The Ancestor's Tale Part 8

2 A couple of teeth which seem to belong to condylarths (a group of extinct placental mammals) have been found, but nothing younger than 55 million years. A couple of teeth which seem to belong to condylarths (a group of extinct placental mammals) have been found, but nothing younger than 55 million years.

3 These more-or-less self-evident terms have become technical terms for taxonomists who habitually lump animals (or plants) into a few large groups, or who habitually split them into lots of small groups. Splitters proliferate names, in extreme cases where fossils are concerned, elevating almost every specimen they discover to species status. These more-or-less self-evident terms have become technical terms for taxonomists who habitually lump animals (or plants) into a few large groups, or who habitually split them into lots of small groups. Splitters proliferate names, in extreme cases where fossils are concerned, elevating almost every specimen they discover to species status.

4 Necrolestes Necrolestes, a South American marsupial of the Miocene Epoch, also appears to have been a 'mole'. Its name, rather inappropriately, translates as 'grave robber'.

Rendezvous 15.

MONOTREMES.

Rendezvous 15 takes place approximately 180 million years ago in the half-monsoonal, half-arid world of the Lower Jura.s.sic. The southern continent of Gondwana was still just about connected to the great northern continent of Laurasia the first time on our backwards journey that we find all major land-ma.s.ses collected into a contiguous 'Pangaea'. In forward time, the split of Pangaea would have momentous consequences for the descendants of Concestor 15, perhaps our 120-million-greats-grandparent. Our rendezvous is a rather one-sided affair. The new pilgrims that join all the rest of the mammals here represent only three genera: takes place approximately 180 million years ago in the half-monsoonal, half-arid world of the Lower Jura.s.sic. The southern continent of Gondwana was still just about connected to the great northern continent of Laurasia the first time on our backwards journey that we find all major land-ma.s.ses collected into a contiguous 'Pangaea'. In forward time, the split of Pangaea would have momentous consequences for the descendants of Concestor 15, perhaps our 120-million-greats-grandparent. Our rendezvous is a rather one-sided affair. The new pilgrims that join all the rest of the mammals here represent only three genera: Ornithorhynchus anatinus Ornithorhynchus anatinus, the duckbilled platypus which lives in Eastern Australia and Tasmania; Tachyglossus aculeatus Tachyglossus aculeatus, the short-beaked echidna which lives all over Australia and New Guinea; and Zaglossus Zaglossus, the long-beaked echidna, which is confined to the highlands of New Guinea.1 Collectively the three genera are known as monotremes. Collectively the three genera are known as monotremes.

Several tales have developed the theme of island continents as nurseries of major animal groups: Africa for the afrotheres, Laurasia for the laurasiatheres, South America for the xenarthrans, Madagascar for the lemurs, Australia for most of the surviving marsupials. But it is looking increasingly as though there was a much earlier continental separation among the mammals. According to one supported theory, long before the demise of the dinosaurs, the mammals were split into two major groups called the australosphenidans and the boreosphenidans. Australo, once again, doesn't mean Australian, it means southern. And boreo means northern, as in the northern aurora borealis. The australosphenidans were those early mammals that evolved in the great southern continent of Gondwana. And the boreosphenidans evolved in the northern continent of Laurasia, in a sort of earlier incarnation long before the evolution of the laurasiatheres we know today. The monotremes are the only surviving representatives of the australosphenidans. All the rest of the mammals, the therians, including the marsupials that we now a.s.sociate with Australia, are descended from the northern boreosphenidans. Those therians who later became a.s.sociated with the south, and with the breakup of Gondwana for instance the afrotheres of Africa and the marsupials of South America and Australia were boreosphenidans who had migrated south into Gondwana long after their northern origins.

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Monotremes join. Living mammals, numbering fewer than 5,000 species, all have fur and suckle their young. Those we have met so far the placental and marsupial mammals are thought to share a common northern hemisphere origin in the Jura.s.sic Period. The five monotreme species are the sole survivors of a once diverse lineage of southern hemisphere mammals which retained the habit of laying eggs. Living mammals, numbering fewer than 5,000 species, all have fur and suckle their young. Those we have met so far the placental and marsupial mammals are thought to share a common northern hemisphere origin in the Jura.s.sic Period. The five monotreme species are the sole survivors of a once diverse lineage of southern hemisphere mammals which retained the habit of laying eggs.

Images, left to right: duck-billed platypus ( duck-billed platypus (Ornithorhynchus anatinus); short-beaked echidna (Tachyglossus aculeatus) Let's now turn to the monotremes themselves. The echidnas live on dry land and eat ants and termites. The platypus lives mostly in water where it feeds on small invertebrates in the mud. Its 'bill' really does look like that of a duck. The echidnas' bill is more tubular. Somewhat surprisingly, by the way, molecular evidence suggests that the concestor of echidnas with platypuses lived more recently than the fossil platypus Obdurodon Obdurodon, which lived and looked essentially like a modern platypus except that it had teeth inside its duckbill. This would mean that echidnas are modified platypuses who left the water within the last 20 million years, lost the webbing between their toes, narrowed the duckbill to make an anteater's probing tube, and developed protective spines.

One respect in which the monotremes resemble reptiles and birds has given them their name. Monotreme means single hole in Greek. As with reptiles and birds, the a.n.u.s, the urinary tract and the reproductive tract empty into a single shared opening, the cloaca. Even more reptilian is that eggs, not babies, emerge from that cloaca. And not microscopic eggs like all other mammals, but two-centimetre eggs with a tough white leathery sh.e.l.l, containing nutriment to feed the baby until it is ready to hatch, which it eventually does like a reptile or bird with the aid of an egg-tooth on the end of its 'bill'.

Monotremes have some other typically reptilian features too, such as the interclavicle bone near the shoulder, which reptiles, but no therian mammals, possess. On the other hand the monotreme skeleton also has a number of standard mammal traits. Their lower jaw consists of a single bone, the dentary. Reptile lower jaws have three additional bones, around the hinge with the main skull. During the evolution of the mammals, these three bones migrated away from the lower jaw into the middle ear, where, renamed the hammer, the anvil and the stirrup, they transmit sound from the eardrum to the inner ear in a cunning way that physicists call impedance-matching. Monotremes are firmly with the mammals on this point. Their inner ear itself, however, is more reptilian or bird-like, in that the cochlea, the tube in the inner ear that detects sounds of different pitch, is more nearly straight than the snail-shaped coil which all other mammals have, and which gives the organ its name.

