation, a gene is shuffled into a new team of companions, meaning the other genes with which it shares a body on any particular occasion. Genes that are habitually good companions, fitting in well with others and co-operating well with them, tend to be in winning teams meaning successful individual bodies that pa.s.s them on to offspring. Genes that are not good co-operators tend to make the teams in which they find themselves become losing teams meaning unsuccessful bodies that die before reproducing.

The proximal set of genes with which a gene has to co-operate are the ones with which it shares a body this body. But in the long term, the set of genes with which it has to co-operate are all the genes of the gene pool, for they are the ones that it repeatedly encounters as it hops from body to body down the generations. This is why I say it is the gene pool of a species that is the ent.i.ty sculpted into shape by the chisels of natural selection. Proximally, natural selection is the differential survival and reproduction of whole individuals the individuals that the gene pool throws up as samples of what it can do. Once again, none of this could be said of the bdelloid rotifers. Nothing like the sculpting of the gene pool goes on, for there is no gene pool to sculpt. A bdelloid rotifer has just one big gene.

What I have just called attention to is a consequence of s.e.x, not a theory for the benefit of s.e.x, nor a theory of why s.e.x arose in the first place. But if I ever were to attempt a theory of the benefit of s.e.x; if I were ever to essay a serious a.s.sault on the 'essential feature of the situation that is being overlooked', it is hereabouts that I would start. And I would listen again and again to the Rotifer's Tale. These tiny, obscure denizens of puddles and mossy moisture may hold the key to the outstanding paradox of evolution. What's wrong with as.e.xual reproduction, if the bdelloid rotifers have run with it for so long? Or, if it's right for them, why don't the rest of us do it and save the ma.s.sive twofold cost of s.e.x?

THE BARNACLE'S TALE When I was at boarding school, it was occasionally necessary to apologise to the housemaster for being late for dinner: 'Sorry I'm late, sir: orchestra practice,' or whatever the excuse might be. On those occasions when there really was no good excuse and we had something to hide, we formed the habit of murmuring, 'Sorry I'm late sir: barnacles.' He always nodded kindly, and I don't know whether he ever wondered what this mysterious out-of-school activity might be. It is possible that we were inspired by the example of Darwin, who devoted years of his life to barnacles so single-mindedly that his children were moved to ask, in innocent puzzlement after being shown round the house of some friends, 'Then where does [your father] do his barnacles?' I'm not sure that we knew the Darwin story then, and I suspect that we invented the excuse because there is something about barnacles that seems too implausible to be a bluff. Barnacles are not what they seem. That applies to other animals too. And it is the theme of the Barnacle's Tale.22 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 sh.e.l.l, they are distorted shrimps lying on their backs, kicking their legs in the air. Their feet bear feathery combs or baskets with which they filter particles of food out of the water. Goose barnacles do the same thing, but instead of sheltering under a conical sh.e.l.l like an acorn barnacle, they sit on the end of a stout stalk. They get their name from yet another misunderstanding of the true nature of barnacles. Their wet filtering 'feathers' give them the appearance of a baby bird in its egg. In the days when people believed in spontaneous generation, a folk belief grew that goose barnacles hatched into geese, specifically Branta leucopsis Branta leucopsis, the barnacle goose.

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 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 relations.h.i.+ps, it is a crustacean, and not just a crustacean but specifically a barnacle. Barnacles are indeed not what they seem. 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 relations.h.i.+ps, it is a crustacean, and not just a crustacean but specifically a barnacle. Barnacles are indeed not what they seem.

Fascinatingly, the embryological development of Sacculina Sacculina's extraordinarily uncrustacean-like body is starting to be understood in terms of the kind of Hox genes that were the subject of the Fruit Fly's Tale. The gene called Abdominal-A Abdominal-A, which normally supervises the development of a typical crustacean abdomen, is not expressed in Sacculina Sacculina. It looks as though you can turn a swimming, kicking, leggy animal into a shapeless fungoid just by suppressing Hox genes.

By the way, Sacculina Sacculina's branching root system is not indiscriminate in its invasion of the crab's tissues. It heads first for the crab's reproductive organs, which has the effect of castrating the crab. Is this just an accidental by-product? Probably not. Castration not only sterilises the crab. Like a fat bullock, the castrated crab, instead of concentrating on becoming a lean, mean, reproducing machine, diverts resources towards getting larger: more food for the parasite.23 [image]

Weird wonder? A whole new Bauplan? Female Female Thaumatoxena andreinii Thaumatoxena andreinii. Drawing by Henry Disney.

To lead into the final tale of this cl.u.s.ter, here's a little fable set in the future. Half a billion years after vertebrate and arthropod life completely perished in the mother of all comet collisions, intelligent life has eventually re-evolved in remote descendants of octopuses. Octopoid palaeontologists come upon a rich fossil bed dating from the twenty-first century AD AD. Not a fair cross-section of contemporary life, this bounteous shale nevertheless impresses the palaeontologists with its variety and diversity. Carefully weighing the fossils up with eight-arm balanced judgement, and expertly sucking the details, one octopodan scholar goes so far as to suggest that life, during this pre-catastrophe dawn age, was more extravagantly profligate in its diversity than it ever would be again, throwing up weird and wonderful new body plans in gleeful experimentation. You can see what he means by thinking of your own animal contemporaries and imagining that a small sampling of them fossilises. Think of the herculean task facing our future palaeontologist, and empathise with his difficulties in trying to discern their affinities from imperfect and sporadic fossil traces.

