The Ancestor's Tale Part 12

Here's another way in which speciation might have happened, and one that seems especially plausible for Lake Victoria. Radiocarbon dating of mud suggests that Lake Victoria dried up about 15,000 years ago. h.o.m.o sapiens h.o.m.o sapiens, not long predating the dawn farmers of Mesopotamia, could walk dry-footed from Kisumu in Kenya straight across to Bukoba in Tanzania a journey that today is a 300-kilometre voyage by the MV Victoria Victoria, a decent-sized ship popularly known as the 'Queen of Africa'. That was an extremely recent drying up, but who knows how many times the Victoria basin has been drained and flooded, flooded and drained in the millennia before that? On the timescale of thousands of years, the lake level may rise and fall like a yo-yo.

Now, hold that thought in mind, together with the theory of speciation by geographical isolation. When the Victoria basin dries up from time to time, what will be left? It could be a desert if the drying up were complete. But a partial drying up would leave a scattering of little lakes and pools, representing the deeper depressions in the basin. Any fish trapped in these little lakes would have the perfect opportunity to evolve away from their colleagues in other little lakes and become separate species. Then, when the basin flooded again and the large lake reconst.i.tuted, the newly distinct species would all swim out and join the larger Victoria fauna. When the yo-yo went down the next time, it would be a different set of species that accidentally found itself separated in each of the smaller refugia. Again, what a wonderful recipe for speciation.

Evidence from mitochondrial DNA supports this theory of rising and falling lake levels for the older Lake Tanganyika. Although a deep rift lake, not a shallow basin like Victoria, there is evidence that Lake Tanganyika's level used to be much lower, and it was at that time separated into three medium-sized lakes. The genetic evidence suggests an early segregation of cichlids into three groupings, presumably one for each of the old lakes, followed by further speciations after the formation of the present large lake.

In the case of Lake Victoria, Erik Verheyen, Walter Salzburger, Jos Snoeks and Axel Meyer have done a very thorough genetic study of the mitochondria of haplochromine cichlid fish, not only in the main lake but in the neighbouring rivers, and the satellite lakes Kivu, Edward, George, Albert and others. They showed that Victoria and its smaller neighbours share a monophyletic 'species flock' that began to diverge about 100,000 years ago. This sophisticated piece of research used the methods of parsimony, maximum likelihood and Bayesian a.n.a.lysis that we met in the Gibbon's Tale. Verheyen and colleagues looked at the distribution in all the lakes and neighbouring rivers of 122 'haplotypes' from the mitochondrial DNA of these fish. A haplotype, as we saw in Eve's Tale, is a length of DNA that lasts long enough to be recognised repeatedly in lots of individuals, who might well belong to lots of different species. For simplicity I shall use the word 'gene' as an approximate synonym for haplotype (although purist geneticists would not). The scientists were, temporarily, ignoring the question of species. They were, in effect, imagining genes swimming around in lakes and rivers, and counting the frequency with which they did so.

It is easy to misunderstand the beautiful diagram (see plate 26) (see plate 26), with which Verheyen and his colleagues summarised their work. It is tempting to think that the circles represent species cl.u.s.tered around parent species, as in a family tree. Or that they represent small lakes cl.u.s.tered around larger lakes, as in a stylised route map of an (amphibious!) airline's network of destinations. Neither of these is even close to what the diagram represents. The circles are neither species nor geographical hubs. Each one is a haplotype: a 'gene', a particular length of DNA that an individual fish might or might not possess.

Each gene, then, is represented by one circle. The area of the circle conveys the number of individuals, regardless of species regardless of species, added up over all lakes and rivers surveyed, who possessed that particular gene. The small circles indicate a gene that was found in only a single individual. Gene 25, to judge from the area of its circle (the largest one), was found in 34 individuals. The number of circles, or blobs, on the line joining two circles represents the minimal number of mutational changes you need to go from one to the other. You will recognise from the Gibbon's Tale that this is a form of parsimony a.n.a.lysis, but slightly easier than parsimony a.n.a.lysis of distantly related genes, because the intermediates are still around. The small black blobs represent intermediate genes that have not been found in real fish, but can be inferred as probably existing in the course of evolution. It is an unrooted tree that doesn't commit itself to the direction of evolution.

Geography enters into the diagram only in the colour coding. Each circle is a pie chart showing the number of times the gene concerned was found in each of the lakes or rivers surveyed (see the colour key at bottom right of the diagram). Of the numerous genes, those labelled 12, 47, 7 and 56 were found only in Lake Kivu (all red circles). Genes 77 and 92 were found only in Lake Victoria (all blue). Gene 25, the most abundant of all, turned up mostly in Lake Kivu but also in significant numbers in the 'Uganda lakes' (a cl.u.s.ter of small lakes close to each other and to the west of Lake Victoria). The pie chart shows that gene 25 was also found in the Victoria Nile river, in Lake Victoria itself, and in Lake Edward/George (these two small and neighbouring lakes are united for purposes of the count). Once again, bear in mind that the diagram contains no information at all about species. The blue slice of pie in gene 25's circle indicates that two individuals from Lake Victoria contained this gene. We are given no indication at all on whether those two individuals were of the same species as each other, or the same species as any of the Lake Kivu individuals bearing that gene. That is not what this diagram is about. It is a diagram to delight any enthusiast for the selfish gene.

The results were powerfully revealing. Little Lake Kivu emerges as the fountainhead of the entire species flock. Genetic signals show that Lake Victoria was 'seeded' with haplochromine cichlids on two separate occasions from Lake Kivu. The great drying of 15,000 years ago by no means extinguished the species flock, and very probably enhanced it in the way we were just imagining, through the Victoria basin becoming a 'Finland' of lakelets. As for the origin of the older population of cichlids in Lake Kivu itself (it now has 26 species, including 15 endemic haplochromines), the genetic oracle says they came from Tanzanian rivers.

This work is only just beginning. The imagination at first quails, and then is uplifted, by the contemplation of what will be achieved when such methods are routinely applied not just to cichlid fish in African lakes, but to any animals, in any 'archipelago' of habitats.

THE BLIND CAVE FISH'S TALE.

Animals of various kinds have found their way into dark caves, where living conditions are obviously very different from outside. Repeatedly, and in many different animal groups including flatworms, insects, crayfish, salamanders and fish, cave dwellers have independently evolved many of the same changes. Some can be thought of as constructive changes for instance, delayed reproduction, fewer but larger eggs, and increased longevity. Apparently in compensation for their useless eyes, cave animals typically have enhanced senses of taste and smell, long feelers and, in the case of fish, improvements to the lateral line system (a pressure-related sense organ beyond our empathy but deeply meaningful to fish). Other changes are referred to as regressive. Cave dwellers tend to lose their eyes and their skin pigment, becoming blind and white.

