Terry Pratchett - The Science of Discworld

by Terry Pratchett

  The Death of Dinosaurs would have been something to see, with volcanoes in one hand and an asteroid in the other, trailing a cloak of ice ...

  They were wonderfully cinematic reptiles, weren’t they? Trust the wizards to get it wrong.

  There is another lesson to be learned from our emphasis on the demise of the dinosaurs. Many other large and/or dramatic reptiles died out at the end of the Cretaceous, notably the plesiosaurs (famous as a possible ’explanation’ of the mythical Loch Ness mon­ster), the ichthyosaurs (enormous fish-shaped predators, reptilian whales and dolphins), the pterosaurs (strange flying forms, of which the pterodactyls appear in all the dinosaur films and are labelled, wrongly, dinosaurs), and especially the mosasaurs .,.

  Mosasaurs?

  What were they? They were as dramatic as the dinosaurs, but they weren’t dinosaurs. They didn’t have as good a PR firm, though, because few non-specialists have heard of them. They are popularly known as fish-lizards - not as good a name as ’terrible lizard’ ­and it describes them well. Some were nearly as fish-like as ichthyosaurs ,

  or dolphins, some were rather crocodile-like, some were fifty-foot predators like the great white shark, some were just a couple of feet long and fed on baby ammonites and other common molluscs. They lasted a good twenty million years, and for much of that time they seem to have been the dominant marine predators. Yet most people meet the word in stories about dinosaurs, assume that the mosasaur was a not-very-interesting kind of dinosaur, and promptly forget them.

  The other really strange thing about the K/T extinction -prob­ably not a ’thing’ in any meaningful sense, because in this context a thing would be an equation of unknowns, whereas what we have is a diversity of related puzzles - is which creatures survived it. In the sea, the ammonites all died out, as did the other shelled forms like belemnites - unrolled ammonites - but the nautilus came through, as did the cuttlefish, squids, and octopuses. Amazingly the croco­diles, which to our eyes are about as dinosaur-like as you can get without actually being one, survived the K/T event with little loss of diversity. And those little dinosaurs called ’birds’ came through pretty well unscathed. (There’s a story here that we need to tell, quickly. Not so long ago, the idea that birds are the living remnants of the dinosaurs was new, controversial, and therefore a hot topic. Then it rapidly turned into the prevailing wisdom. New fossil dis­coveries, however, have shown conclusively that the major families of modern birds diverged, in an evolutionary sense, long before the K/T event. So they aren’t remnants of the dinosaurs that otherwise died - they got out early by ceasing to be dinosaurs at all.)

  Myths, not least Jurassic Park itself, have suggested that dinosaurs are not ’really’ extinct at all. They survive, or so semi-fact semi-fic­tion accounts lead us to believe, in Lost World South American valleys, on uninhabited islands, in the depths of Loch Ness, on other planets, or more mystically as DNA preserved inside blood­sucking insects trapped and encased in amber. Alas, almost certainly not. In particular, ’ancient DNA’ reportedly extracted from insects fossilized in amber comes from modern contaminants, not prehistoric organisms - at least if the amber is more than a hundred thousand years old.

  Significantly, no one has made a film bringing back dodos, moas, pygmy elephants, or mosasaurs - only dinosaurs and Hitler are popular for the reawakening myth. Both at the same time would be a good trick.

  Dinosaurs are the ultimate icon for an evolutionary fact which we generally ignore, and definitely find uncomfortable to think about: nearly all species that have ever existed are extinct. As soon as we realize that, we are forced to look at conservation of animal species in new ways. Does it really matter that the lesser spotted pogo-bird is down to its last hundred specimens, or that a hundred species of tree-snail on a Pacific island have been eaten out of exis­tence by predators introduced by human activity? Some issues, like the importation of Nile Perch into Lake Victoria in order to improve the game fishing - which has resulted in the loss of many hundreds of fascinating ’cichlid’ fish species - are regretted even by the people responsible, if only because the new lake ecosystem seems to be much less productive. Everyone (except purveyors of bizarre ancient ’medicines’, their even more foolish customers, and some unreconstructed barbarians) seems to agree that the loss of magnificent creatures like the great whales, elephants, rhinos and of course plants like ginkgoes and sequoias would be a tragedy. Nevertheless, we persist in reducing the diversity of species in ecosystems all around the planet, losing many species of beetles and bacteria with hardly any regrets.