Just to take one example, how on earth would you cla.s.sify the animal above? Evidently a new 'weird wonder', probably deserving to have a previously unnamed phylum coined in its honour? A whole new Bauplan, hitherto unknown to zoology?

Well, no. To return from futuristic fantasy to the present, this weird wonder is actually a fly, Thaumatoxena andreinii Thaumatoxena andreinii. Not only that, it is a fly that belongs to the perfectly respectable family Phoridae. A more typical member of the Phoridae is pictured above, Megaselia scalaris Megaselia scalaris.

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How a fly ought to be? Phorid fly, Phorid fly, Megaselia scalaris Megaselia scalaris (Loew). Drawing by Arthur Smith. (Loew). Drawing by Arthur Smith.

What happened to Thaumatoxena Thaumatoxena, the 'weird wonder', is that it took up residence in a termite nest. The demands of life in that claustral world are so different that probably in rather a short time it lost all resemblance to a fly. The boomerang-shaped front end is what is left of the head. Then comes the thorax, and you can see the remains of the wings tucked in between the thorax and the abdomen, which is the hairy bit at the back.

The moral is that of the barnacle again. But the parable of the palaeontologist of the future, and his seduction by the rhetoric of weird wonders gleefully carousing in morphos.p.a.ce, was not idly spun. It was intended as a softening-up for the next tale, which is all about the 'Cambrian Explosion'.

THE VELVET WORM'S TALE If modern zoology admits of anything approaching a full-blown origin myth, it is the Cambrian Explosion. The Cambrian is the first period of the Phanerozoic Eon, the last 545 million years, during which animal and plant life as we know it suddenly became manifest in fossils. Before the Cambrian, fossils were either tiny traces or enigmatic mysteries. From the Cambrian onwards, there has been a clamorous menagerie of multicellular life, more or less plausibly presaging our own. It is the suddenness with which multicellular fossils appear at the base of the Cambrian that prompts the metaphor of explosion.

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.24 At the other extreme, romantically overheated zoologists love the Cambrian Explosion for its aura of Arcadian Dreamtime, a zoological age of innocence in which life danced to a frenzied and radically different evolutionary tempo: a prelapsarian baccha.n.a.lia of leaping improvisation before a fall into the earnest utilitarianism that has prevailed since. In At the other extreme, romantically overheated zoologists love the Cambrian Explosion for its aura of Arcadian Dreamtime, a zoological age of innocence in which life danced to a frenzied and radically different evolutionary tempo: a prelapsarian baccha.n.a.lia of leaping improvisation before a fall into the earnest utilitarianism that has prevailed since. In Unweaving the Rainbow Unweaving the Rainbow I quoted the following words of a distinguished biologist who may, by now, have thought better of it: I quoted the following words of a distinguished biologist who may, by now, have thought better of it: Soon after multicelled forms were invented, a grand burst of evolutionary novelty thrust itself outward. One almost gets the sense of multicellular life gleefully trying out all its possible ramifications, in a kind of wild dance of heedless exploration.

If there is one animal, more than any other, that stands for this feverish vision of the Cambrian, it is Hallucigenia Hallucigenia. Stands? Hallucinations apart, you might suspect that such an unlikely creature never stood in its life. And you would be right. It seems that Hallucigenia Hallucigenia and Simon Conway Morris chose its name advisedly was originally reconstructed upside down. That is why it stands on improbably spiky toothpick stilts. The single row of 'tentacles' along the back were legs, according to the more recent, inverted interpretation. A single row of legs did it balance as if on a tightrope? No, new fossils discovered in China suggest a second row, and modern reconstructions look as though they might just have been at home in the real world (see opposite page). and Simon Conway Morris chose its name advisedly was originally reconstructed upside down. That is why it stands on improbably spiky toothpick stilts. The single row of 'tentacles' along the back were legs, according to the more recent, inverted interpretation. A single row of legs did it balance as if on a tightrope? No, new fossils discovered in China suggest a second row, and modern reconstructions look as though they might just have been at home in the real world (see opposite page). Hallucigenia Hallucigenia is no longer cla.s.sified as a 'weird wonder' of uncertain and probably long-vanished affinities. Instead, together with many other Cambrian fossils, it is now tentatively placed in the phylum Lobopodia, which has modern representatives in the form of is no longer cla.s.sified as a 'weird wonder' of uncertain and probably long-vanished affinities. Instead, together with many other Cambrian fossils, it is now tentatively placed in the phylum Lobopodia, which has modern representatives in the form of Peripatus Peripatus and the other 'onychophorans' or 'velvet worms' whom we met at and the other 'onychophorans' or 'velvet worms' whom we met at Rendezvous 26 Rendezvous 26.

In the days when annelid worms were thought to be close relatives of arthropods, the Onychophora were often touted as 'intermediate' 'bridging the gap' between them, although that is not an entirely helpful concept if you think carefully about how evolution works. The annelids are now placed in the Lophotrochozoa, while the Onychophora are ecdysozoans with the arthropods. Peripatus Peripatus, with its ancient affinities, is well placed among modern pilgrims to tell the tale of the Cambrian Explosion.

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Hallucigenia - modern reconstruction. - modern reconstruction.

The modern Onychophora (see plate 36) (see plate 36) are widely distributed in the tropics and especially in the southern hemisphere. are widely distributed in the tropics and especially in the southern hemisphere. Peripatus, Peripatopsis Peripatus, Peripatopsis, and all the modern onychophorans live on land, in leaf litter and humid places, where they hunt snails, worms, insects and other small prey. In the Cambrian, of course, Hallucigenia Hallucigenia and the remote forebears of and the remote forebears of Peripatus Peripatus and and Peripatopsis Peripatopsis lived along with everybody else in the sea. lived along with everybody else in the sea.