The Mexican tetra Astyanax mexica.n.u.s Astyanax mexica.n.u.s (also known as (also known as A. fasciatus A. fasciatus) is particularly remarkable because different populations within within the one species of fish have independently followed streams into caves and very rapidly evolved a common pattern of cave-related regressive changes, which can be directly contrasted with fellow species members still living outside. These 'Mexican blind cave fish' are found only in Mexican caves mostly limestone caves in a single valley. Once understandably thought to belong to their own separate species, they are now cla.s.sified as a race of the same species, the one species of fish have independently followed streams into caves and very rapidly evolved a common pattern of cave-related regressive changes, which can be directly contrasted with fellow species members still living outside. These 'Mexican blind cave fish' are found only in Mexican caves mostly limestone caves in a single valley. Once understandably thought to belong to their own separate species, they are now cla.s.sified as a race of the same species, Astyanax mexica.n.u.s Astyanax mexica.n.u.s, which is common in surface waters from Mexico to Texas. The blind race has been found in 29 separate caves and, to repeat, it looks strongly as though at least some of these cave populations evolved their regressive eyes and white coloration independently of each other: surface-dwelling tetras have on many occasions taken up residence in caves, and independently lost their eyes and their colour on each occasion.

Intriguingly, it appears that some populations have been in their caves longer than others, and this shows itself as a gradient in the extent to which they have pushed in the typical cave-specific direction. The extreme is found in the Pachon cave, believed to hold the oldest cave population. At the 'young' end of the gradient is the Micos cave, whose population is relatively unchanged from the normal surface-dwelling form of the species. None of the populations can have been in their caves very long because this is a South American species which could not have crossed into Mexico before the formation of the Isthmus of Panama 3 million years ago the Great American Interchange. My guess is that the cave populations of tetras are far younger than that.

It is easy to see why dwellers in darkness might never have evolved eyes in the first place; less easy to see why, given that their recent ancestors certainly had normal, functioning eyes, the cave fish should 'bother' to get rid of them. If there is a possibility, however slight, of a cave fish finding itself washed out of its cave into the light of day, wouldn't there be some benefit in keeping the eyes 'just in case'? That isn't how evolution works, but it can be rephrased in respectable terms. Building eyes indeed, building anything is not free of cost. Individual fish that divert resources into some other part of the animal's economy would have an advantage over rival fish that retain full-sized eyes.5 If a cave-dweller has insufficient probability of needing eyes to offset the economic costs of making them, eyes will disappear. Where natural selection is concerned, even very slight advantages are significant. Other biologists leave economics out of their reckoning. For them, it is sufficient to invoke an acc.u.mulation of random changes in eye development, which are not penalised by natural selection because they make no difference. There are many more ways of being blind than of being sighted, so random changes, for purely statistical reasons, tend towards blindness. If a cave-dweller has insufficient probability of needing eyes to offset the economic costs of making them, eyes will disappear. Where natural selection is concerned, even very slight advantages are significant. Other biologists leave economics out of their reckoning. For them, it is sufficient to invoke an acc.u.mulation of random changes in eye development, which are not penalised by natural selection because they make no difference. There are many more ways of being blind than of being sighted, so random changes, for purely statistical reasons, tend towards blindness.

And this leads us to the main point of the Blind Cave Fish's Tale. It is a tale of Dollo's Law, which states that evolution is not reversed. Is Dollo's Law disproved by the cave fish's apparent reversal of an evolutionary trend, shrinking again the eyes that grew, so painstakingly, over past evolutionary time? Is there, in any case, some general theoretical reason to expect evolution to be irreversible? The answer to both questions is no. But Dollo's Law has to be correctly understood, and that is the purpose of this tale.

Except in the very short term, evolution cannot be precisely and exactly reversed, but the emphasis is on 'precisely and exactly'. It is very improbable that any particular evolutionary pathway, specified in advance, will be followed. There are too many possible pathways. An exact reversal of evolution is just a special case of a particular evolutionary pathway, specified in advance. With such a large number of possible paths that evolution might follow, the odds are heavily against any one particular path, and that includes an exact reversal of the forward one just travelled. But there is no law against evolutionary reversal as such.

Dolphins are descended from land-dwelling mammals. They returned to the sea and resemble, in many superficial respects, large, fast-swimming fish. But evolution has not reversed itself. Dolphins resemble fish in certain respects, but most of their internal features clearly label them as mammals. If evolution had truly reversed itself, they would simply be fish. Maybe some 'fish' really are dolphins the reversion to fish being so perfect and far-reaching that we haven't noticed? Want a bet? That is the sense in which you can bet heavily on Dollo's Law. Especially if you look at evolutionary change at the molecular level.

This interpretation of Dollo's Law could be called the thermodynamic interpretation. It is reminiscent of the Second Law of Thermodynamics, which states that entropy (or disorder or 'mixedupness') increases in a closed system. A popular a.n.a.logy (or it may be more than an a.n.a.logy) for the Second Law is a library. Without a librarian energetically reshelving books in their correct places, a library tends to become disordered. The books become mixed up. People leave them on the table, or put them on the wrong shelf. As time goes by, the library's equivalent of entropy inevitably increases. That's why all libraries need a librarian, constantly working to restore the books to order.

The great misunderstanding of the Second Law is to a.s.sume that there is a driving urge towards some particular goal state of disorder. It isn't like that at all. It is just that there are far more ways of being disordered than of being ordered. If the books are shuffled at random by sloppy borrowers, the library will automatically move away from the state (or the small minority of states) that anybody would recognise as ordered. There is no drive towards a state of high entropy. Rather, the library meanders in some random direction away from the initial state of high order and, no matter where it wanders in the s.p.a.ce of all possible libraries, the vast majority of possible pathways will const.i.tute an increase in disorder. Similarly, of all the evolutionary pathways that a lineage could follow, only one out of a vast number of possible pathways will be an exact reversal of the path by which it has come into being. Dollo's Law turns out to be no more profound than the 'law' that if you toss a coin 50 times, you won't get all heads nor all tails, nor strict alternation, nor any other particular, prespecified sequence nor any other particular, prespecified sequence. The same 'thermodynamic' law would also state that any particular particular evolutionary pathway in a 'forward' direction (whatever that might mean!) will not be precisely followed twice. evolutionary pathway in a 'forward' direction (whatever that might mean!) will not be precisely followed twice.