  From the point of view of the majority of humans, there are ’good’ species, unimportant species, and ’bad’ species like smallpox and mosquitoes, which we would clearly be better off without. Unless you take an extreme view on the ’rights’ of all living crea­tures to a continued existence, you find yourself having to pass judgment about which species should be conserved. And if you do take such an extreme view, you’ve got a real problem trying to pre­serve the rights of cheetahs and those of their prey, such as gazelles. On the other hand, if you take the task of passing judgment seri­ously, you can’t just assume that, say, mosquitoes are bad and should be eliminated. Ecosystems are dynamic, and the loss of a species in one place may cause unexpected trouble elsewhere. You have to examine the unintended consequences of your methods as well as the intended ones. When worldwide efforts were made to eradicate mosquitoes, with the aim of getting rid of malaria, the preferred route was mass sprayings of the insecticide DDT. For a time this appeared to be working, but the result in the medium term was to destroy all manner of beneficial insects and other creatures, and to produce resistant strains of mosquitoes which if anything were worse than their predecessors. DDT is now banned world­wide - which unfortunately doesn’t stop some people continuing to use it.

  In the past, the environment was a context for us - we evolved to suit it. Now we’ve become a context for the environment - we change it to suit us. We need to learn how to do that, but going back to some imaginary golden age in which primitive humans allegedly lived in harmony with nature isn’t the answer. It may not be politi­cally correct to say so, but most primitive humans did as much environmental damage as their puny technology would allow. When humans came to the Americas from Siberia, by way of Alaska, they slaughtered their way right down to the tip of South America in a few tens of thousands of years, wiping out dozens of species - giant tree sloths and mastodons (ancient elephants, like mammoths but different), for example. The Anasazi Indians in the southern part of today’s USA cut down forests to build their cliff dwellings, creating some of the most arid areas of the United States. The Maoris killed off the moas. Modern humans may be even more destructive, but there are more of us and technology can amplify our actions. Nevertheless, by the time humans were able to articulate the term ’natural environment’, there wasn’t one. We had changed the face of continents, in ways big and small.

  To live in harmony with nature, we must know how to sing the same song as nature. To do that, we must understand nature. Good intentions aren’t enough. Science might be if we use it wisely.

  THIRTY-FIVE

  BACKSLIDERS

  GLOOM HAD SETTLED OVER THE WIZARDS. Some of them had even refused a third helping at dinner. ’It’s not as if they were very advanced,’ said the Dean, in an attempt to cheer everyone up. ’They weren’t even using metal. And their writing was frankly nothing but pictograms.’

  ’Why doesn’t that sort of this thing happen here?’ said the Senior Wrangler, merely toying with his trifle.

  ’Well, there have been historical examples of mass extinction,’ said Ponder

  ’Yes, but only as a result of argumentative wizardry. That’s quite different. You don’t expect rocks to drop out of the sky.’

  ’You don’t expect them to stay up? said Ridcully. ’In a proper universe, the turtle snaps up most of them and the elephants get the rest. Protects the world. Y’know, it seems to me that the most sen­sible thing an
y intelligent lifeform could do on that little world would be to get off it.’

  ’Nowhere to go,’ said Ponder.

  ’Nonsense! There’s a big moon. And there’s other balls floating around this star.’

  ’All too hot, too cold, or completely without atmosphere,’ said Ponder

  ’People would just have to make their own entertainment. Anyway ... there’s plenty of other suns, isn’t there?’

  ’All far too far away. It would take ... well, lifetimes to get there.’

  ’Yes, but being extinct takes forever.’

  Ponder sighed. ’You’d set out not even knowing if there’s a world you could live on, sir,’ he said.

  ’Yes, but you’d be leavin’ one that you’d know you couldn’t,’ said Ridcully calmly. ’Not for any length of time, anyway.’

  ’There are new lifeforms turning up, sir. I went and checked before dinner.’

  ’Tell that to the lizards,’ sighed the Senior Wrangler. ’Any of the new ones any good?’ said Ridcully. ’They’re ... more fluffy, sir.’ ’Doin’ anything interesting?’

