‘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 any 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.’
FORTY
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 half an ounce (15 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, the southern half of the original supercontinent Pangaea. 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. T
he 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.1 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.
From this we can deduce more. With only two sets of teeth, the morganucodontids had to have some special trick for feeding their young, something different from the reptiles with their continuous succession of teeth. There isn’t room for a full set of adult teeth in a baby shrew, and if teeth only come in two stages, you can’t add the odd one every so often as the jaw grows bigger. The easy solution is to have babies with no teeth at all, to start with. But what can they then eat? Something nutritious and easily digested – milk. So we think that milk-production evolved before those high-precision interlocking teeth. This is one reason why the morganucodontids are definitely placed among the mammals.
Amazing what you can learn from a few teeth.
As they prospered and diversified, mammals evolved into two main types: placental mammals, where the mother carries the young in her uterus, and marsupials, where she carries them in a pouch. The marsupial that springs most readily to mind is the kangaroo – possibly because it springs most readily to almost anything, as for example in The Last Continent:
‘And … what’s kangaroo for “You are needed for a quest of the utmost importance”?’ said Rincewind, with guileful innocence.
‘You know, it’s funny you should ask that –’
The sandals barely moved. Rincewind rose from them like a man leaving the starting blocks, and when he landed his feet were already making running movements in the air.
After a while the kangaroo came alongside and accompanied him in a series of easy bounds.
‘Why are you running away without even listening to what I have to say?’
I’ve had long experience of being me,’ panted Rincewind. ‘I know what’s going to happen. I’m going to be dragged into things that shouldn’t concern me. And you’re just a hallucination caused by rich food on an empty stomach, so don’t try to stop me!’
‘Stop you?’ said the kangaroo. ‘When you’re heading in the right direction?’
Australia alone has over a hundred species of marsupials – in fact most native Australian mammals are marsupials. Another seventy or so are found in the same general region – Tasmania, New Guinea, Timor, Sulawesi, various smaller neighbouring islands. The rest are opossums and some diminutive ratlike creatures, mainly in South America, though ranging into Central America and for one species of opossum right up into Canada.
It looks as though placental mammals generally win out against marsupials, but the difference isn’t so great, and if there aren’t any competing placental mammals then marsupials do very well indeed. There are even some close parallels between marsupials and placental – a good example is the koala ‘bear’, which isn’t a true bear but looks like an unusually cuddly one.
Most marsupials resemble ‘parallel’ placentals; a very curious case is the thylacine, otherwise known as the Tasmanian tiger or Tasmanian wolf, which is distinctly wolflike and has a striped rear. The thylacine was officially declared extinct in 1936, but there are persistent reports of occasional sightings, and suitable habitat still exists, so don’t be surprised if the thylacine makes a comeback. National Park Ranger Charlie Beasley reported watching one for two minutes in Tasmania in 1995. Similar sightings have been reported from Queensland’s Sunshine Coast since 1993: if these sightings are genuine, they are probably of thylacines whose recent ancestors escaped from zoos.
Why such a concentration of marsupials in Australia? The fossil record makes it clear that marsupials originated in the Americas – most probably North America, but that’s not so certain. Placentals arose in what is now Asia, but was then linked to the other continents, so they spread into Europe and the Americas. Before placental mammals really got going in the Americas, marsupials migrated to Australia by way of Antarctica, which in those days wasn’t the frozen wasteland it is now. Australia was already moving away from South America, but hadn’t yet gone all that far, and neither had Antarctica, so presumably the migration involved ‘island hopping’, or taking advantage of land bridges that temporarily rose from the ocean. By 65 million years ago – oddly enough, the time that the dinosaurs died out, though that’s probably not significant – Australia was well separated from the other continents, Antarctica included, and Australian evolution was pretty much on its own.
In the absence of serious competition, the marsupials thrived – just as ground birds did in New Zealand, and for the same reason. But back in the Americas and elsewhere, the superior placental mammals ousted the marsupials almost completely.
Until a few years ago it was assumed that the placentals never made it to Australia at all – except for the very late arrival of rodents and bats from South East Asia about 10 million years ago, and subsequent human introduction of species like dogs and rabbits. This theory was demolished when Mike Archer found a single fossil tooth at a place called Tingamarra. The tooth is from a placental mammal, and it is 55 million years old.
From the form of the tooth it is clear that this mammal had ho
oves.
Did a lot of placental mammals accompany the marsupials on their migration Down Under? Or was it just a few? Either way, why did the placentals die out and the marsupials thrive?
We have no idea.
Early marsupials probably lived in trees, to judge by their forepaws. Early placentals probably lived on the ground, especially in burrows. This difference in habitat allowed them to coexist for a long time. Marsupial extinctions in the Americas were helped along by humans, who found marsupials especially easy to kill.
In Australia, about 40,000-50,000 years ago, there were sudden extinctions of Genyornis, the heaviest bird ever, and of the marsupial equivalent of a lion. Again, evidence suggests that humans were responsible, but the theory was hotly disputed because it was difficult to date the events accurately. And, we suspect, because many people wish to believe that all ‘primitive’ humans lived in exquisite harmony with their environment. In 2001 Linda Ayliffe and Richard Roberts used two accurate dating methods to find out when 45 species, found as fossils at 28 separate sites, disappeared. They all went extinct 46,000 years ago – just after the Aborigines, the first humans to reach Australia, arrived.
Later arrivals were no better. When European settlers turned up, from 1815 onwards, they very nearly wiped out numerous marsupial species.
The evolutionary history of the placental mammals is controversial and has not been mapped out in detail. An early branch of the family tree was the sloths, anteaters, and armadillos – all animals that look ‘primitive’, even though there’s no earthly reason why they should, because today’s sloths, anteaters, and armadillos have evolved just as much as today’s everything else’s, having survived over the same period.
Mammals really got going during the early Tertiary period, about 66 to 57 million years ago. The climate then was mild, with deciduous forests at both poles. It looks as if whatever killed the dinosaurs also changed the climate, so that in particular it was much more rainy than it had been during dinosaur times, and the rainfall was distributed more evenly throughout the year, instead of all coming at once in a rainy season. Tropical forests covered much of the planet, but they were mainly inhabited by tiny tree-dwelling mammals. No big carnivores, not even big plant-eaters … no leopards, no deer, no elephants. It took the mammals several million years to evolve bigger bodies. Possibly the forests were much denser than they had been when there were dinosaurs around, because there weren’t any big animals to trample paths through them. If so, there was less incentive for a big animal to evolve, because it wouldn’t be able to move easily through the forest.
The Science of Discworld Revised Edition Page 34