In this thermodynamic sense, Dollo's Law is true but unremarkable. It doesn't deserve the t.i.tle of law at all, any more than there is a 'law' against tossing a coin 100 times and getting heads every time. One could imagine a 'real law' interpretation of Dollo's Law which stated that evolution could not return to anything that was vaguely like an ancestral state, as a dolphin is vaguely like a fish. This interpretation would indeed be remarkable and interesting but it is (ask any dolphin) false. And I cannot imagine any sensible theoretical rationale that would expect it to be true.

THE FLOUNDER'S TALE.

An endearing quality of Chaucer is the naive perfectionism of his General Prologue, where he introduces his pilgrims. It wasn't enough to have a Doctour of Physik on the pilgrimage he had to be the finest doctor in the land: In all this world ne was ther noon hym lik,To speke of physik and of surgerye.

The 'verray, parfit gentil knyght' was, it seemed, unmatched in Christendom for bravery, loyalty and even temper. As for his squire and son, he was 'A lovere and a l.u.s.ty bacheler ... wonderly delyvere, and of greet strengthe'. To top it all, he was 'as fressh as is the month of May'. Even the knight's yeoman knew all there was to know of woodcraft. The reader comes to take it for granted that, if a profession is mentioned, its pract.i.tioner will automatically turn out to be unrivalled in all England.

Perfectionism is a vice of evolutionists. We are so used to the wonders of Darwinian adaptation, it is tempting to believe there could be nothing better. Actually, it is a temptation that I can almost recommend. A surprisingly strong case can be built for evolutionary perfection, but it must be done with circ.u.mspection and sophisticated attention.6 Here I shall give just one example of a historical constraint, the so-called 'jet engine effect': imagine how imperfect a jet engine would be if, instead of being designed on a clean drawing board, it had to be changed one step at a time, screw by screw and rivet by rivet, from a propeller engine. Here I shall give just one example of a historical constraint, the so-called 'jet engine effect': imagine how imperfect a jet engine would be if, instead of being designed on a clean drawing board, it had to be changed one step at a time, screw by screw and rivet by rivet, from a propeller engine.

A skate is a flat fish that might have been designed on a drawing board to be flat, resting on the belly, with wide 'wings' reaching symmetrically out to both sides. Teleost flatfish do it in a different way. They rest on one side, either the left (e.g. plaice) or the right (e.g. turbot and flounder). Whichever the side, the shape of the whole skull is distorted so that the eye on the lower side moves over to the upper side, where it can see. Pica.s.so would have loved them (see plate 27) (see plate 27). But, by the standards of any drawing board, they are revealingly imperfect. They have precisely the kind of imperfection you would expect from being evolved rather than designed.

1 Spiders eat the large prey not so much in bits as in liquid form. They inject it with digestive juices and then suck it in as if through a straw. Spiders eat the large prey not so much in bits as in liquid form. They inject it with digestive juices and then suck it in as if through a straw.

2 Snakes do it by disarticulating their skulls. For a snake, eating a meal must be an ordeal comparable to giving birth for a woman. Snakes do it by disarticulating their skulls. For a snake, eating a meal must be an ordeal comparable to giving birth for a woman.

3 Lake Victoria has been the victim of a man-made catastrophe. In 1954, the British colonial administration, hoping to improve fisheries, introduced the Nile perch ( Lake Victoria has been the victim of a man-made catastrophe. In 1954, the British colonial administration, hoping to improve fisheries, introduced the Nile perch (Lates niloticus) to the lake. This decision was opposed by biologists, who predicted that the perch would disrupt the lake's unique ecosystems. Their prediction came disastrously true. The cichlids had never evolved to cope with a big predator like the Nile perch. Probably 50 cichlid species have gone for good, and another 130 are critically endangered. In a mere half-century, completely avoidable ignorance has devastated local economies around the lake and irreversibly wiped out a priceless scientific resource.

4 The whole topic is treated in detail in Dolph Schluter's recent book, The whole topic is treated in detail in Dolph Schluter's recent book, The Ecology of Adaptive Radiation The Ecology of Adaptive Radiation.

5 Eyes can be an even more costly extravagance if they become infected or irritated, which is probably why burrowing moles have reduced them as much as possible. Eyes can be an even more costly extravagance if they become infected or irritated, which is probably why burrowing moles have reduced them as much as possible.

6 I have set out the pitfalls in a chapter of I have set out the pitfalls in a chapter of The Extended Phenotype The Extended Phenotype called 'Constraints on Perfection'. called 'Constraints on Perfection'.

Rendezvous 21.

SHARKS AND THEIR KIN.

'Out of the murderous innocence of the sea ...' The context of Yeats's poem was completely other but I can't help it the phrase always makes me think of a shark. Murderous, but innocent of deliberate cruelty, just making a living as perhaps the world's most effective killing machine. I know people for whom the great white shark is their worst nightmare. If you are one of them, you may not wish to know that the Miocene shark Carcharocles megalodon Carcharocles megalodon was three times the size of a great white, with jaws and teeth to scale. was three times the size of a great white, with jaws and teeth to scale.

My own recurrent nightmare, having grown up as an exact contemporary of the atomic bomb, is not a shark but a huge, black, futuristic, delta-winged aircraft bristling with high-tech missile launchers, filling the sky with its shade and my heart with foreboding. Almost exactly the shape of a manta ray in fact. The dark shape that roars over the treetops of my dreams, with its twin gun turrets so enigmatically menacing, is a sort of technological cousin to Manta birostris Manta birostris. I always found it hard to accept that these seven-metre monsters are harmless filter-feeders, straining plankton through their gills. They are also extremely beautiful.

What of the sawfish, what on earth is that all about? And the hammerhead shark? Hammerheads occasionally attack people, but that is not why they might invade your dreams. It is the bizarre T-shaped head, the eyes set wider than you expect outside science fiction, as though this shark were designed by an artist with a drugged imagination (see plate 28) (see plate 28). And the thresher shark, Alopias Alopias, isn't that another work of art, another candidate for a dream? The upper lobe of the tail is nearly as long as the rest of the body. Threshers use their prodigious tailblades first to herd prey, then to thresh them to death. A thresher, hara.s.sed by fishermen in a boat, has been known to decapitate a man with a single swipe of that magnificent tail.