Hallucigenia's connection with the modern Onychophora is still controversial, and we must remember what a lot of imagination necessarily intervenes between a blurred and squashed fossil in a rock, and the reconstruction that is eventually drawn, often in daring colour, on the page. It has even been suggested that Hallucigenia Hallucigenia might not be a whole animal at all, but a part of some unknown animal. It would not be the first time such a mistake had been made. Some early artists' reconstructions of Cambrian scenes included a swimming jellyfish-like creature, seemingly inspired by tinned pineapple rings, which turned out to be part of the jaw apparatus of the mysterious predatory animal might not be a whole animal at all, but a part of some unknown animal. It would not be the first time such a mistake had been made. Some early artists' reconstructions of Cambrian scenes included a swimming jellyfish-like creature, seemingly inspired by tinned pineapple rings, which turned out to be part of the jaw apparatus of the mysterious predatory animal Anomalocaris Anomalocaris (see page 452). Other Cambrian fossils, for example (see page 452). Other Cambrian fossils, for example Aysheaia Aysheaia, certainly seem quite like marine versions of Peripatus Peripatus, and this reinforces Peripatus Peripatus's ent.i.tlement to tell this Cambrian tale.

Most fossils, in any era, are the remains of hard parts of animals: vertebrate bones, the carapaces of arthropods or the sh.e.l.ls of molluscs or brachiopods. But there are three Cambrian fossil beds one in Canada, one in Greenland and one in China where freak conditions, with almost miraculous good fortune for us, preserved soft parts as well. These are the Burgess Shale of British Columbia, Sirius Pa.s.set of northern Greenland, and the Chengjiang site of southern China.25 The Burgess Shale was first discovered in 1909 and was made famous 80 years later by Stephen Gould in The Burgess Shale was first discovered in 1909 and was made famous 80 years later by Stephen Gould in Wonderful Life Wonderful Life. The Sirius Pa.s.set site in northern Greenland was discovered in 1984 but is so far less studied than the other two. In the same year, the Chengjiang fossils were discovered by Hou Xian-guang. Dr Hou is one of those who have collaborated on a beautifully ill.u.s.trated monograph, The Cambrian Fossils of Chengjiang, China The Cambrian Fossils of Chengjiang, China, published in 2004 fortunately for me just before this book went to press.

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Anomalocaris saron.

The Chengjiang fossils are now dated at 525 million years old. That's roughly contemporary with Sirius Pa.s.set, and some 10 or 15 million years older than the Burgess Shale, but these outstanding fossil sites have a similar fauna. There are lots of lobopods, many looking more or less like marine versions of Peripatus Peripatus. There are algae, sponges, worms of various kinds, brachiopods looking pretty much like modern ones, and enigmatic animals of uncertain kins.h.i.+p. There are large numbers of arthropods, including crustaceans, trilobites and lots of others that loosely resemble crustaceans or trilobites but may have belonged in their own rather separate groups. The large (over a metre in some cases), apparently predatory Anomalocaris Anomalocaris and its kind are found in Chengjiang as well as the Burgess Shale. n.o.body is quite sure what they were probably distant relations of the arthropods but they must have been spectacular. Not all the 'weird wonders' of the Burgess Shale have been found at Chengjiang, for example and its kind are found in Chengjiang as well as the Burgess Shale. n.o.body is quite sure what they were probably distant relations of the arthropods but they must have been spectacular. Not all the 'weird wonders' of the Burgess Shale have been found at Chengjiang, for example Opabinia Opabinia, with its famous five eyes.

The Sirius Pa.s.set fauna from Greenland includes a beautiful creature called Halkieria Halkieria. It has been thought to be an early mollusc but Simon Conway Morris, who has described many of the strange creatures of the Cambrian, believes it has affinities with three major phyla: molluscs, brachiopods and annelid worms. This gladdens my heart because it helps to break down the almost mystical reverence with which zoologists regard the great phyla (see plate 37) (see plate 37). If we take our evolution seriously, it has to be the case that, as we go back in time and approach their rendezvous points, they will become more and more like each other, more and more closely related. Whether or not Halkieria Halkieria fits the bill, it would be worrying if there were fits the bill, it would be worrying if there were not not an ancient animal that united annelids, brachiopods and molluscs. Note the sh.e.l.ls, one at each end, in the ill.u.s.trations in plate 37. an ancient animal that united annelids, brachiopods and molluscs. Note the sh.e.l.ls, one at each end, in the ill.u.s.trations in plate 37.