  ’Eating leaves, mainly,’ said Ponder. ’There are some much more realistic trees now.’

  ’Billions of years of history and we’ve got a better tree,’ sighed the Senior Wrangler

  ’No, no, that’s got to be a step in the right direction,’ said Ridcully, thoughtfully.

  ’Oh? How so?’

  ’You can make paper out of trees.’

  The wizards stared into the omniscope.

  ’Oh, how nice,’ said the Lecturer in Recent Runes. ’Ice again. It’s a long time since we’ve had a really big freeze.’

  ’Well, look at the universe,’ said the Dean. ’It’s mainly freezing cold with small patches of boiling hot. The planet’s only doing what it knows.’

  ’You know, we’re certainly learning a lot from this project,’ said Ridcully. ’But it’s mainly that we should be grateful we’re living on a proper world.’

  A few million years passed, as they do.

  The Dean was on the beach and almost in tears. The other wiz­ards appeared nearby and wandered over to see what the fuss was about.

  Rincewind was waist deep in water, apparently struggling with a medium-sized dog.

  ’That’s right,’ the Dean shouted. ’Turn it round! Use a stick if you have to!’

  ’What the thunder is going on here?’ said Ricully.

  ’Look at them!’ said the Dean, beside himself with rage. ’Backsliders! Caught them trying to return to the ocean!’

  Ridcully glanced at one of the creatures, which was lying in the shallows and chewing on a crab.

  ’Didn’t catch them soon enough, did you,’ he said. ’They’ve got webbed paws.’

  ’There’s been too much of this sort of thing lately!’ snapped the Dean. He waved his finger at one of the creatures, who watched it carefully in case it turned out to be a fish.

  ’What would your ancestors say, my friend, if they saw you rush­ing into the water just because times are a bit tough on land?’ he said.

  ’Er ... ’"Welcome back"?’ suggested Rincewind, trying to avoid the snapping jaws.

  "Long time no sea’?’ said the Senior Wrangler, cheerfully.

  The creature begged, uncertainly.

  ’Oh, go on, if you must,’ said the Dean. ’Fish, fish, fish ... you’ll turn into a fish one of these days!’

  ’Y’know, going back to the sea might not be a bad idea,’ said Ridcully, as they strolled away along the beach. ’Beaches are edges. You always get interestin’ stuff on the edge. Look at those lizards we saw on the islands. Their world was all edges.’

  ’Yes, but giving up the land to just go swimming around in the water? I don’t call that evolution.’

  ’But if you go on land where you have to grow a decent brain and some cunning and a bit of muscle in order to get anything done, and then you go back to see the sea where the fish have never had to think about anything very much, you could really, er, kick butt.’

  ’Do fish have-?’

  ’All right, all right. I meant, in a manner of speaking. It was just a thought, anyway.’ Uncharacteristically, the Archchancellor frowned.

  ’Back to the sea,’ he said. ’Well, you can’t blame them.’

  THIRTY-SIX

  MAMMALS ON THE MAKE

  AFTER THE DINOSAURS CAME THE MAMMALS -Not exactly.

  Mammals constitute the most obvious class of animal alive on Earth today. When we say ’animal’ in ordinary conversation, we’re mostly referring to mammals - cats, dogs, elephants, cows, mice, rabbits, whatever. There are about 4,000 species of mammals, and they are astonish­ingly diverse in shape, size, and behaviour. The largest mammal, the blue whale, lives in the ocean and looks like a fish but isn’t; it can weigh 150 tons (136,000 kg). The smallest mammals, various species of shrew, live in holes in the ground and weigh about an ounce (30 g). Roughly in the middle come humans which, paradox­ically, have specialized in being generalists. We are the most intelligent of the mammals

  -sometimes.

  The main distinguishing feature of mammals is that when they are young their mother feeds them on milk, produced by special glands. Other features that (nearly) all mammals have in common include their ears, specifically the three tiny bones in the middle ear known as the anvil, stirrup, and hammer, which send sound to the eardrum; hair (except on adult whales); and the diaphragm, which separates the heart and lungs from the rest of the internal organs. Virtually all mammals bear live young: the exceptions are the duck­billed platypus and the echidna, which lay eggs. Another curious feature is that mammalian red blood cells lack a nucleus, whereas the red cells of all other vertebrates possess a nucleus. All is this is evidence for a lengthy common evolutionary history, subject to a few unusual events of which the most significant was the early sep­aration of Australia from the rest of Gondwanaland. Modern studies of mammalian DNA confirm that basically we are all one big happy family.