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Could your ancestor have looked like this? Drawing of Drawing of Henkelotherium Henkelotherium, a eupantothere, by Elke Groning. (The leaf form shown is that of modern ginkgos; the leaves of Jura.s.sic ginkgos would have been more finely divided.) Monotremes are again with the mammals in secreting milk for their young: that most proverbially mammalian of substances. But again, they slightly spoil the effect by lacking discrete nipples. Instead, the milk oozes out from pores over a wide area of skin on the ventral surface, where it is licked up by the baby clinging to the hairs on the mother's belly. Our ancestors probably did the same. Monotreme limbs sprawl sideways a little more than those of a typical mammal. You can see this in the weird rolling gait of echidnas: not quite lizard-like, but not entirely mammal-like either. It adds to the impression that the monotremes are sort of intermediate between reptiles and mammals.

What did Concestor 15 look like? There is of course no reason to think it was like either an echidna or a platypus. It was, after all, our our ancestor, as well as theirs, and we've all had a very long time to evolve since. Fossils of the right vintage in the Jura.s.sic Period belong to various types of small shrew-like or rodent-like animals such as ancestor, as well as theirs, and we've all had a very long time to evolve since. Fossils of the right vintage in the Jura.s.sic Period belong to various types of small shrew-like or rodent-like animals such as Morganucodon Morganucodon and the large group known as mult.i.tuberculates. The charming picture on page 241 is of another of these early mammals, a eupantothere, up a ginkgo tree. and the large group known as mult.i.tuberculates. The charming picture on page 241 is of another of these early mammals, a eupantothere, up a ginkgo tree.

THE DUCKBILL'S TALE.

An early Latin name of the platypus was Ornithorhynchus paradoxus Ornithorhynchus paradoxus. It seemed so weird when first discovered that a specimen sent to a museum was thought to be a hoax: bits of mammal and bits of bird st.i.tched together. Others have wondered whether G.o.d was having a bad day when he created the platypus. Finding some spare parts left over on the workshop floor, he decided to unite rather than waste them. More insidiously (because they aren't joking) some zoologists write the monotremes off as 'primitive', as though sitting around being primitive was a full-time way of life. To question this is a purpose of the Duckbill's Tale.

Since Concestor 15, platypuses have had exactly the same time to evolve as the rest of the mammals. There is no reason why either group should be more primitive than the other (primitive, remember, precisely means 'resembling the ancestor'). Monotremes might be more primitive than us in some respects, such as laying eggs. But there is no reason at all why primitiveness in one respect should dictate primitiveness in another. There is no substance called Essence of Antiquity that pervades the blood and soaks into the bones. A primitive bone is a bone that has not changed much for a long time. There is no rule that says the neighbouring bone has to be primitive too, not even a faint presumption in that direction at least unless a further case is made. There's no better ill.u.s.tration than the eponymous duck bill itself. It has evolved far, even if other parts of the platypus have not.

The platypus bill seems comical, its resemblance to that of a duck made the more incongruous by its relatively large size, and also because a duck's bill has a certain intrinsic laughableness, perhaps borrowed from Donald. But humour does an injustice to this wondrous apparatus. If you want to think in terms of an incongruous graft, forget all about ducks. A more telling comparison is the extra nose grafted onto a Nimrod reconnaissance aircraft. The American equivalent is AWACS, more familiar but less appropriate for my comparison in that the AWACS 'graft' is on top of the fuselage rather than at the front like a bill.

The point is that the platypus bill is not just a pair of jaws for dabbling and feeding, as in a duck. It is that too, though it is rubbery rather than h.o.r.n.y like a duck's bill. But far more interestingly, the platypus bill is a reconnaissance device, an AWACS organ. Platypuses hunt crustaceans, insect larvae and other small creatures in the mud at the bottom of streams. Eyes aren't much use in mud, and the platypus keeps them tight shut while hunting. Not only that, it closes its nostrils and its ears as well. See no prey, hear no prey, smell no prey: yet it finds prey with great efficiency, catching half its own weight in a day.

If you were a sceptical investigator of somebody claiming a 'sixth sense', what would you do? You'd blindfold him, stop his ears and his nostrils, and then set him some task of sensory perception. Platypuses go out of their way to do the experiment for you. They switch off three senses which are important to us (and perhaps to them on land), as if to concentrate all their attention on some other sense. And the clue is given by one further feature of their hunting behaviour. They swing the bill in movements called saccades, side to side, as they swim. It looks like a radar dish scanning ...

One of the first scientific descriptions of the platypus, Sir Everard Home's publication in the Philosophical Transactions of the Royal Society Philosophical Transactions of the Royal Society for 1802, was farsighted. He noticed that the branch of the trigeminal nerve that innervates the face is for 1802, was farsighted. He noticed that the branch of the trigeminal nerve that innervates the face is uncommonly large. We should be led by this circ.u.mstance to believe that the sensibility of the different parts of the bill is very great, and therefore that it answers the purpose of a hand, and is capable of nice discrimination in its feeling.Sir Everard didn't know the half of it. It's the reference to a hand that tells. The great Canadian neurologist Wilder Penfield published a famous picture of a human brain, together with a diagram showing the proportions given over to different parts of the body. The map of a part of the brain given over to controlling muscles in different parts of the body, on one side, is shown on page 244. Penfield made a similar map of parts of the brain concerned with the sense of touch in different parts of the body. The striking thing about both maps is the huge prominence given to the hand. The face, too, is prominent, especially the parts controlling jaw movements, in chewing and speaking. But it is the hand that you really notice when you see a Penfield 'homunculus'. The image reproduced in plate 13 is another way of representing the same thing. This grotesque has his body distorted in proportion to the amount of brain given over to different parts. Again it shows that the human brain is hand-heavy.

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Penfield brain map. Adapted from Penfield and Rasmussen [ Adapted from Penfield and Rasmussen [222].

Where is all this leading? My account of the Duckbill's Tale is indebted to the distinguished Australian neurobiologist Jack Pettigrew and his colleagues, including Paul Manger, and one of the fascinating things they did was to prepare a 'platypunculus', the platypus equivalent of a Penfield homunculus. The first thing to say is that it is far more accurate than the Penfield homunculus, which was based on very scanty data. The platypunculus is a very thorough piece of work. You can see three little platypus maps on the upper part of the brain: separate representations, in different parts of the brain, of sensory information from the body surface. What matters to the animal is that there is an orderly spatial mapping between each part of the body and the corresponding part of the brain.

Notice that the hands and feet, coloured black on the three maps, are approximately in proportion to the body itself, unlike the case of the Penfield homunculus with its vast hands. What is not in proportion in the platypunculus is the bill. The bill's maps are the huge areas reaching down from the maps of the rest of the body. Where the human brain is hand-heavy, the platypus brain is bill-heavy (see plate 14) (see plate 14). Sir Everard Home's guess is looking good. But, as we shall see, in one respect the bill is even better than a hand: it can reach out and 'feel' things that it is not touching. It can feel at a distance. It does it by electricity.