As we saw at Rendezvous 22 Rendezvous 22, Chengjiang has fossils that appear to be true vertebrates, pre-dating the amphioxus-like Pikaia Pikaia of the Burgess Shale and other Cambrian chordates. Traditional zoological wisdom never had vertebrates arising so early. Yet of the Burgess Shale and other Cambrian chordates. Traditional zoological wisdom never had vertebrates arising so early. Yet Myllokunmingia Myllokunmingia, of which more than 500 specimens have now been discovered at Chengjiang, looks pretty much like a good jawless fish, such as had previously been thought not to arise until 50 million years later in the middle of the Ordovician. At first, two new genera were described Myllokunmingia Myllokunmingia, which was described as relatively close to the lampreys, and Haikouichthys Haikouichthys (alas, not named after the j.a.panese verse form), which was believed to have hagfish affinities. Some revisionist taxonomists now place the two in one species, (alas, not named after the j.a.panese verse form), which was believed to have hagfish affinities. Some revisionist taxonomists now place the two in one species, Myllokunmingia fengjiaoa Myllokunmingia fengjiaoa. This controversial updating of the status of Haikouichthys Haikouichthys is eloquent of how difficult it is to discern the details of very old fossils. On the following page is a photograph of an individual is eloquent of how difficult it is to discern the details of very old fossils. On the following page is a photograph of an individual Myllokunmingia Myllokunmingia fossil, together with a drawing of it made with a camera lucida. I find myself filled with admiration for the patience that goes into reconstructing ancient animals like these. fossil, together with a drawing of it made with a camera lucida. I find myself filled with admiration for the patience that goes into reconstructing ancient animals like these.

The pus.h.i.+ng of the vertebrates back into the middle of the Cambrian only strengthens the idea of sudden explosion that is the basis of the myth. It really does appear that most of today's major animal phyla first appear as fossils in a narrow span within the Cambrian. This doesn't mean that there were no representatives of those phyla before the Cambrian. But they have mostly not fossilised. How should we interpret this? We can distinguish various combinations of three main hypotheses, rather like the three hypotheses for the explosion of the mammals after the extinction of the dinosaurs.

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Vertebrates were not supposed to be this old. Fossil Fossil Myllokunmingia fengjiaoa Myllokunmingia fengjiaoa, Chengjiang. From D-G Shu et al et al. [264].

1. NO REAL EXPLOSION NO REAL EXPLOSION. On this view there was only an explosion of fossilisability, not of actual evolution. The phyla actually go back a long way before the Cambrian, with concestors spread out through hundreds of millions of years in the Precambrian. This view is supported by some molecular biologists who have used molecular clock techniques to date key concestors. For example, G. A. Wray, J. S. Levinton and L. H. Shapiro, in a famous paper of 1996, estimated that the concestor uniting vertebrates and echinoderms lived about a billion years ago, and the concestor uniting vertebrates and molluscs was 200 million years earlier still, more than twice the age of the so-called Cambrian Explosion. Molecular clock estimates have in general tended to push these deep branchings way back into the Precambrian, far further than most palaeontologists are happy with. On this view, fossils were, for unknown reasons, not readily formed before the Cambrian. Perhaps they lacked readily fossilisable hard parts, such as sh.e.l.ls, carapaces and bones. After all, the Burgess Shale and the Chengjiang beds are extremely unusual, among all geological layers, in recording soft parts as fossils. Perhaps Precambrian animals, although long existing in a wide range of complex body plans, were simply too small to fossilise. In favour of this idea, there are some small animal phyla that have left no fossils at all after the Cambrian, until they appear today as live 'orphans'. Why then should we feel ent.i.tled to expect fossils before the Cambrian? In any case, some of the Precambrian fossils that have been found, including the Ediacaran fauna (see page 460) and trace fossils of tracks and burrows, indicate the presence of real Precambrian metazoans.

2. MEDIUM-FUSE EXPLOSION. MEDIUM-FUSE EXPLOSION. The concestors uniting the various phyla really did live reasonably close to each other in time, but still spread out over several tens of millions of years before the observed explosion of fossils. From the great distance of the present, Chengjiang at 525 million years seems at first sight rather close to a putative concestor at, say, 590 million. But a full 65 million years separates them, which is the same time as has elapsed today since the death of the dinosaurs the entire time during which modern mammals have radiated and radiated again to produce the spectacularly diverse ranges that we see today. Even 10 million years is a long time in the light of the extremely rapid evolutionary bursts of the Galapagos Finch's Tale and the Cichlid's Tale. It is all too easy, with hindsight, to think that because we recognise two ancient fossils as belonging to different modern phyla, those two fossils must have been as different from each other as modern representatives of the two phyla are. It is too easy to forget that the modern representatives have had half a billion years in which to diverge. There is no good reason to believe that a Cambrian taxonomist, blessedly free of 500 million years' worth of zoological hindsight, would have placed the two fossils in separate phyla. He might have placed them only in separate orders, notwithstanding the then-unknowable fact that their descendants were destined eventually to diverge so far as to warrant separate phylum status. The concestors uniting the various phyla really did live reasonably close to each other in time, but still spread out over several tens of millions of years before the observed explosion of fossils. From the great distance of the present, Chengjiang at 525 million years seems at first sight rather close to a putative concestor at, say, 590 million. But a full 65 million years separates them, which is the same time as has elapsed today since the death of the dinosaurs the entire time during which modern mammals have radiated and radiated again to produce the spectacularly diverse ranges that we see today. Even 10 million years is a long time in the light of the extremely rapid evolutionary bursts of the Galapagos Finch's Tale and the Cichlid's Tale. It is all too easy, with hindsight, to think that because we recognise two ancient fossils as belonging to different modern phyla, those two fossils must have been as different from each other as modern representatives of the two phyla are. It is too easy to forget that the modern representatives have had half a billion years in which to diverge. There is no good reason to believe that a Cambrian taxonomist, blessedly free of 500 million years' worth of zoological hindsight, would have placed the two fossils in separate phyla. He might have placed them only in separate orders, notwithstanding the then-unknowable fact that their descendants were destined eventually to diverge so far as to warrant separate phylum status.