The sharks, rays and other cartilaginous fish or chondrichthyans join us at Rendezvous 21 Rendezvous 21, 460 million years ago, in seas off the icycold and barren lands of the Middle Ordovician. The most noticeable difference between the new pilgrims and all the others so far is that sharks have no bone. Their skeleton is made of cartilage. We too use cartilage for special purposes like lining our joints, and all of our skeleton starts out as flexible cartilage in the embryo. Most of it later becomes ossified when mineral crystals, mostly calcium phosphate, incorporate themselves. Except for the teeth, the shark skeleton never undergoes this transformation. Nevertheless, their skeleton is quite rigid enough to sever your leg in a single bite.

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Sharks and their kin join. The cartilaginous fish, who join us here, include the sharks and rays. Fossils leave no doubt about the early split of the jawed vertebrates into bony fish and these cartilaginous fish. Recent, robust data strongly support this scheme of relationships within the 850 or so species of cartilaginous fish. The cartilaginous fish, who join us here, include the sharks and rays. Fossils leave no doubt about the early split of the jawed vertebrates into bony fish and these cartilaginous fish. Recent, robust data strongly support this scheme of relationships within the 850 or so species of cartilaginous fish.

Images, left to right: grey reef shark ( grey reef shark (Carcharhinus amblyrhynchos); manta ray (Manta birostris); elephant fish (Callorhynchus milii).

Sharks lack the swim bladder that contributes to the success of the bony fish, and many of them have to swim continuously to maintain their desired level in the water. They a.s.sist their buoyancy by retaining the waste product urea in the blood and by having a large, oilrich liver. Incidentally, some bony fish use oil instead of gas in their swim bladder.

If you should be so incautiously affectionate as to stroke a shark, you would find that its whole skin feels like sandpaper, at least if you stroke it 'against the grain'. It is covered with dermal denticles sharp, tooth-like scales. Not only are they tooth-like, but the formidable teeth of a shark are themselves evolutionary modifications of dermal denticles.

Sharks and rays almost all live in the sea, although a few genera venture up estuaries and rivers. Freshwater shark attacks on humans used to be common in Fiji, but that was when humans were cannibals. All but the choicest cuts were discarded into rivers, and it would seem that sharks were attracted upstream by the smell of leftovers from cannibal feasts. When Europeans arrived they put a stop to cannibalism, but at the same time inadvertently brought new diseases against which the Fijians had not evolved immunity. Corpses of diseased victims were also disposed of in rivers, so sharks continued to be attracted. Nowadays bodies are no longer tossed into rivers, and shark attacks have decreased accordingly. Unlike the bony fish, no sharks have ever shown any inclination to come on land.

The cartilaginous fish are divided into two main groups: the rather weird-looking chimaeras or ratfish, which are not numerous enough to be a significant part of the fauna; and the sharks, skates and rays, which are. Skates and rays are flattened sharks. Dogfish are small sharks, but they are still not very small: no whitebait-sized sharks exist. The spined pygmy shark Squaliolus laticaudus Squaliolus laticaudus grows up to about 20 centimetres. The shark body plan seems to lend itself to large size, and the biggest of all, the whale shark grows up to about 20 centimetres. The shark body plan seems to lend itself to large size, and the biggest of all, the whale shark Rhincodon typus Rhincodon typus can be up to 12 metres long and weigh 12 tonnes. Like the second largest, the basking shark can be up to 12 metres long and weigh 12 tonnes. Like the second largest, the basking shark Cetorhinus maximus Cetorhinus maximus, and like the largest whales, the whale shark is a plankton feeder. Carcharocles megalodon Carcharocles megalodon, already mentioned as the stuff of nightmares, was not to use a calculated understatement a filter-feeder. That Miocene monster had teeth, each one as big as your face. It was a voracious predator, like the majority of sharks today, and they have topped the food chains of the sea for hundreds of millions of years with relatively little change.

If manta rays feature in nightmares as bombers, the smaller role of jump-jet fighter might be played by the chimaeras (see plate 29) (see plate 29), also known as ratfish or ghost sharks. These strange deep-sea fish occupy the cla.s.s Holocephali (whole head), where all the rest of the cartilaginous fish, the sharks and rays combined, belong in the Elasmobranchii. They can be recognised by their unusual gill covers, which completely encase the separate gills, providing a single opening for all of them. Unlike sharks and rays, their skin is not covered with dermal denticles but is 'naked'. This may be what gives them their 'ghostly' appearance. Their resemblance to a nightmare plane comes from the fact that their tails are not prominent and they swim by 'flying' with their large pectoral fins. There are only about 35 species of living chimaeras.

Successful as sharks certainly are and over a spectacularly long time too teleost fish outnumber them thirtyfold when it comes to species numbers. There have been two major radiations of sharks. The first flourished mightily in the Palaeozoic seas, especially during the Carboniferous Period. This ancient domination of sharks had come to an end by the beginning of the Mesozoic Era (the age of dinosaurs on land). After a lull of about 100 million years, the sharks enjoyed another major resurgence in the Cretaceous, which has continued to this day.

A word a.s.sociation test that mentioned 'shark' would very probably elicit the response 'jaws', so it is appropriate that Concestor 21, perhaps our 200-million-greats-grandparent, is the grand ancestor of all the vertebrates that have true jaws, the gnathostomes. Gnathos Gnathos in Greek means 'lower jaw', and that is specifically what sharks and all the rest of us share. It was one of the triumphs of cla.s.sical comparative anatomy to demonstrate that jaws evolved from modified parts of the gill skeleton. The next pilgrims to join us, at in Greek means 'lower jaw', and that is specifically what sharks and all the rest of us share. It was one of the triumphs of cla.s.sical comparative anatomy to demonstrate that jaws evolved from modified parts of the gill skeleton. The next pilgrims to join us, at Rendezvous 22 Rendezvous 22, are the jawless vertebrates, the Agnatha, well endowed with gills but with no lower jaw. Once numerous, diverse and heavily armoured, the Agnatha are now reduced to the eel-shaped lampreys and hagfish.

Rendezvous 22.

LAMPREYS AND HAGFISH.