When any animal, such as a freshwater shrimp which is a typical platypus prey, uses its muscles, weak electric fields are inevitably generated. With sufficiently sensitive apparatus these can be detected, especially in water. Given dedicated computer power to handle data from a large array of such sensors, the source of the electric fields can be calculated. Platypuses don't, of course, calculate as a mathematician or a computer would. But at some level in their brain the equivalent of a calculation is done, and the result is that they catch their prey.

Platypuses have about 40,000 electrical sensors distributed in longitudinal stripes over both surfaces of the bill. As the platypunculus shows, a large proportion of the brain is given over to processing the data from these 40,000 sensors. But the plot thickens. In addition to the 40,000 electrical sensors, there are about 60,000 mechanical sensors called push rods, scattered over the surface of the bill. Pettigrew and his co-workers have found nerve cells in the brain that receive inputs from mechanical sensors. And they have found other brain cells that respond to both electrical and mechanical sensors (so far they have found no brain cells that respond to electrical sensors only). Both kinds of cell occupy their correct position on the spatial map of the bill, and they are layered in a way that is reminiscent of the human visual brain, where layering a.s.sists binocular vision. Just as our layered brain combines information from the two eyes to construct a stereo percept, the Pettigrew group suggests that the platypus might be combining the information from electrical and mechanical sensors in some similarly useful way. How might this be done?

They propose the a.n.a.logy of thunder and lightning. The flash of lightning and the crack of thunder happen at the same moment. We see the lightning instantaneously, but the thunder takes longer to reach us, travelling at the relatively slow speed of sound (and incidentally the bang becomes a rumble because of echoes). By timing the lag between lightning and thunder, we can calculate how far away the storm is. Perhaps the electrical discharges from the prey's muscles are the platypus's lightning, while the thunder is the waves of disturbance in the water caused by the prey animal's movements. Is the platypus brain set up to compute the time lag between the two, and hence calculate how far away the prey is? It seems likely.

As for pinpointing the prey's direction, this must be done by comparing the inputs from different receptors all over the map, presumably aided by the scanning side-to-side movements of the bill, just as a man-made radar uses the rotation of the dish. With such a huge array of sensors projecting to mapped arrays of brain cells, the platypus very likely forms a detailed three-dimensional image of any electrical disturbances in its vicinity.

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Remote pins and needles. The electric sensory world of the platypus. From Manger and Pettigrew [ The electric sensory world of the platypus. From Manger and Pettigrew [181].

Pettigrew and his colleagues prepared this contour map of lines of equal electrical sensitivity around the bill of the platypus. When you think of a platypus, forget duck, think Nimrod, think AWACS; think huge hand feeling its way, by remote pins and needles; think lightning flashing and thunder rumbling, through the watery mud of Australia.

The platypus is not the only animal to use this kind of electrical sense. Various fish do it, including paddlefish such as Polyodon spathula Polyodon spathula. Technically 'bony' fish, paddlefish have secondarily, with their relatives the sturgeons, evolved a cartilaginous skeleton like a shark. Unlike sharks, however, paddlefish live in freshwater, often turbid rivers where again eyes are not much use. The 'paddle' is shaped pretty much like the upper jaw of a platypus's bill, though it is not a jaw at all but an extension of the cranium. It can be extremely long, often as much as one-third of the body length. It reminds me of a Nimrod aircraft even more than the platypus does.

The paddle is obviously doing something important in the life of the fish, and it has in fact been clearly demonstrated that it is doing the same job as the platypus bill detecting electric fields from prey animals. As with the platypus, the electrical sensors are set into pores deployed in longitudinal lines. The two systems are independently evolved, however. Platypus electrical pores are modified mucus glands. Paddlefish electric pores are so similar to the pores used by sharks for electrical sensing, called ampullae of Lorenzini, that they have been given the same name. But where the platypus arranges its sensory pores in a dozen or so narrow stripes along the length of the bill, the paddlefish has two broad stripes, on either side of the midline of the paddle. Like the platypus, the paddlefish has an enormous number of sensory pores actually even more than the platypus. Both the paddlefish and the platypus are far more sensitive to electricity than any one of their sensors by itself. They must be doing some sort of sophisticated signal summation from different sensors.

There is evidence that the electrical sense is more important for juvenile paddlefish than for adults. Adults who have accidentally lost their paddle have been found alive and apparently healthy, but no juveniles have been found to survive up any creek without a paddle. This may be because juvenile paddlefish, like adult platypuses, target and catch individual prey. Adult paddlefish feed more like planktivorous baleen whales, sieving their way through the mud, catching prey en ma.s.se en ma.s.se. They grow big on this diet, too not as big as whales, but as long and as heavy as a man, larger than most animals that swim in freshwater. Presumably if you are sieving plankton as an adult, you have less need of an accurate prey-locator than if you are darting after individual prey as a juvenile.

Platypus and paddlefish, then, have independently hit upon the same ingenious trick (see plate 15) (see plate 15). Has any other animal discovered it? Whilst doing his D.Phil. work in China, my research a.s.sistant Sam Turvey encountered an extremely unusual trilobite called Reedocalymene Reedocalymene. Otherwise a 'bog-standard' trilobite (similar to the Dudley Bug, Calymene Calymene, which features on the coat of arms of the town of Dudley), Reedocalymene Reedocalymene has one unique and remarkable feature: a huge flattened rostrum, like that of a paddlefish, sticking out a whole body length in front. It can't have been for streamlining, since this trilobite, unlike many others, was obviously unfitted for swimming above the sea bed. A defensive purpose is also unlikely for various reasons. Like a paddlefish, sturgeon or platypus bill, the trilobite's rostrum is studded with what look like sensory receptors, probably used for detecting prey. Turvey knows of no modern arthropods with an electrical sense (interesting in itself, given the versatility of the arthropods), but he would put money on has one unique and remarkable feature: a huge flattened rostrum, like that of a paddlefish, sticking out a whole body length in front. It can't have been for streamlining, since this trilobite, unlike many others, was obviously unfitted for swimming above the sea bed. A defensive purpose is also unlikely for various reasons. Like a paddlefish, sturgeon or platypus bill, the trilobite's rostrum is studded with what look like sensory receptors, probably used for detecting prey. Turvey knows of no modern arthropods with an electrical sense (interesting in itself, given the versatility of the arthropods), but he would put money on Reedocalymene Reedocalymene being yet another 'paddlefish' or 'platypus'. He is hoping to start work on it soon. being yet another 'paddlefish' or 'platypus'. He is hoping to start work on it soon.

Other fish, though lacking the Nimrod-like 'antenna' of the platypus and the paddlefish, have an even more sophisticated electrical sense. Not content with picking up electrical signals inadvertently given off by prey, these fish generate their own electric fields. They navigate and detect prey by reading the distortions in these self-generated fields. Along with various cartilaginous rays, two groups of bony fish, the gymnotid family of South America and the mormyrid family of Africa, have independently developed this to a high art.