  When the dinosaurs died out, the mammals had a field day. Released from dinosaurian thrall, they could occupy environmental niches that, only a few million years before, would merely have pre­sented a dinosaur with an easy meal. It seems likely that the current diversity of mammals has a lot to do with the suddenness with which they came into their kingdom ­for a while, almost any lifestyle was good enough to make a living. However, it would be wrong to imagine that the mammals came into existence to fill the gaps left by the vanished dinosaurs. Mammals had coexisted with dinosaurs for at least 150 million years.

  Harry Jerison has suggested that before the dinosaurs became really dominant, many mammals were able to make their living in daylight, and they evolved good eyesight to do so. As the dinosaurs became a bigger and bigger problem, the mammals adopted a lower profile, mostly staying hidden undergound during the day. If you’re a nocturnal animal, you rely on a really good sense of hearing, so evolutionary pressures then equipped the mammals with excellent ears - including those three little bones. However, they retained their eyesight. So when the mammals again dared to venture out into the daylight, they had good eyesight and good hearing. The combination gave them a substantial advantage over most remaining competitors.

  Mammals evolved from an order of Triassic reptiles known as therapsids - small, quick-moving hunters, mostly, though some were herbivores. Compared to other reptiles, the therapsids were not especially impressive, but their low-profile lifestyle led, in stages, to the distinctive features of mammals. A diaphragm leads to more efficient breathing, useful if you need to run fast. It also lets the young animals continue to breathe while mother is feeding them her milk - changes to animals ’co-evolve’ as a whole suite of coop­erative attributes, not one at a time. Hair keeps you warm, and the warmer you are, the faster all your bodily parts can move ... and so on.

  All this makes it difficult to decide when the mammal-like rep­tilian ancestors of the therapsids became reptile-like mammals ... but, as we’ve said, humans have problems
with becomings. There was no such point: instead, there was a mostly gradual, but occa­sionally bumpy, transition.[50] The earliest fossils that can definitely be identified as mammals come from 210 million years ago - creatures rejoicing in the name ’morganucodontids’. These were shrews, probably nocturnal, probably insect-eaters, probably egg-layers. Darwin’s detractors objected to having apes as their ancestors: heaven knows what they would have thought about bug-eating egg-laying shrews. But there’s good news too, if you’re of that turn of mind, because morganucodontids were brainy. Not especially brainy for a shrew, but brainy compared to the reptiles from which they evolved. Admittedly, this was largely because the therapsids were as thick as two short ... er, slices of tree-fern, but it was a start.

  How do we know that these early shrews were true mammals? One of the bits of an animal that survives as a fossil far more often than any other bit is the tooth. This is why palaeontologists use teeth, above all else, to identify species of long-dead animals. There are plenty of species for which the sole evidence is a tooth or two. Fortunately, you can tell a lot about an animal by its teeth. On the whole, the bigger the tooth is, the bigger the animal - an elephant’s tooth today is a lot bigger than an entire mouse, so whatever animal it came from, it couldn’t be mouse-sized. If you can find a jawbone, a whole array of teeth, all the better. The shape of a tooth tells us a lot about what the animal ate - grinding teeth are for plants, slicing teeth are for meat. The arrangement of teeth in a jawbone tells us a lot more. The morganucodontids made a major breakthrough in tooth design: teeth that interlocked when the jaws were brought together, very effective at cutting bits off meat or insects. They also paid a heavy price for their teeth, one that we still pay today. Reptiles continually produce new teeth: as old ones wear down, they get replaced. We produce just two set of teeth: milk teeth as children and the real thing as adults. When our adult teeth wear out, the only replacements available are artificial. Blame the mor­ganucodontids for this: if you want to take advantage of precisely interlocking teeth, you have to maintain that precision, which is impractical if you keep discarding teeth and growing new ones. So they grew only two sets of teeth, and we have to do likewise.