Rendezvous 22, where we meet the lampreys and hagfish, occurs somewhere in the warm seas of the early Cambrian, say 530 million years ago, and I would very roughly guess that Concestor 22 was our 240-million-greats-grandparent. The lampreys and hagfish survive as pivotal messengers from the dawn of vertebrates. Although it is convenient to treat them together, as the jawless and limbless fish, I have to admit that many morphologists think that lampreys are closer cousins to us than they are to hagfish. According to this school, we should greet the lamprey pilgrims at Rendezvous 22 Rendezvous 22, and the hagfish at 23. On the other hand, molecular biologists are equally insistent that both join us at one rendezvous, and this is the opinion I am provisionally adopting here. In any case, it is fair to say that neither lampreys nor hagfish do justice to the jawless fish as a whole, most of whom are extinct.

Lampreys and hagfish have a superficially eel-like appearance, with soft bodies but when the jawless fish dominated the seas, in the Devonian 'Age of Fish', many of them, known as ostracoderms, had hard, bony armour plating, and some had paired fins, unlike lampreys and hagfish. They give the lie to any suggestion that bone is an 'advanced' feature of vertebrates that 'took over' from cartilage. Sturgeons and some other 'bony' fish resemble sharks and lampreys in possessing a skeleton almost entirely made of cartilage, but they are descended from far more bony ancestors indeed from fish with heavy armour plating and it is not unlikely that sharks and lampreys are too.

Even more heavily armoured were the placoderms, a wholly extinct group of jaw-bearing and limb-bearing fish of uncertain affinities, who also lived in the Devonian Period, contemporary with some of the jawless ostracoderms and presumably descended from earlier jawless fish. Some of the placoderms were so heavily armoured that even their limbs had a tubular, jointed exoskeleton, superficially similar to a crab's leg. If you encountered one in a poor light and an imaginative frame of mind, you could be forgiven for thinking you had stumbled on a strange kind of lobster or crab. As a rather young undergraduate, I used to dream about discovering a living placoderm it was my equivalent of the scoring-a-century-for-England fantasy.

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Jawless fish join. There is still a great deal of argument about evolutionary relationships at the base of the vertebrate lineage, particularly with respect to the living jawless fish: the 41 species of lamprey and the 43 species of hagfish. Fossils argue that the earliest divergence occurred between hagfish and the other vertebrates, followed by the divergence of the lamprey lineage. However, the molecular data strongly argue that lampreys and hagfish group together, as shown here. There is still a great deal of argument about evolutionary relationships at the base of the vertebrate lineage, particularly with respect to the living jawless fish: the 41 species of lamprey and the 43 species of hagfish. Fossils argue that the earliest divergence occurred between hagfish and the other vertebrates, followed by the divergence of the lamprey lineage. However, the molecular data strongly argue that lampreys and hagfish group together, as shown here.

Images, left to right: New Zealand hagfish ( New Zealand hagfish (Eptatretus cirrhatus); sea lamprey (Petromyzon marinus).

Why did both the jawed placoderms and the jawless ostracoderms develop such heavily fortified bodies? What was it about those Palaeozoic seas that demanded such formidable protection? The presumed answer is equally formidable predators, and the obvious candidates, apart from other placoderms, are the eurypterids or sea scorpions, some more than two metres in length the largest arthropods that ever lived. Whether or not any of the eurypterids had venomous stings like modern scorpions (recent evidence suggests not), they still must have been fearsome predators, capable of driving the Devonian fish, both jawless and jawed, to evolve costly armour plating.

Lampreys are not armoured, and they are easy to eat, as King Henry I had good reason to regret (school history books never fail to remind us that he died of a surfeit of them). Most lampreys are parasitic on other fish. Instead of jaws they have a circular sucker around the mouth, looking a bit like an octopus sucker but with concentric rings of tiny teeth. The lamprey fastens its sucker to the outside of another fish, the little teeth rasp through the skin, and the lamprey sucks the blood of its victim, like a leech. Lampreys have had serious effects on fisheries, for example in the North American Great Lakes.

n.o.body knows what Concestor 22 was like but, living as it probably did in the Cambrian Period, long before the Devonian Age of Fish and the dreaded sea scorpions, it probably wasn't armour-plated like the ostracoderms of the jawless fish's heyday. Nevertheless, the ostracoderms seem to be closer cousins to us jawed vertebrates than the lampreys are. In other words, 'before' our pilgrims join the lampreys at Rendezvous 22 Rendezvous 22, we have already incorporated the ostracoderms into our pilgrimage. Our concestor with the ostracoderms, whom we don't number because they are all extinct, was presumably jawless.

Modern hagfish resemble lampreys in their long, eel-like shape, their lack of a lower jaw, their lack of paired limbs, their row of gill port-holes on either side, and their notochord retained into the adult (this stiffening rod, running the length of the back, is in most vertebrates present only in the embryo). But hagfish are not parasites. They rummage with their mouth hole around the bottom of the sea for small invertebrates, or they scavenge on dead fish or whales, often wriggling inside to eat from the inside out. They are exceedingly slimy, and they use their surprising talent for tying themselves in knots in order to get a purchase when burrowing into carca.s.ses.

Vertebrates were once thought to have arisen long after the Cambrian Period. Maybe it was an aspect of our sn.o.bbish desire to arrange the animal kingdom on a ladder of progress. Somehow it seemed right and fitting that there was an age where animal life was limited to invertebrates, setting the scene for the eventual arrival of the mighty vertebrates. Zoologists of my generation were taught that the earliest known vertebrate was a jawless fish called Jamoytius Jamoytius (named, somewhat freely, after J. A. Moy-Thomas) that lived in the middle of the Silurian Period, 100 million years after the Cambrian when most of the invertebrate phyla arose. Obviously vertebrates must have had ancestors living in the Cambrian, but they were a.s.sumed to be invertebrate forerunners of the true vertebrates protochordates. (named, somewhat freely, after J. A. Moy-Thomas) that lived in the middle of the Silurian Period, 100 million years after the Cambrian when most of the invertebrate phyla arose. Obviously vertebrates must have had ancestors living in the Cambrian, but they were a.s.sumed to be invertebrate forerunners of the true vertebrates protochordates. Pikaia Pikaia has been heavily promoted as the oldest fossil protochordate. has been heavily promoted as the oldest fossil protochordate.1 It was a delicious surprise, therefore, when apparently true vertebrate fossils started turning up in the Cambrian strata of China, and the Lower Cambrian at that. This has robbed It was a delicious surprise, therefore, when apparently true vertebrate fossils started turning up in the Cambrian strata of China, and the Lower Cambrian at that. This has robbed Pikaia Pikaia of some of its mystique. There were true vertebrates, jawless fish, living before of some of its mystique. There were true vertebrates, jawless fish, living before Pikaia Pikaia. The vertebrates hark back to the deep Cambrian.