How do these fish make their own electricity? The same way the shrimps and insect larvae and other prey of the platypus inadvertently do it: with their muscles. But whereas the shrimps can't help making a little electricity because that is what muscles just do, the electric fish gang their blocks of muscle together just like batteries in series.2 A gymnotid or mormyrid electric fish has a battery of muscle blocks arranged in series along its tail, each generating a low voltage and adding up to a higher voltage. The electric eel (not a true eel but another South American freshwater gymnotid) takes it to an extreme. It has a very long tail into which it can pack a much larger battery of electrical cells than a fish of normal length. It stuns its prey with electric shocks which may exceed 600 volts and can be fatal to people. Other freshwater fish, such as the African electric catfish A gymnotid or mormyrid electric fish has a battery of muscle blocks arranged in series along its tail, each generating a low voltage and adding up to a higher voltage. The electric eel (not a true eel but another South American freshwater gymnotid) takes it to an extreme. It has a very long tail into which it can pack a much larger battery of electrical cells than a fish of normal length. It stuns its prey with electric shocks which may exceed 600 volts and can be fatal to people. Other freshwater fish, such as the African electric catfish Malapterurus Malapterurus and the marine electric ray and the marine electric ray Torpedo Torpedo also generate enough volts to kill, or at least knock out, their prey. also generate enough volts to kill, or at least knock out, their prey.

These high-voltage fish seem to have pushed, to a literally stunning extreme, a capacity which was originally a kind of radar used by the fish to find its way around and detect prey. Weakly electric fish such as the South American Gymnotus Gymnotus and the unrelated African and the unrelated African Gymnarchus Gymnarchus have an electrical organ like the electric eel's but much shorter their battery consists of fewer modified muscle plates in series and a weakly electric fish typically generates less than one volt. The fish holds itself like a rigid stick in the water, for a very good reason as we shall see, and electric current flows along curved lines that would have delighted Michael Faraday. All along the sides of the body are pores containing electrical sensors tiny voltmeters. Obstacles or prey items distort the field in various ways, which are detected by these little voltmeters. By comparing the readings of the different voltmeters and correlating them with the fluctuations of the field itself (sinusoidal in some species, pulsed in others) the fish can calculate the location of obstacles and prey. They also use their electric organs and sensors to communicate with one another. have an electrical organ like the electric eel's but much shorter their battery consists of fewer modified muscle plates in series and a weakly electric fish typically generates less than one volt. The fish holds itself like a rigid stick in the water, for a very good reason as we shall see, and electric current flows along curved lines that would have delighted Michael Faraday. All along the sides of the body are pores containing electrical sensors tiny voltmeters. Obstacles or prey items distort the field in various ways, which are detected by these little voltmeters. By comparing the readings of the different voltmeters and correlating them with the fluctuations of the field itself (sinusoidal in some species, pulsed in others) the fish can calculate the location of obstacles and prey. They also use their electric organs and sensors to communicate with one another.

A South American electric fish such as Gymnotus Gymnotus is remarkably similar to is remarkably similar to Gymnarchus Gymnarchus, its African opposite number, but there is one revealing difference. Both have a single long fin running the length of the midline, and both use it for the same purpose. They can't throw the body into the normal sinuous waves of a swimming fish because it would distort their electrical sense. Both are obliged to keep the body rigid, so they swim by means of the longitudinal fin, which waves sinuously just like a normal fish should. It means they swim slowly, but presumably it is worth it to get the benefits of a good clear signal. The beautiful fact is that Gymnarchus Gymnarchus has its longitudinal fin on its back, while has its longitudinal fin on its back, while Gymnotus Gymnotus and the other South American electric fish, including the electric 'eel', keep their longitudinal fin on their belly. It is for such cases that 'the exception that proves the rule' was coined. and the other South American electric fish, including the electric 'eel', keep their longitudinal fin on their belly. It is for such cases that 'the exception that proves the rule' was coined.

Returning to the platypus, the sting in the tale is actually in the hind claws of the male platypus. True venomous stings, with hypodermic injection, are found in various invertebrate phyla, and in fish and reptiles among vertebrates but never in birds or mammals other than the platypus (unless you count the toxic saliva of solenodons and some shrews that makes their bites slightly venomous). Among mammals, the male platypus is in a cla.s.s of its own, and it may be in a cla.s.s of its own among venomous animals too. The fact that the sting is found only in males suggests, rather surprisingly, that it is aimed not at predators (as in bees) nor at prey (as in snakes) but at rivals. It is not dangerous but is extremely painful, and is unresponsive to morphine. It looks as though platypus venom works directly on pain receptors themselves. If scientists could understand how this is done, there is a hope that it might give a clue to how to resist the pain caused by cancer.

This tale began by chiding those zoologists who call the platypus 'primitive' as though that were any kind of explanation for the way it is. At best it is a description. Primitive means 'resembling the ancestor' and there are many respects in which this is a fair description of a platypus. The bill and the sting are interesting exceptions. But the more important moral of the tale is that even an animal that is genuinely primitive in all respects is primitive for a reason. The ancestral characteristics are good for its way of life, so there is no reason to change. As Professor Arthur Cain of Liverpool University liked to say, an animal is the way it is because it needs to be.

WHAT THE STAR-NOSED MOLE SAID TO THE DUCKBILLED PLATYPUS.

The star-nosed mole, who had joined the pilgrimage along with the other laurasiatheres at Rendezvous 11 Rendezvous 11, listened to the Duckbill's Tale with close attention, and with growing recognition in what was left of his vestigial, pin-p.r.i.c.k eyes. 'Yes!' he squeaked, too high for some of the larger pilgrims to hear, and he clapped his spades with excitement. 'That's just the way it is for me ... well, sort of.'

No, it won't do, I wanted to follow Chaucer in having at least one section devoted to what one pilgrim said to another, but I'll limit it to the heading and first paragraph, and now revert to my practice of telling the tale itself in my own words. Bruce Fogle (101 Questions Your Dog Would Ask Its Vet) or Olivia Judson (Dr Tatiana's s.e.x Advice to All Creation) might get away with it, but not me.

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Pushes the envelope of touch beyond our dreams. An in-your-face view of a star-nosed mole, An in-your-face view of a star-nosed mole, Condylura cristata Condylura cristata.

The star-nosed mole, Condylura cristata Condylura cristata, is a North American mole which, in addition to burrowing and hunting for worms like other moles, is a good swimmer too, hunting for underwater prey it often tunnels deep into river banks. It is also more at home above ground than other moles, where it still prefers damp, soggy places. It has large spade hands like other moles.