3. OVERNIGHT EXPLOSION. OVERNIGHT EXPLOSION. This third school of thought is, in my opinion, bonkers. Or, to use more parliamentary language, wildly and irresponsibly unrealistic. But I must spend some time on it because it has recently become unaccountably popular, following the rhetoric I attributed to romantically overheated zoologists. This third school of thought is, in my opinion, bonkers. Or, to use more parliamentary language, wildly and irresponsibly unrealistic. But I must spend some time on it because it has recently become unaccountably popular, following the rhetoric I attributed to romantically overheated zoologists.

The third school believes that new phyla sprang into existence overnight, in a single macromutational leap. Here are some quotations I have used before, in Unweaving the Rainbow Unweaving the Rainbow, from otherwise reputable scientists.

It was as if the facility for making evolutionary leaps that produced major functional novelties the basis of new phyla had somehow been lost when the Cambrian period came to an end. It was as if the mainspring of evolution had lost some of its power ... Hence, evolution in Cambrian organisms could take bigger leaps, including phylum-level leaps, while later on it would be more constrained, making only modest jumps, up to the cla.s.s level.

Or this, from the same distinguished scientist from whom we heard at the beginning of the tale.

Early on in the branching process, we find a variety of long-jump mutations that differ from the stem and from one another quite dramatically. These species have sufficient morphological differences to be categorized as founders of distinct phyla. These founders also branch, but do so via slightly closer long-jump variants, yielding branches from each founder of a phylum to dissimilar daughter species, the founders of cla.s.ses. As the process continues, fitter variants are found in progressively more nearby neighborhoods, so founders of orders, families, and genera emerge in succession.

Those preposterous quotations moved me to retort that it is as though a gardener looked at an old oak tree and remarked, wonderingly: Isn't it strange that no major new boughs have appeared on this tree for many years? These days, all the new growth appears to be at the twig level!

Here's another quotation, which this time I will attribute because it was published after Unweaving the Rainbow Unweaving the Rainbow and I have therefore not used it before. Andrew Parker's and I have therefore not used it before. Andrew Parker's In the Blink of an Eye In the Blink of an Eye is mainly concerned with advocating his interesting and original theory that the Cambrian Explosion was triggered by animals' sudden discovery of eyes. But before coming to his theory itself, Parker begins by falling hook, line and sinker for the 'wild and irresponsible' version of the Cambrian Explosion myth. He first expresses the myth itself in the most frankly 'explosive' version I have read: is mainly concerned with advocating his interesting and original theory that the Cambrian Explosion was triggered by animals' sudden discovery of eyes. But before coming to his theory itself, Parker begins by falling hook, line and sinker for the 'wild and irresponsible' version of the Cambrian Explosion myth. He first expresses the myth itself in the most frankly 'explosive' version I have read: 544 million years ago there were indeed three animal phyla with their variety of external forms, but at 538 million years ago there were thirtyeight, the same number that exists today.

He goes on to make it clear that he is not talking about extremely rapid gradualistic evolution compressed into a period of 6 million years, which would be an extreme version of our Hypothesis Two, and just barely acceptable. Nor is he saying, as I would, that near the initial divergence of a pair of (what are destined to become) phyla, they would not have been very different would, indeed, have pa.s.sed through successive stages of being a pair of species, then genera, and so on until eventually their separation warranted recognition at the phylum level. No, Parker gives every appearance of regarding his 38 phyla, at 538 million years, as fully fledged phyla that sprang into existence overnight, at the drop of a macromutational hat: Thirty-eight animal phyla have evolved on Earth. So only thirty-eight monumental genetic events have taken place, resulting in thirty-eight different internal organisations.

Monumental genetic events are not utterly out of the question. Control genes of the various Hox families that we met in the Fruit Fly's Tale can certainly mutate in dramatic ways. But there's monumental and monumental. A fruit fly with a pair of legs where the antennae should be is about as monumental as it gets, and even then there is a big question mark over survival. There is a powerful general reason for this, which I shall briefly explain.

A mutant animal has a certain probability of being better off as a consequence of its new mutation. 'Better off' means better when compared to the premutated parental type. The parent must have been at least good enough to survive and reproduce, otherwise it wouldn't be a parent. It is easy to see that the smaller the mutation, the more likely it is to be an improvement. 'It is easy to see' was a favourite phrase of the great statistician and biologist R. A. Fisher, and he sometimes used it when it was anything but easy for ordinary mortals to see. In this particular case, however, I think it is genuinely easy to follow Fisher's argument for the case of a simple metric feature something such as thigh length, which varies in one dimension: some number of millimetres that could grow larger or could grow smaller.

Imagine a set of mutations of increasing magnitude. At one extreme, a mutation of zero magnitude is by definition exactly as good as the parent's copy of the gene which, as we've seen, must have been at least good enough to survive childhood and reproduce. Now imagine a random mutation of small magnitude: the leg, say, gets one millimetre longer or one millimetre shorter. a.s.suming that the parental gene is not perfect, a mutation that is infinitesimally different from the parental version has a 50 per cent chance of being better and a 50 per cent chance of being worse: it'll be better if it is a step in the right direction, worse if it is a step in the opposite direction, relative to the parental condition. But a very large mutation will probably be worse than the parental version, even if it is a step in the right direction even if it is a step in the right direction, because it will overshoot. To push to the extreme, imagine an otherwise normal man with thighs two metres long.