Not surprisingly given their immense age, these fossils, called Myllokunmingia Myllokunmingia and and Haikouichthys Haikouichthys (although they may belong to the same species) are not in mint condition, and much is still unknown about these primeval fish. They seem to have had most of the features you'd expect from a relative of the lampreys and hagfish, including gills, segmented muscle blocks and a notochord. (although they may belong to the same species) are not in mint condition, and much is still unknown about these primeval fish. They seem to have had most of the features you'd expect from a relative of the lampreys and hagfish, including gills, segmented muscle blocks and a notochord. Myllokunmingia Myllokunmingia, whom we shall meet again in the Velvet Worm's Tale, is perhaps not too far from being a plausible model for Concestor 22.

Rendezvous 22 is a major milestone. From now on, for the first time, all the vertebrates are united in a single pilgrim band. It is a big event because, traditionally, animals were divided into two major groups, the vertebrates and the invertebrates. As a division of convenience, the distinction has always been useful in practice. From a strictly cladistic point of view, however, the vertebrate/invertebrate distinction is an odd one, nearly as unnatural as the ancient Jewish cla.s.sification of humanity into themselves and 'gentiles' (literally everybody else). Important though we vertebrates think ourselves, we don't const.i.tute even a whole phylum. We are a subphylum of the phylum Chordata, and the phylum Chordata should be thought of as on a par with, say, the phylum Mollusca (snails, limpets, squids, etc.) or the phylum Echinodermata (starfish, sea urchins, etc.). The phylum Chordata includes other vertebrate-like creatures that nevertheless lack a backbone for example, the amphioxus, whom we are about to meet at Rendezvous 23 Rendezvous 23.

Strict cladism notwithstanding, there really is something rather special about vertebrates. Professor Peter Holland has made to me the powerful point that there is a ma.s.sive difference in genome complexity between (all) vertebrates and (all) invertebrates. 'It is, at the genetic level, perhaps the biggest change in our metazoan2 ancestry.' Holland thinks the traditional divide between vertebrates and invertebrates needs to be revived, and I see what he means. ancestry.' Holland thinks the traditional divide between vertebrates and invertebrates needs to be revived, and I see what he means.

The chordates get their name from the already mentioned notochord, the cartilage rod that runs along the back of the animal, in the embryo if not in the adult.3 Other characteristics of chordates (including vertebrates), which in ourselves are seen only in the embryo, include gill openings near the front end on both sides, and a tail that reaches back beyond the a.n.u.s. All chordates have a dorsal nerve cord (runs along the back), unlike many invertebrates where the nerve cord is ventral (runs along the belly). Other characteristics of chordates (including vertebrates), which in ourselves are seen only in the embryo, include gill openings near the front end on both sides, and a tail that reaches back beyond the a.n.u.s. All chordates have a dorsal nerve cord (runs along the back), unlike many invertebrates where the nerve cord is ventral (runs along the belly).

Vertebrate embryos all have a notochord but it is replaced in the adult, to a greater or lesser extent, by the segmented, articulated backbone. In most vertebrates the notochord itself survives into the adult only in fragments, such as the intervertebral discs whose tendency to slip can cause us so much grief. The lampreys and hagfish are unusual among vertebrates in retaining the notochord more or less intact into the adult. In this respect they are, I suppose, borderline vertebrates, but everyone calls them vertebrates anyway.

THE LAMPREY'S TALE.

The reason it falls to the lamprey to tell this tale will be revealed at the end. It is a reprise on a theme we have met before: there is a separate gene's-eye view of ancestry and pedigree that is surprisingly independent of the view we get when we think about family trees in more traditional ways.

Haemoglobin is well known as the vitally important molecule that carries oxygen to our tissues and gives our blood its spectacular colour. Human adult haemoglobin is actually a composite of four protein chains called globins, knotted around each other. Their DNA sequences show that the four globin chains are closely related to each other, but they are not identical. Two of them are called alpha globins (each a chain of 141 amino acids), and two are beta globins (each a chain of 146 amino acids). The genes coding for the alpha globins are on our chromosome 11; those coding for the beta globins are on chromosome 16. On each of these chromosomes there is a cl.u.s.ter of globin genes in a row, interspersed with some junk DNA that is never transcribed. The alpha cl.u.s.ter, on chromosome 11, contains seven globin genes. Four of these are pseudogenes disabled versions of alpha with faults in their sequence, never translated into protein. Two are true alpha globins, used in the adult. The final one is called zeta, and it is used only in embryos. The beta cl.u.s.ter, on chromosome 16, has six genes, some of which are disabled, and one of which is used only in the embryo. Adult haemoglobin, as we've seen, contains two alpha and two beta chains, wrapped around each other to form a beautifully functioning parcel.

Never mind all this complexity. Here's the fascinating point. Careful letter-by-letter a.n.a.lysis shows that the different kinds of globin genes are literally cousins of each other members of a family. But these distant cousins still co-exist inside you and me. They still sit side by side with their cousins inside every cell of every warthog and every wombat, every owl and every lizard.

On the scale of whole organisms, of course, all vertebrates are cousins of each other too. The tree of vertebrate evolution is the family tree we are all familiar with, its branch-points representing speciation events the splitting of species into daughter species. In reverse, they are the rendezvous points that punctuate this pilgrimage. But there is another family tree occupying the same timescale, whose branches represent not speciation events but gene duplication events within genomes. And the branching pattern of the globin tree looks very different from the branching pattern of the family tree, if we trace it in the usual, orthodox way, with species branching to form daughter species. There is not just one evolutionary tree in which species divide and give rise to daughter species. Every gene has its own tree, its own chronicle of splits, its own catalogue of close and distant cousins.

The dozen or so different globins inside you and me have come down to us through the entire lineage of our vertebrate ancestors. About half a billion years ago, in a jawless fish perhaps like a lamprey, an ancestral globin gene accidentally split in two, both copies remaining in different parts of that fish's genome. There were then two copies of it, in different parts of the genome of all descendant animals. One copy was destined to give rise to the alpha cl.u.s.ter, on what would eventually become chromosome 11 in our genome, the other to the beta cl.u.s.ter, now on our chromosome 16. There is no point in trying to guess which chromosome either of them sat on in the intermediate ancestors. The locations of recognisable DNA sequences, indeed the number of chromosomes into which the genome is divided, are shuffled and changed with surprisingly gay abandon. Chromosome numbering systems, therefore, do not generalise across animal groups.