What sets it apart is the remarkable nose that gives it its name. Surrounding the two forwards-pointing nostrils, there is an extraordinary ring of fleshy tentacles, like a baby sea anemone with 22 arms. The tentacles are not used to grasp things. Nor are they an aid to smelling, which is the next hypothesis that might occur to us. Nor, despite the beginning of this section, are they an electrical radar like that of the platypus. Their true nature has been beautifully worked out by Kenneth Catania and Jon Kaas of Vanderbilt University, Tennessee. The star is a touch-sensitive organ, like a super-sensitive human hand, but lacking the grasping function of the hand and emphasising its sensitivity instead. But it isn't just any ordinary touch-sensitive organ. The star-nosed mole pushes the envelope of touch beyond our dreams. The skin of its nose is more sensitive than any other area of skin anywhere among the mammals, not excluding the human hand.

There are 11 tentacles arcing round each nostril, labelled 1 to 11 in order. Tentacle 11, which lies close to the midline and just below the level of the nostril, is special, as we shall see in a moment. Although they are not used for grasping, the tentacles are moved, independently or in particular groupings. The surface of each tentacle is carpeted with a regular array of little round b.u.mps called Eimer's Organs, each one a unit of touch sensitivity, and each one wired up by between seven nerve fibres (for tentacle 11) and four nerve fibres (most of the other tentacles).

The density of Eimer's organs is the same for all tentacles. Tentacle 11, being smaller, has fewer of them, but it has more nerves supplying each one. Catania and Kaas were able to map the tentacles to the brain. They found (at least) two independent maps of the nose star in the cerebral cortex. In each of these two brain areas, the parts of the brain corresponding to each tentacle are laid out in order. And tentacle 11 again is special. It is more sensitive than the rest. Once an object has been first detected by any of the tentacles, the animal then moves the star so that tentacle 11 can examine it carefully. Only then is the decision taken whether to eat it or not. Catania and Kaas refer to tentacle 11 as the 'fovea' of the star.3 More generally, they say: More generally, they say: Although the nose of the star-nosed mole acts as a tactile sensory surface, there are anatomical and behavioral similarities between the mole's sensory system and the visual system of other mammals.

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You can see where its priorities lie. 'Molunculus' brain map of the star-nosed mole. From Catania and Kaas [ 'Molunculus' brain map of the star-nosed mole. From Catania and Kaas [41].

If the star is not an electrical sensor, whence the empathy with the platypus with which I opened this section? Catania and Kaas constructed a schematic model of the relative amount of brain tissue given over to different parts of the body surface. It is a molunculus, by a.n.a.logy with Penfield's homunculus and Pettigrew's platypunculus. And just look at it!4 You can see where the star-nosed mole's priorities lie. You can get a feel for the world of the star-nosed mole. And feel feel is the right word. This animal lives in a tactile world, dominated by the tentacles of the nose, with a subsidiary interest in the large spade hands and the whiskers. is the right word. This animal lives in a tactile world, dominated by the tentacles of the nose, with a subsidiary interest in the large spade hands and the whiskers.

What is it like to be a star-nosed mole? I am tempted to propose the star-nosed counterpart to an idea I once offered for bats. Bats live in a world of sound, but what they do with their ears is pretty much the same as what, say, insect-hawking birds like swallows do with their eyes. In both cases the brain needs to construct a mental model of a three-dimensional world, to be navigated at high speed, with obstacles to be avoided and small moving targets to catch. The model of the world needs to be the same, whether it is constructed and updated with the aid of light rays or sound echoes. My conjecture was that a bat probably 'sees' the world (using echoes) in pretty much the same way as a swallow, or a person, sees the world using light.

I even went so far as to speculate that bats hear in colour. The hues that we perceive have no necessary link with the particular wavelengths of light that they represent. The sensation that I call red (and n.o.body knows if my red is the same as yours) is an arbitrary label for light of long wavelengths. It could equally well have been used for short wavelengths (blue), and the sensation that I call blue used for long wavelengths. Those hue sensations are available in the brain for tying to whatever, in the outside world, is most convenient. In bat brains those vivid qualia would be wasted on light. They are more likely to be used as labels tied to particular qualities of echo, perhaps textures of surfaces on obstacles or prey.

My conjecture now is that a star-nosed mole 'sees' with its nose. And my speculation is that it uses those same qualia that we call colour, as labels for tactile sensations. Similarly, I want to guess that duckbilled platypuses 'see' with the bill, and use the qualia we call colour as internal labels for electrical sensations. Could this be why platypuses close their eyes tight shut when they are hunting electrically with the bill? Could it be because the eyes and the bill are competing, in the brain, for internal qualia labels, and to use both senses at once would lead to confusion?

1 Three species of Three species of Zaglossus Zaglossus have been distinguished, one of them called, I am delighted to say, have been distinguished, one of them called, I am delighted to say, Z. attenboroughi Z. attenboroughi.

2 Of course, the word battery in its original electrical sense means a battery of cells in series, as opposed to a single cell. If your transistor radio takes six 'batteries', a pedant would insist that it takes one battery of six cells. Of course, the word battery in its original electrical sense means a battery of cells in series, as opposed to a single cell. If your transistor radio takes six 'batteries', a pedant would insist that it takes one battery of six cells.

3 The fovea is the small area in the middle of the human retina where cone cells are concentrated so that acuity, and colour vision, are both maximal. We read with our fovea, recognise each other's faces, and do everything that needs fine visual discrimination. The fovea is the small area in the middle of the human retina where cone cells are concentrated so that acuity, and colour vision, are both maximal. We read with our fovea, recognise each other's faces, and do everything that needs fine visual discrimination.

4 Note that parts of the 'molunculus' are hidden behind parts that we can see. Note that parts of the 'molunculus' are hidden behind parts that we can see.

MAMMAL-LIKE REPTILES.

The monotremes having joined us, the entire company of mammal pilgrims now walks back 130 million unbroken years, the longest gap yet between any two milestones, to Rendezvous 16 Rendezvous 16 where we are to meet an even larger band of pilgrims than our own, the sauropsids: reptiles and birds. That pretty much means all vertebrates that lay large eggs with a waterproof sh.e.l.l on land. I have to say 'pretty much', partly because monotremes, who have already joined us, also lay that kind of egg. Even turtles that are otherwise wholly marine haul themselves up the beach to lay their eggs. Plesiosaurs may have done the same. Ichthyosaurs, however, were so specialised for swimming that, like the dolphins who later resembled them, they presumably couldn't come on sh.o.r.e at all. They independently discovered how to give birth to live young as we know from mothers fossilised in the act. where we are to meet an even larger band of pilgrims than our own, the sauropsids: reptiles and birds. That pretty much means all vertebrates that lay large eggs with a waterproof sh.e.l.l on land. I have to say 'pretty much', partly because monotremes, who have already joined us, also lay that kind of egg. Even turtles that are otherwise wholly marine haul themselves up the beach to lay their eggs. Plesiosaurs may have done the same. Ichthyosaurs, however, were so specialised for swimming that, like the dolphins who later resembled them, they presumably couldn't come on sh.o.r.e at all. They independently discovered how to give birth to live young as we know from mothers fossilised in the act.1 I said that our pilgrims walked through 130 million years without milestones, but of course 'without milestones' is true only within the conventions of this book: we are recognising as milestones only rendezvous with living pilgrims. Our ancestral lineage indulged in fertile evolutionary branching during that time, as we know from the rich fossil record of 'mammal-like reptiles', but none of the branches among these counts as a 'rendezvous' because, as it turned out, none of them survived. There are therefore no modern representatives to set off as pilgrims from the present. When we met a similar problem with the hominids, we decided to give certain fossils honorary status as 'shadow pilgrims'. Since we are pilgrims seeking our ancestors, pilgrims who actually want to know what our 100-million-greats-grandparent looked like, we cannot ignore the mammal-like reptiles and jump straight to Concestor 16. Concestor 16, as we shall see, looked like a lizard. The gap from Concestor 15, which looked like a shrew, is too great to leave unbridged. We have to examine the mammal-like reptiles as shadow pilgrims, as though they were living pilgrims joining our march although they shall not actually tell tales. But first, some background information on the timespan involved, because it is very long.