Fisher's argument was more general than this. When we are talking about macromutational leaps into new phylum territory, we are no longer dealing with simple metric characters like leg length, and we need another version of the argument. The essential point, as I have put it before, is that there are many more ways of being dead than of being alive. Imagine a mathematical landscape of all possible animals. I have to call it mathematical, because it is a landscape in hundreds of dimensions and it includes an almost infinitely large range of conceivable monstrosities, as well as the (relatively) small number of animals that have actually ever lived. What Parker calls a 'monumental genetic event' would be equivalent to a macromutation of huge effect, not just in one dimension as with our thigh example, but in hundreds of dimensions simultaneously. That is the scale of change we are talking about if we imagine, as Parker seems to, an abrupt and immediate change from one phylum to another.

In the multidimensional landscape of all possible animals, living creatures are islands of viability separated from other islands by gigantic oceans of grotesque deformity. Starting from any one island, you can evolve away from it one step at a time, here inching out a leg, there shaving the tip of a horn, or darkening a feather. Evolution is a trajectory through multidimensional s.p.a.ce, in which every step of the way has to represent a body capable of surviving and reproducing about as well as the parental type reached by the preceding step of the trajectory. Given enough time, a sufficiently long trajectory leads from a viable starting point to a viable destination so remote that we recognise it as a different phylum, say, molluscs. And a different step-by-step trajectory from the same starting point can lead, through continuously viable intermediates, to another viable destination, which we recognise as yet another phylum, say, annelids. Something like this must have happened for each of the forks leading to each pair of animal phyla from their respective concestors.

The point we are leading up to is this. A random change of sufficient magnitude to initiate a new phylum at one fell swoop will be so large, in hundreds of dimensions simultaneously, that it would have to be preposterously lucky to land on another island of viability. Almost inevitably, a megamutation of that magnitude will land in the middle of the ocean of inviability: probably unrecognisable as an animal at all.

Creationists foolishly liken Darwinian natural selection to a hurricane blowing through a junkyard and having the luck to a.s.semble a Boeing 747. They are wrong, of course, for they completely miss the gradual, c.u.mulative 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.

We can, then, with complete confidence, reject the third of our three hypotheses, the bonkers one. That leaves the other two, or some compromise between them, and here I find myself agnostic and eager for more data. As we shall see in the epilogue to this tale, it seems to be increasingly accepted that the early molecular clock estimates were exaggerating when they pushed the major branch points hundreds of millions of years back into the Precambrian. On the other hand, the mere fact that there are few, if any, fossils of most animal phyla before the Cambrian should not stampede us into a.s.suming that those phyla must have evolved extremely rapidly. The hurricane in a junkyard argument tells us that all those Cambrian fossils must have had continuously evolving antecedents. Those antecedents had to be there, but they have not been discovered. Whatever the reason, and whatever the timescale, they failed to fossilise, but they must have been there. On the face of it, it is harder to believe that a whole lot of animals could be invisible for 100 million years than that they could be invisible for only 10 million years. This leads some people to prefer the short-fuse Cambrian Explosion theory. On the other hand, the shorter you make the fuse, the harder it is to believe all that diversification could be crammed into the time available. So this argument cuts both ways and doesn't decisively choose between our two surviving hypotheses.

The fossil record is not completely void of metazoan life before Chengjiang and Sirius Pa.s.set. Around 20 million years earlier, almost plumb on the Cambrian/Precambrian boundary, start to appear a variety of microscopic fossils that look rather like tiny sh.e.l.ls together they are known as the 'small sh.e.l.ly fauna'. It came as a surprise to most palaeontologists when some of these were identified as armour plating from lobopods relatives of the velvet worm. That means that the divergences between different groups of protostomes must must have occurred in the Precambrian, before the visible 'explosion'. have occurred in the Precambrian, before the visible 'explosion'.

And there are hints of older animal diversity. Twenty million years before the start of the Cambrian, in the Ediacaran Period of the late Precambrian, there was a worldwide flouris.h.i.+ng of a mysterious group of animals called the Ediacaran fauna, named after the Ediacara Hills in South Australia where they were first found. It is hard to know quite what most of them were, but they were among the first large animals to be fossilised. Some of them are probably sponges. Some are a bit like jelly-fish. Others somewhat resemble sea anemones, or sea pens (feather-like relatives of sea anemones). Some look a bit worm-like or slug-like, and could conceivably represent true Bilateria. Others are just plain mysterious. What are we to make of this creature d.i.c.kinsonia d.i.c.kinsonia? (See plate 38) (See plate 38). Is it a coral? Or a worm? Or a fungus? Or something completely different from anything that survives today? There is even one tadpole-like fossil from Australia, still not formally described, that is suspected of being a chordate (that's the phylum, remember, to which the vertebrates belong). If this turns out to be right, it would be very exciting, but we must wait and see. In spite of such tantalising straws in the wind, the consensus among zoologists is that the Ediacaran fauna, though intriguing, doesn't help us much one way or the other in tracing the ancestry of most modern animals.

There are also fossil imprints that appear to be the trails or burrows of Precambrian animals. These traces tell us of the early existence of crawling animals large enough to make them. Unfortunately, they don't tell us much about what those animals looked like. There are also some even older, mostly microscopic fossils found at Doushantuo in China which appear to be embryos, though it is not clear what kind of animal they might have grown into. Older still are small, disc-shaped impressions from northwest Canada, dated between about 600 and 610 million years ago, but these animals are, if anything, even more enigmatic than the Ediacaran forms.