As the ages pa.s.sed, there were further duplications, and doubtless some deletions as well. Around 400 million years ago the ancestral alpha gene duplicated again, but this time the two copies remained near neighbours of each other, in a cl.u.s.ter on the same chromosome. One of them was destined to become the zeta of our embryos, the other became the alpha globin genes of adult humans (further branchings gave rise to the non-functional pseudogenes I mentioned). It was a similar story along the beta branch of the family, but with duplications at other moments in geological history.

Now here's a fascinating point. Given that the split between the alpha cl.u.s.ter and the beta cl.u.s.ter took place half a billion years ago, it will of course not be just our human genomes that show the split, and possess both alpha genes and beta genes in different parts of our genomes. We should see the same within-individual split if we look at the genomes of any other mammals, at birds, reptiles, amphibians or bony fish for our common ancestor with all of them lived less than 500 million years ago. Wherever it has been investigated, this expectation has proved correct. Our greatest hope of finding a vertebrate that does not share with us the ancient alpha/beta split would be a jawless fish like a lamprey or a hagfish, for they are our most remote cousins among surviving vertebrates. They are the only surviving vertebrates whose common ancestor with the rest is sufficiently ancient that it could have predated the alpha/beta split. Sure enough, these jawless fish are the only known vertebrates that lack the alpha/beta divide. Rendezvous 22 Rendezvous 22 is so ancient, in other words, that it predated the split between alpha and beta globin. is so ancient, in other words, that it predated the split between alpha and beta globin.

Something like the Lamprey's Tale could be told for each one of our genes, for they all, if you go back far enough, owe their origin to the splitting of some ancient gene. And something like this entire book could be written for each gene. We arbitrarily decided that this should be a human pilgrimage, and we defined our milestones as meeting points with other lineages, which means, in the forward direction, speciation events at which our human ancestors split away from the others. I've already made the point that we could equally have begun our pilgrimage with a modern dugong, or a modern blackbird, and counted a different set of concestors back to Canterbury. But I am now making a more radical point. We could also write a backward pilgrimage for any gene gene.

We could choose to follow the pilgrimage of alpha haemoglobin, or cytochrome-c, or any other named gene. Rendezvous 1 Rendezvous 1 would have been the milestone at which our chosen gene most recently duplicated to make a copy of itself elsewhere in the genome. would have been the milestone at which our chosen gene most recently duplicated to make a copy of itself elsewhere in the genome. Rendezvous 2 Rendezvous 2 would have been the previous duplication event, and so on. Each of the rendezvous milestones would have taken place inside some particular animal or plant, just as the Lamprey's Tale has identified a Cambrian jawless fish as the likely receptacle for the split between alpha and beta haemoglobin. would have been the previous duplication event, and so on. Each of the rendezvous milestones would have taken place inside some particular animal or plant, just as the Lamprey's Tale has identified a Cambrian jawless fish as the likely receptacle for the split between alpha and beta haemoglobin.

The gene's eye view of evolution keeps forcing itself upon our attention.

1 This Cambrian fossil, originally cla.s.sified as an annelid worm, was later recognised as a protochordate, in which role it starred in S. J. Gould's This Cambrian fossil, originally cla.s.sified as an annelid worm, was later recognised as a protochordate, in which role it starred in S. J. Gould's Wonderful Life Wonderful Life.

2 Metazoa means many-celled animals, and we shall be meeting the term further on in the pilgrimage. Metazoa means many-celled animals, and we shall be meeting the term further on in the pilgrimage.

3 The word is calculated to confuse, because chord with an h, in modern English, means only something musical, as in The word is calculated to confuse, because chord with an h, in modern English, means only something musical, as in The Lost Chord The Lost Chord, one of my favourite songs. The notochord is a cord, without an h, meaning rope. However, chord is a recognised archaic spelling of cord (rope), and the connection with music may be that chorda chorda is the Latin for the string of an instrument. is the Latin for the string of an instrument.

Rendezvous 23.

LANCELETS.

And now here's a tidy little pilgrim, wriggling up all on its own to join the pilgrimage. It is the amphioxus or lancelet. Amphioxus Amphioxus used to be its Latin name, but the rules of nomenclature imposed used to be its Latin name, but the rules of nomenclature imposed Branchiostoma Branchiostoma on it. Nevertheless, it had become so well known as on it. Nevertheless, it had become so well known as Amphioxus Amphioxus that the name lives on. The lancelet or amphioxus is a protochordate, not a vertebrate, but it is clearly related to the vertebrates, and placed with them in the phylum Chordata. There are a few other related genera, but they are very similar to that the name lives on. The lancelet or amphioxus is a protochordate, not a vertebrate, but it is clearly related to the vertebrates, and placed with them in the phylum Chordata. There are a few other related genera, but they are very similar to Branchiostoma Branchiostoma, and I shall not distinguish them but call them all, informally, amphioxus.

I call amphioxus tidy because it elegantly lays out the features that proclaim it to be a chordate. It is a living, swimming (well, mostly buried in sand, actually) textbook diagram. There is the notochord running the length of the body, but not a trace of a vertebral column. There is the nerve tube on the dorsal side of the notochord, but no brain unless you count the small swelling at the front end of the nerve tube (where there is also an eye spot), and no skeletal brain case. There are the gill slits at the sides, which are used for filter-feeding, and the segmental muscle blocks along the length of the body, but no trace of limbs. There is the tail, stretching back behind the a.n.u.s, unlike a typical worm, which has the a.n.u.s at the posterior tip of the body. Amphioxus is also unlike a worm, but like many fish, in being shaped like a vertical blade, rather than cylindrical. It swims like a fish, with side-to-side undulations of the body, using the fishlike muscle blocks. The gill slits are part of the feeding apparatus, not primarily for breathing at all. Water is drawn in through the mouth and pa.s.sed out through the gill slits, which act as filters to catch food particles. This is very likely how Concestor 23 used its gill slits, which would mean that gills for breathing came later, as an afterthought. If so, it is a pleasing reversal that, when the lower jaw eventually evolved, it was modified from a part of the gill apparatus.

We are now approaching the point where dating becomes so difficult and controversial that my courage fails me. If forced to put a date on Rendezvous 23 Rendezvous 23, I would guess about 560 million years ago, the vintage of our 270-million-greats-grandparent. But I could easily be wrong, and for this reason I shall from now on abandon my attempts to describe the state of the world at the time of the concestor. As for what it looked like, I don't think we shall ever know for certain, but it is not implausible that Concestor 23 really may have been quite like a lancelet (see plate 30) (see plate 30). If that is so, it is equivalent to saying that the lancelet is primitive. But that demands an immediate cautionary tale the Lancelet's Tale.