The intervening years without rendezvous milestones span half the Jura.s.sic, the whole of the Tria.s.sic, the whole of the Permian and the final 10 million years of the Carboniferous. As the pilgrimage moves from the Jura.s.sic back into the hotter and drier world of the Tria.s.sic one of the hottest periods in the planet's history, when all the landma.s.ses were joined together, forming Pangaea we pa.s.s the late Tria.s.sic ma.s.s extinction, when three-quarters of all species went extinct. But this is nothing compared to the next transition, from the Tria.s.sic Period back into the Permian. At the PermoTria.s.sic boundary, a staggering 90 per cent of all species perished without descendants, including all the trilobites and several other major groups of animals. The trilobites, to be fair, had already been declining over a long period. But the end-Permian ma.s.s extinction was the most devastating of all time. There is some evidence from Australia that this extinction, like the Cretaceous one, was caused by a ma.s.sive bolide collision. Even the insects took a severe knock, the only one in their history. At sea, bottom-dwelling communities were almost wiped out. On land, the Noah among the mammal-like reptiles was Lystrosaurus Lystrosaurus. Immediately after the catastrophe, the squat, short-tailed Lystrosaurus Lystrosaurus became extremely abundant over the whole world, rapidly occupying vacant niches. became extremely abundant over the whole world, rapidly occupying vacant niches.

The natural a.s.sociation with apocalyptic carnage needs to be tempered. Extinction is the eventual fate of nearly all species. Perhaps 99 per cent of all species that have ever existed have gone extinct. Nevertheless, the rate of extinctions per million years is not fixed and only occasionally rises above 75 per cent, the threshold arbitrarily recognised for a 'ma.s.s' extinction. Ma.s.s extinctions are spikes in the rate of extinction, rising above the background rate.

The diagram on the next page shows rates of extinction per million years.2 Something happened at the time of those spikes. Something bad. Perhaps a single catastrophic event, such as the collision with a ma.s.sive celestial rock that killed the dinosaurs 65 million years ago in the CretaceousPalaeogene extinction. Or, in other cases among the five spikes, the agony may have been drawn out. What Richard Leakey and Roger Lewin have called the Sixth Extinction is the one now being perpetrated by Something happened at the time of those spikes. Something bad. Perhaps a single catastrophic event, such as the collision with a ma.s.sive celestial rock that killed the dinosaurs 65 million years ago in the CretaceousPalaeogene extinction. Or, in other cases among the five spikes, the agony may have been drawn out. What Richard Leakey and Roger Lewin have called the Sixth Extinction is the one now being perpetrated by h.o.m.o sapiens h.o.m.o sapiens or or h.o.m.o insipiens h.o.m.o insipiens as my old German teacher William Cartwright preferred to say. as my old German teacher William Cartwright preferred to say.3 [image]

Percent extinction of marine genera throughout the Phanerozoic Eon. Adapted from Sepkoski [ Adapted from Sepkoski [260].

Before we get to the mammal-like reptiles, we face a somewhat tiresome point of terminology. Terms like reptile and mammal can refer to 'clades' or 'grades' the two are not exclusive. A clade is a set of animals consisting of an ancestor and all its descendants. The 'birds' const.i.tute a good clade. 'Reptile', as traditionally understood, is not a good clade because it excludes birds. Biologists consequently refer to the reptiles as 'paraphyletic'. Some reptiles (e.g. crocodiles) are closer cousins of some non-reptiles (birds) than they are of other reptiles (turtles). To the extent that reptiles all have something in common, they are members of a grade grade, not a clade. A grade is a set of animals that have reached a similar stage in a recognisably progressive evolutionary trend.

Yet another informal grade name, favoured by American zoologists, is 'herp'. Herpetology is the study of reptiles (except birds) and amphibians. 'Herp' is a rare kind of word: an abbreviation for which there is no long form. A herp is simply the kind of animal studied by a herpetologist, and that is a pretty lame way to define an animal. The only other name that comes close is the biblical 'creeping thing'.

Another grade name is fish. 'Fish' include sharks, various extinct fossil groups, teleosts (bony fish such as trout and pike) and coelacanths. But trout are closer cousins to humans than they are to sharks (and coelacanths are even closer cousins to humans than trout are). So 'fish' is not a clade because it excludes humans (and all mammals, birds, reptiles and amphibians). Fish is a grade name for animals that sort of look fishy. It is more or less impossible to make grade terminology precise. Ichthyosaurs and dolphins look sort of fishy, and very possibly would taste fishy if we were to eat them, but they don't count as members of the fish 'grade', because they reverted reverted to fishiness via ancestors that were non-fishy. to fishiness via ancestors that were non-fishy.

Grade terminology works well for you if you have a strong belief in evolution marching progressively in one direction, in parallel lines from a shared starting point. If, say, you think that a whole lot of related lineages were all independently evolving in parallel from amphibianhood through reptilehood towards mammalhood, you could speak of pa.s.sing through the reptile grade on the way to the mammal grade. Something like that parallel march may have happened. It was the view that I was brought up with, by my own respected teacher of vertebrate palaeontology Harold Pusey. I have a lot of time for it, but it is not something to be taken for granted in general, nor necessarily enshrined in terminology.