This book is hung upon a series of 39 rendezvous points and it seemed desirable to make some sort of guess as to the date of each one. Most of the rendezvous points can now be dated with some confidence, using a combination of datable fossils and molecular clocks calibrated by datable fossils. Not surprisingly, the fossils start to let us down when we reach the older rendezvous points. This means that the molecular methods can no longer be reliably calibrated, and we enter a wilderness of undatability. For completeness I have forced myself to put some sort of date on these wilderness concestors, roughly Concestors 23 to 39. The most recently available evidence seems to me to favour, even if only slightly, a view closer to a medium-fuse explosion. This goes against my earlier bias in favour of no real explosion. When more evidence comes in, as I hope it will, I shall not be in the least surprised if we find ourselves pushed the other way again into the deep Precambrian in our quest for the concestors of modern animal phyla. Or we might be pulled back to an impressively short explosion, in which the concestors of most of the great animal phyla are compressed into a period of 20 or even 10 million years around the beginning of the Cambrian. In this case, my strong expectation would be that even if we correctly place two Cambrian animals in different phyla on the basis of their resemblance to modern animals, back in the Cambrian they would have been much closer to each other than the modern descendants of one are to the modern descendants of the other. Cambrian zoologists would not have placed them in separate phyla but only in, say, separate subcla.s.ses.

I wouldn't be surprised to see either of the first two hypotheses vindicated. I'm not sticking my neck out. But I'll eat my hat if any evidence is ever found in favour of Hypothesis Three. There is every reason to suppose that evolution in the Cambrian was essentially the same kind of process as evolution today. All that over-excited rhetoric about the mainspring of evolution running down after the Cambrian; all that euphoric shouting about wild, heedless dances of extravagant invention, with new phyla leaping into existence in a blissful dawn of zoological irresponsibility now here's something I am prepared to stick my neck out for: all that stuff is just plain dotty.

I hasten to say I have nothing against prose-poetry on the Cambrian. But give me Richard Fortey's version, on page 120 of his beautiful book Life: An Unauthorised Biography: Life: An Unauthorised Biography: I can imagine standing upon a Cambrian sh.o.r.e in the evening, much as I stood on the sh.o.r.e at Spitsbergen and wondered about the biography of life for the first time. The sea lapping at my feet would look and feel much the same. Where the sea meets the land there is a patch of slightly sticky, rounded stromatolite pillows, survivors from the vast groves of the Precambrian. The wind is whistling across the red plains behind me, where nothing visible lives, and I can feel the sharp sting of wind-blown sand on the back of my legs. But in the muddy sand at my feet I can see worm casts, little curled wiggles that look familiar. I can see trails of dimpled impressions left by the scuttling of crustacean-like animals ... Apart from the whistle of the breeze and the crash and suck of the breakers, it is completely silent, and nothing cries in the wind ...

EPILOGUE TO THE VELVET WORM'S TALE Written with Yan Wong For much of this book I have tossed rendezvous dates around with insouciance, and even been rash enough, when introducing many of the concestors, to stick a specific number of 'greats' before 'grandparent'. My dates have mostly been based upon fossils which, as we shall see in the Redwood's Tale, can be dated to a precision commensurate with the vast timescales involved. But fossils never helped us much with tracing the ancestry of soft-bodied animals such as flatworms. Coelacanths went missing from the record for the past 70 million years, which was why the discovery of a live one in 1938 was such an exhilarating surprise. The fossil record, even at the best of times, can be a fickle witness. And now, having reached the Cambrian Period, we are sadly running out of fossils. Whatever interpretation we place on 'explosion', everyone agrees that almost all the predecessors of the great Cambrian fauna have, for uncertain reasons, failed to fossilise. As we seek concestors that predate the Cambrian, we find no more help in the rocks. Fortunately, fossils are not our only recourse. In the Elephant Bird's Tale, the Lungfish's Tale and other places, we have made use of the ingenious technique known as the molecular clock. The time has come to explain the molecular clock properly.

Wouldn't it be wonderful if measurable, or countable, evolutionary changes happened at a fixed rate? We could then use evolution itself as its own clock. And this needn't involve circular reasoning because we could calibrate the evolutionary clock on parts of evolution where the fossil record is good, then extrapolate to parts where it isn't. But how do we measure rates of evolution? And, even if we could measure them, why on earth should we expect that any aspect of evolutionary change should go at a fixed rate like a clock?

There is not the slightest hope that leg length, or brain size, or number of whiskers will evolve at a fixed rate. Such features are important for survival, and their rates of evolution will surely be hideously inconstant. As clocks they are doomed by the very principles of their own evolution. In any case, it is hard to imagine an agreed standard for measuring rates of visible evolution. Do you measure evolution of leg length in millimetres per million years, as percentage change per million years, or what? J. B. S. Haldane proposed a unit of evolutionary rate, the darwin, which is based upon proportional change per generation. Wherever it has been used on real fossils, results vary from millidarwins to kilo-darwins and megadarwins, and n.o.body is surprised.

Molecular change looks like a much more promising clock. First, because it is obvious what you must measure. Since DNA is textual information written in a four-letter alphabet, there is an entirely natural way to measure its rate of evolution. You just count letter differences. Or, if you prefer, you can go to the protein products of the DNA code and count subst.i.tutions of amino acids.26 There are reasons to hope that the majority of evolutionary change at the molecular level is neutral rather than being steered by natural selection. Neutral is not the same as useless or functionless it only means that different versions of the gene are equally good, therefore change from one to the other is not noticed by natural selection. This is good for the clock. There are reasons to hope that the majority of evolutionary change at the molecular level is neutral rather than being steered by natural selection. Neutral is not the same as useless or functionless it only means that different versions of the gene are equally good, therefore change from one to the other is not noticed by natural selection. This is good for the clock.