[image]

Lancelets join. The closest living relatives of the vertebrates are the 25 known species of fish-like animals commonly known as lancelets. There is little dispute over this. However, the The closest living relatives of the vertebrates are the 25 known species of fish-like animals commonly known as lancelets. There is little dispute over this. However, the dates dates of rendezvous points from now on backwards are often disputed (see the Epilogue to the Velvet Worm's Tale). of rendezvous points from now on backwards are often disputed (see the Epilogue to the Velvet Worm's Tale).

Image: Branchiostoma Branchiostoma sp. (formerly sp. (formerly Amphioxus Amphioxus).

THE LANCELET'S TALE.

If just one touch of sunlight more should make his gonads grow The lancelet's claim to ancestry would get a nasty blow.WALTER GARSTANG (18681949) (18681949) We have already met Walter Garstang, the distinguished zoologist who idiosyncratically expressed his theories in verse. I quote the couplet above not to develop Garstang's own theme, which, though interesting enough to be the subject of the Axolotl's Tale, is irrelevant to my purpose here.1 I am concerned only with the last line, and especially the phrase 'claim to ancestry'. The lancelet, I am concerned only with the last line, and especially the phrase 'claim to ancestry'. The lancelet, Branchiostoma Branchiostoma or amphioxus has enough features in common with true vertebrates to have been long regarded as a surviving relative of some remote ancestor of the vertebrates. Or even which is the real b.u.t.t of my criticism as the ancestor itself. or amphioxus has enough features in common with true vertebrates to have been long regarded as a surviving relative of some remote ancestor of the vertebrates. Or even which is the real b.u.t.t of my criticism as the ancestor itself.

I am being unfair to Garstang, who knew perfectly well that the lancelet, as a surviving animal, could not be literally ancestral. Nevertheless, such talk really does sometimes mislead. Students of zoology delude themselves into imagining that when they look at some modern animal, which they call 'primitive', they are seeing a remote ancestor. This delusion is betrayed by phrases such as 'lower animal', or 'at the bottom of the evolutionary scale', which are not only sn.o.bbish but evolutionarily incoherent. Darwin's advice to himself would serve us all: 'Never use the words higher and lower.'

Lancelets are live creatures, our exact contemporaries. They are modern animals who have had exactly the same time as we have in which to evolve. Another telltale phrase is 'a side branch, off the main line of evolution'. All living animals are side branches. No line of evolution is more 'main' than any other, except with the conceit of hindsight.

Modern animals like lancelets, then, should never be revered as ancestors, nor patronised as 'lower', nor, for that matter, flattered as 'higher'. Slightly more surprisingly and here we come to the second main point of the Lancelet's Tale it is probably in general safest to say the same of fossils. It is theoretically conceivable that a particular fossil really is the direct ancestor of some modern animal. But it is statistically unlikely, because the tree of evolution is not a Christmas tree or a Lombardy poplar, but a densely branched thicket or bush. The fossil you are looking at probably isn't your ancestor, but it may help you to understand the kind kind of intermediate stage your real ancestors went through, at least in respect of some particular bit of the body, such as the ear, or the pelvis. A fossil, therefore, has something like the same status as a modern animal. Both can be used to illuminate our guesses about some ancestral stage. Under normal circ.u.mstances, neither should be treated as though it really is ancestral. Fossils as well as living creatures are usually best treated as cousins, not ancestors. of intermediate stage your real ancestors went through, at least in respect of some particular bit of the body, such as the ear, or the pelvis. A fossil, therefore, has something like the same status as a modern animal. Both can be used to illuminate our guesses about some ancestral stage. Under normal circ.u.mstances, neither should be treated as though it really is ancestral. Fossils as well as living creatures are usually best treated as cousins, not ancestors.

Members of the cladistic school of taxonomists can become positively evangelical about this, proclaiming the non-specialness of fossils with the zeal of a puritan or a Spanish inquisitor. Some go right over the top. They take the sensible statement, 'It is unlikely that any particular fossil is an ancestor of any surviving species', and interpret it to mean 'There never were any ancestors!' Obviously this book stops short of such an absurdity. At every single moment in history there must have been at least one human ancestor (contemporary with, or identical to, at least one elephant ancestor, swift ancestor, octopus ancestor, etc.), even if any particular fossil almost certainly isn't it.

The upshot is that, on our backward journey towards the past, the concestors we have been meeting have not, in general, been particular fossils. The best we can normally hope for is to put together a list of attributes that the ancestor probably had. We have no fossil of the common ancestor we share with the chimpanzees, even though that was less than 10 million years ago. But we were able to guess, with misgivings, that the ancestor was most likely to have been, in Darwin's famous words, a hairy quadruped, because we are the only ape that walks on its hind legs and has bare skin. Fossils can help us with our inferences, but mostly in the same kind of indirect way that living animals help us.

The moral of the Lancelet's Tale is that it is vastly harder to find an ancestor than a cousin. If you want to know what your ancestors looked like 100 million years ago, or 500 million years ago, it is no use reaching down to the appropriate depth in the rocks and hoping to come up with a fossil labelled 'Ancestor', as if from some Mesozoic or Palaeozoic bran tub. The most we can normally hope for is a series of fossils that, some with respect to one part, others with respect to another part, represent the kind kind of thing the ancestors probably looked like. Perhaps this fossil tells us something about our ancestors' teeth, while that fossil a few million years later gives us an inkling about our ancestors' arms. Any particular fossil is almost certainly not our ancestor but, with luck, some parts of it may resemble the corresponding parts of the ancestor just as, today, the shoulder-blade of a leopard is a reasonable approximation to the shoulder-blade of a puma. of thing the ancestors probably looked like. Perhaps this fossil tells us something about our ancestors' teeth, while that fossil a few million years later gives us an inkling about our ancestors' arms. Any particular fossil is almost certainly not our ancestor but, with luck, some parts of it may resemble the corresponding parts of the ancestor just as, today, the shoulder-blade of a leopard is a reasonable approximation to the shoulder-blade of a puma.

1 In the Garstang poem, 'his' gonads doesn't refer to the lancelet's but to the 'ammocoete' larva of a lamprey. In the Garstang poem, 'his' gonads doesn't refer to the lancelet's but to the 'ammocoete' larva of a lamprey.