If we swing to the other extreme and adopt strict cladistic terminology, the word reptile can be rescued only if it is deemed to include birds. This is the course favoured by the authoritative 'Tree of Life' project founded by the Maddison brothers.4 There's a lot to be said for following them, and also for the very different tactic of replacing 'mammal-like reptile' by 'reptile-like mammal'. But the word reptile has become so ingrained in its traditional sense that I fear it would confuse to change it now. Also, there are times when strict cladistic purism can give ludicrous results. Here's a There's a lot to be said for following them, and also for the very different tactic of replacing 'mammal-like reptile' by 'reptile-like mammal'. But the word reptile has become so ingrained in its traditional sense that I fear it would confuse to change it now. Also, there are times when strict cladistic purism can give ludicrous results. Here's a reductio ad absurdum reductio ad absurdum. Concestor 16 must have had an immediate descendant on the mammal side and an immediate descendant on the lizard/crocodile/dinosaur/bird, or 'sauropsid', side. These two must have been all but identical to each other. In fact there must have been a time when they could hybridise with each other. Yet the strict cladist would insist on calling one of them a sauropsid and the other one a mammal. Fortunately we don't often reach such a reductio reductio in practice, but such hypothetical cases are good to quote when cladistic purists start getting above themselves. in practice, but such hypothetical cases are good to quote when cladistic purists start getting above themselves.

We are so used to the idea of mammals as successors to the dinosaurs, that we may find it surprising that the mammal-like reptiles flourished before the rise of the dinosaurs. They filled the same range of niches as the dinosaurs were later to fill, and as the mammals themselves were to fill even later still. Actually they filled those niches not once but several times in succession, separated by large-scale extinctions. In the absence of milestones supplied by rendezvous with living pilgrims, I shall recognise three shadowy milestones to bridge the gap between the shrew-like Concestor 15 (which unites us to the monotremes) and the lizard-like Concestor 16 (which unites us to birds and dinosaurs).

Your 150-million-greats-grandmother might have been something a bit like a Thrinaxodon Thrinaxodon, which lived in the Middle Tria.s.sic and whose fossils have been found in Africa and Antarctica, then joined to each other within Gondwana. It is too much to hope that it was Thrinaxodon Thrinaxodon itself, or any other particular fossil that we happen to have found. itself, or any other particular fossil that we happen to have found. Thrinaxodon Thrinaxodon, like any fossil, should be thought of as a cousin of our ancestor, not the ancestor itself. It was a member of a group of mammal-like reptiles called the cynodonts. The cynodonts were so mammal-like, it is tempting to call them mammals. But who cares what we call them? They are almost perfect intermediates. Given that evolution has happened, it would be weird if there were not intermediates like the cynodonts.

The cynodonts were among several groups that radiated from an earlier group of mammal-like reptiles called the therapsids. Your 160-million-greats-grandfather was probably a therapsid, living in the Permian Period, but it is hard to pick out a particular fossil to represent it. The therapsids dominated the land trades before the dinosaurs arrived in the Tria.s.sic Period, and even in the Tria.s.sic itself they gave the dinosaurs a run for their money. They included some huge animals: herbivores three metres long, with large and probably ferocious carnivores to prey on them. But our therapsid ancestor was probably a smaller and more insignificant creature. It seems to be a rule that large or specialised animals, such as the fearsome fanged gorgonopsids or the tusk-bearing herbivorous dicynodonts (see plate 16) (see plate 16), don't have a long-term evolutionary future but belong to the 99 per cent of species destined for extinction. The Noah species, the one per cent from which we later animals are all descended whether we ourselves are large and spectacular in our own time or not tend to be smaller and more retiring.

The early therapsids were a bit less mammal-like than their successors, the cynodonts, but more mammal-like than their predecessors, the pelycosaurs, who const.i.tuted the early radiation of mammal-like reptiles. Before the therapsids, your 165-million-greats-grandmother was almost certainly a pelycosaur although, once again, it would be foolhardy to attempt to single out a particular fossil for that honour. The pelycosaurs were the earliest wave of mammal-like reptiles. They flourished in the Carboniferous Period, when the great coalfields were being laid down. The best-known pelycosaur is Dimetrodon Dimetrodon, the one with the great sail on its back. n.o.body knows how Dimetrodon Dimetrodon used its sail. It may have been a solar panel to help the animal warm up to a temperature where it could use its muscles, and/or perhaps it was a radiator to cool down in the shade, when things got too hot. Or it could have been a s.e.xual advertis.e.m.e.nt, a bony equivalent of a peac.o.c.k's fan. The pelycosaurs mostly went extinct during the Permian all except for the Noah-pelycosaurs who sprouted the second wave of mammal-like reptiles, the therapsids. The therapsids then spent the early part of the Tria.s.sic Period 'reinventing many of the lost body forms of the Late Permian'. used its sail. It may have been a solar panel to help the animal warm up to a temperature where it could use its muscles, and/or perhaps it was a radiator to cool down in the shade, when things got too hot. Or it could have been a s.e.xual advertis.e.m.e.nt, a bony equivalent of a peac.o.c.k's fan. The pelycosaurs mostly went extinct during the Permian all except for the Noah-pelycosaurs who sprouted the second wave of mammal-like reptiles, the therapsids. The therapsids then spent the early part of the Tria.s.sic Period 'reinventing many of the lost body forms of the Late Permian'.5 The pelycosaurs were considerably less mammal-like than the therapsids, which in turn were less mammal-like than the cynodonts. For example, the pelycosaurs sprawled on their bellies like lizards, with legs splayed out sideways. They probably had a fish-like wiggle to their gait. The therapsids, and then the cynodonts and finally the mammals, raised their bellies progressively higher off the ground their legs became more vertical and their gait less reminiscent of a fish on land. Other 'mammalisation' trends perhaps recognised as progressive only with the hindsight of the mammals that we are include the following. The lower jaw became reduced to a single bone, the dentary, as its other bones were commandeered by the ear (as discussed at Rendezvous 15 Rendezvous 15). At some point, though fossils aren't much help in pinning it down, our ancestors developed hair and a thermostat, milk and advanced parental care, and complex teeth specialised for different purposes.

I have dealt with the evolution of our mammal-like reptile anchors 'shadow pilgrims' as three successive waves: pelycosaurs, therapsids and cynodonts. The mammals themselves are the fourth wave, but their evolutionary invasion into the familiar range of ecotypes was postponed 150 million years. First, the dinosaurs had to have their go, which lasted twice as long as all three waves of mammal-like reptiles put together.

On our backwards march, the earliest of our three groups of 'shadow pilgrims' have brought us to a rather lizard-like pelycosaur 'Noah', our 165-million-greats-grandparent, who lived in the Tria.s.sic Period, about 300 million years ago. We have almost penetrated back to Rendezvous 16 Rendezvous 16.

1 Some extant lizards have also discovered live birth. Some extant lizards have also discovered live birth.

2 The absolute figures are lower than 75 per cent, because they refer to genera not species. Absolute figures for species are higher than for genera, because each genus contains lots of species so it's harder to extinguish a genus than a species. The absolute figures are lower than 75 per cent, because they refer to genera not species. Absolute figures for species are higher than for genera, because each genus contains lots of species so it's harder to extinguish a genus than a species.