Contrary to my rather ludicrous reputation as an 'ultra-Darwinist' (a slander I would protest more vigorously if the name sounded less of a compliment than it does), I do not think that the majority of evolutionary change at the molecular level is favoured by natural selection. On the contrary, I have always had a lot of time for the socalled neutral theory a.s.sociated with the great j.a.panese geneticist Motoo Kimura, or its extension, the 'nearly neutral' theory of his collaborator Tomoko Ohta. The real world has no interest in human tastes, of course, but as it happens I positively want want such theories to be true. This is because they give us a separate, independent chronicle of evolution, unlinked to the visible features of the creatures around us, and they hold out the hope that some kind of molecular clock might really work. such theories to be true. This is because they give us a separate, independent chronicle of evolution, unlinked to the visible features of the creatures around us, and they hold out the hope that some kind of molecular clock might really work.

Just in case the point is misunderstood, I must emphasise that the neutral theory does not in any way denigrate the importance of selection in nature. Natural selection is all-powerful with respect to those visible changes that affect survival and reproduction. Natural selection is the only explanation we know for the functional beauty and apparently 'designed' complexity of living things. But if there are any changes that have no visible effect changes that pa.s.s right under natural selection's radar they can acc.u.mulate in the gene pool with impunity and may supply just what we need for an evolutionary clock.

As ever, Charles Darwin was way ahead of his time with respect to neutral changes. In the first edition of The Origin of Species The Origin of Species, near the beginning of Chapter 4, he wrote: This preservation of favourable variations and the rejection of injurious variations, I call natural selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic.

By the sixth and last edition, the second sentence had an even more modern-sounding addendum: ... as perhaps we see in certain polymorphic species, or would ultimately become fixed ...

'Fixed' is a genetic technical term and Darwin surely cannot have meant it in quite the modern sense, but it gives me a lovely lead-in to the next point. A new mutation, whose frequency in the population begins near zero by definition, is said to become 'fixed' when it has reached 100 per cent in the population. The rate of evolution that we seek to measure, for purposes of a molecular clock, is the rate at which a succession of mutations of the same genetic locus become fixed in the population. The obvious way for fixation to happen is if natural selection favours the new mutation over the previous 'wild type' allele, and therefore drives it to fixation it becomes the norm, 'the one to beat'. But a new mutation can also go to fixation even if it is exactly as good as its predecessor true neutrality. This is nothing to do with selection: it happens by sheer chance. You can simulate the process by tossing pennies, and can calculate the rate at which it will happen. Once a neutral mutation has drifted to 100 per cent, it will become the norm, the so-called 'wild type' at that locus, until another mutation has the luck to drift to fixation.

If there is a strong component of neutrality, we could potentially have a marvellous clock. Kimura himself wasn't particularly concerned with the molecular clock idea. But he believed it now seems rightly that the majority of mutations in DNA are indeed neutral 'neither useful nor injurious'. And, in a remarkably neat and simple piece of algebra, which I shall not spell out here, he calculated that, if this is true, the rate at which genuinely neutral genes should 'ultimately become fixed' is exactly equal to the rate at which the variations are generated in the first place: the mutation rate.

You see how perfect this is for anybody who wants to date bifurcation ('rendezvous') points using a molecular clock. As long as the mutation rate at a neutral genetic locus remains constant over time, the fixation rate will also be constant. You can now compare the same gene in two different animals, say a pangolin and a starfish, whose most recent common ancestor was Concestor 25. Count the number of letters by which the starfish gene differs from the pangolin gene. a.s.sume that half the differences acc.u.mulated in the line leading from concestor to starfish, and the other half in the line leading from concestor to pangolin. That gives you the number of ticks of the clock since Rendezvous 25 Rendezvous 25.

But it isn't as simple as that, and the complications are interesting. First, if you listened to the ticking of the molecular clock, it would not be regular like a pendulum clock or a hairspring watch; it would sound like a Geiger counter near a radioactive source. Completely random! Each tick is the fixation of yet another mutation. Under the neutral theory, the interval between successive ticks could be long or it could be short, by chance 'genetic drift'. In a Geiger counter, the timing of the next tick is unpredictable. But and this is really important the average average interval over a large number of ticks is highly predictable. The hope is that the molecular clock is predictable in the same way as a Geiger counter, and in general this is true. interval over a large number of ticks is highly predictable. The hope is that the molecular clock is predictable in the same way as a Geiger counter, and in general this is true.

Second, the tick rate varies from gene to gene within a genome. This was noticed early, when geneticists could look only at the protein products of DNA, not DNA itself. Cytochrome-c evolves at its own characteristic rate, which is faster than histones but slower than globins, which in turn are slower than fibrinopeptides. In the same way, when a Geiger counter is exposed to a very slightly radioactive source such as a lump of granite, versus a highly radioactive source such as a lump of radium, the timing of the next tick is always unpredictable but the average rate of ticking is predictably and dramatically different as you move from granite to radium. Histones are like granite, ticking at a very slow rate; fibrinopeptides are like radium, buzzing like a dementedly randomised bee. Other proteins such as cytochrome-c (or rather the genes that make them) are intermediate. There is a spectrum of gene clocks, each running at its own speed, and each useful for different dating purposes, and for cross-checking with each other.

Why do different genes run at different speeds? What distinguishes 'granite' genes from 'radium' genes? Remember that neutral doesn't mean useless, it means equally good. Granite genes and radium genes are