3 A remarkable, bushy-browed, slow-spoken man who called a spade a spade and was seldom seen without one, Mr Cartwright discovered environmental activism long before its vogue, and filled his lessons with ecology, to the detriment of our German but the advantage of our humane education. A remarkable, bushy-browed, slow-spoken man who called a spade a spade and was seldom seen without one, Mr Cartwright discovered environmental activism long before its vogue, and filled his lessons with ecology, to the detriment of our German but the advantage of our humane education.

4 This excellent resource is continually updated at This excellent resource is continually updated at http://tolweb.org/tree. The website has a delightful disclaimer: 'The Tree is under construction. Please have patience: the real Tree took over 3,000,000,000 years to grow.'

5 This happy turn of phrase is from This happy turn of phrase is from Ma.s.s Extinctions and Their Aftermath Ma.s.s Extinctions and Their Aftermath by A. Hallam and P. B. Wignall. by A. Hallam and P. B. Wignall.

Rendezvous 16.

SAUROPSIDS.

Concestor 16, our approximately 170-million-greats-grandparent, lived some 310 million years ago in the second half of the Carboniferous, a time of vast swamps of giant club moss trees in the tropics (the origin of most coal) and an extensive ice cap at the South Pole. This rendezvous point is where a huge throng of new pilgrims joins us: the sauropsids. The sauropsids are by far the largest contingent of newcomers we have yet had to deal with along our Pilgrims' Way. For most of the years since Concestor 16 lived, sauropsids, in the form of dinosaurs, dominated the planet. Even today, with the dinosaurs gone, there still are more than three times as many sauropsid species as mammals. At Rendezvous 16 Rendezvous 16, approximately 4,600 mammal pilgrims greet 9,600 bird pilgrims and 7,770 pilgrims from the rest of the reptiles: crocodiles, snakes, lizards, tuataras, turtles. They are the main group of land vertebrate pilgrims. The only reason I am regarding them as joining us, rather than we joining them, is that we arbitrarily chose to see the journey through human eyes.

Seen through sauropsid eyes, the last to join their pilgrimage 'before' the rendezvous with us were the turtles (using the word in its American sense to include tortoises as well as aquatic turtles and terrapins). The sauropsid contingent, therefore, consists of the turtles and the rest. 'The rest' are a union of two major groups: the lizard-like reptiles which include snakes, chameleons, iguanas, Komodo dragons and tuataras; and the dinosaur-like reptiles or archosaurs, which include pterodactyls, crocodiles and birds. The great aquatic reptile groups such as ichthyosaurs and plesiosaurs are not dinosaurs and seem to be, if anything, closer to the lizard-like reptiles. Pterodactyls have less claim to be called dinosaurs than birds do. Birds are an offshoot of one particular order of dinosaurs, the saurischians. The saurischian dinosaurs, such as Tyrannosaurus Tyrannosaurus and the gigantic sauropods, are closer to birds than they are to the other main group of dinosaurs, the unfortunately-named ornithischians such as and the gigantic sauropods, are closer to birds than they are to the other main group of dinosaurs, the unfortunately-named ornithischians such as Iguanodon, Triceratops Iguanodon, Triceratops, and the duckbilled hadrosaurs. Ornithischian means 'bird-hipped', but the resemblance is superficial and confusing.

The relationship of birds to saurischian dinosaurs is made secure by recent spectacular finds of feathered dinosaurs in China. Tyrannosaurs are closer cousins to birds than they are even to other saurischians such as the large plant-eating sauropods Diplodocus Diplodocus and and Brachiosaurus Brachiosaurus.

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Reptiles (including birds) join. A breakthrough in the evolution of terrestrial vertebrates was the A breakthrough in the evolution of terrestrial vertebrates was the amnion amnion, a waterproof yet breathable egg membrane. Two early-diverging lineages of these 'amniotes' survive today: the synapsids (represented by the mammals), and the sauropsids (17,000 living species of 'reptiles' and birds) who join us here. The phylogeny shown here is reasonably secure.

Images, left to right: medium ground finch ( medium ground finch (Geospiza fortis); Indian peafowl (Pavo cristatus); mandarin duck (Aix galericulata); solitary tinamou (Tinamus solitarius); Nile crocodile (Crocodylus niloticus); red-sided garter snake (Thamnophis sirtalis parietalis); Mediterranean chameleon (Chamaeleo chamaeleon); tuatara (Sphenodon punctatus); green turtle (Chelonia mydas).

These, then, are the sauropsid pilgrims, the turtles, lizards and snakes, crocodiles, and birds, together with the huge concourse of shadowpilgrims the pterosaurs in the air, the ichthyosaurs, plesiosaurs and mosasaurs in water, and above all the dinosaurs on land. Focused as this book is on pilgrims from the present, it is not appropriate to expatiate on the dinosaurs, who dominated the planet for so long, and who would dominate it yet, but for the cruel no, indifferent bolide that laid them low. It seems added cruelty to treat them now so indifferently.1 They do survive after a fashion the special and beautiful fashion of birds and we shall do them homage by listening to four tales of birds. But first, They do survive after a fashion the special and beautiful fashion of birds and we shall do them homage by listening to four tales of birds. But first, in memoriam in memoriam, Sh.e.l.ley's well-known Ode to a Dinosaur: Ode to a Dinosaur: I met a traveller in an antique landWho said: 'Two vast and trunkless legs of stoneStand in the desert ... Near them, on the sand,Half-sunk, a shattered visage lies, whose frown,And wrinkled lip, and sneer of cold command,Tell that its sculptor well those pa.s.sions readWhich yet survive, stamped on these lifeless things,The hand that mocked them and the heart that fed:And on the pedestal these words appear:"My name is Ozymandias, king of kings:Look on my works, ye Mighty, and despair!"Nothing beside remains. Round the decayOf that colossal wreck, boundless and bareThe lone and level sands stretch far away.'

PROLOGUE TO THE GALAPAGOS FINCH'S TALE.

The human imagination is cowed by antiquity, and the magnitude of geological time is so far beyond the ken of poets and archaeologists it can be frightening. But geological time is large not only in comparison to the familiar timescales of human life and human history. It is large on the timescale of evolution itself. This would surprise those, from Darwin's own critics on, who have complained of insufficient time for natural selection to wreak the changes the theory requires of it. We now realise that the problem is, if anything, opposite. There has been too much time! If we measure evolutionary rates over a short time, and then extrapolate, say, to a million years, the potential amount of evolutionary change turns out to be hugely greater than the actual amount. It is as though evolution must have been marking time for much of the period. Or, if not marking time, wandering around this way and that, with meandering fluctuations drowning out, in the short term, whatever trends there might be in the long.

Evidence of various kinds, and theoretical calculations, all point towards this conclusion. Darwinian selection, if we impose it artifi