The Science of Discworld I tsod-1

by Terry Pratchett

  There are many scientists who insist that an organism's DNA determines everything about it, even though it manifestly does not, and they argue that the mother's temperature-control system is included in her DNA recipe. This may well be true, but even if it is, 'this organism's' DNA has somehow migrated to another organism (mother, not her offspring). As soon as two generations are involved in implementing the genetic blueprint, a gap opens up into which things can be inserted that are not genetic at all. We've already men­tioned several, for example prions in the reproduction of yeast.

  Our mammalian ancestry may even be responsible for one of the more bizarre modern myths, persistent tales of people being abducted by aliens. Ufologists allege that one American in twenty now claims to have undergone such an experience (but they would, wouldn't they?). If true, this figure is a remarkable and not very happy comment either on the critical faculties of that great nation or on the habits of an unknown spacefaring species. Be that as it may, a lot of people are convinced that strange aliens, usually with big black eyes and pear-shaped heads like the ones in Close Encounters of the Third Kind, landed a UFO near them, loaded them on board, and took them for a flight round the solar system while carrying out weird experiments, often of a sexual nature, on them. After which they were calmly returned to the very spot from which they had been abducted, as if absolutely nothing had happened.

  The first thing to say is that without doubt many of these expe­riences are false. Ianonce did a radio broadcast which included a woman who had undergone a convincing experience of being abducted, except that she knew she hadn't really been, because her family told her she'd been asleep beside the fire the whole time. Jack once met a woman who claimed that the aliens abducted her and took away her baby. So he asked a question that nobody else had thought to ask, the woman included: 'Were you pregnant?'

  'No.'

  The point is that to the victims, the experience felt real. Even though logic told them it couldn't have happened, they either did­n't apply the logic, or they did but still remembered the experience vividly. We deduce that the human mind sometimes has vivid mem­ories that do not correspond to real events. Of course we must also observe that just because some alien abductions aren't real, that doesn't imply they all aren't. However, if we can find a sensible mechanism for otherwise reasonable people believing that they really were carted off in a UFO, then the burden of proof shifts dra­matically and evidence of abduction stronger than sincere expressions of belief becomes necessary.

  Reports of alien abductions are not new. Back in the Middle Ages, however, they would have been either flights on witches' broomsticks or encounters with fabulous creatures like the succubus, a demon in a woman's body who allegedly had sex with men while they slept. The witches of Discworld employ broomsticks for transport only. The sex bit doesn't appeal to them at all, except for Nanny Ogg, of course.

  Folk tales of succubi and their like can be found worldwide. In Newfoundland people tell of an ancient hag sitting on their chests at night, and in Vietnam they speak of the 'grey ghost'. What seems to be going on is some common mental pattern, overlaid with cul­tural influences. That's why abductions by witches riding broomsticks have gone out of vogue, but abductions by aliens rid­ing UFOs are flavour-of-the-decade.

  Susan Blackmore thinks that all of these experiences are, and were, caused by 'sleep paralysis'. This is a feature of the mind that prevents sleeping people from moving their limbs as they would if they were acting out their dreams. Such a 'mental switch' is impor­tant for any animal that dreams: you don't really want to go sleepwalking out of your cosy burrow and straight down a preda­tor's throat. Plenty of mammals dream, most of us have seen a cat or dog asleep with its legs twitching, and the evidence from record­ings of the brain's electrical activity is that the animals are engaged in something that closely resembles the brain activity of a dreaming human. We can't be sure whether cats have visual dreams like we do, but sleep and dreaming take place in primitive parts of the brain, so they probably go back a long way in our evolutionary his­tory. At any rate, if the sleep paralysis system malfunctions, people who are partially awake may undergo sleep paralysis. Experiments show that in such cases they typically get a strong impression that 'somebody is there'.

  This feature of the human mind may go back to the time, just after the meteorite hit, when the nocturnal mammals suddenly awoke in a world without dinosaurs. Their senses of hearing and sight, previously separate from each other because they had evolved at very different periods and in very different circumstances, would have become linked together. When their ears heard something strange, their visual sense would kick in and make them feel that they could see what was causing it. We inherited this tendency, but we interpret it in terms of the current culture: bogeymen, witches, maybe even dragons a few centuries ago, aliens with big black eyes today. The sexual link is straightforward, too: dreams about sex are very common anyway.

  Oh, yes, one more thing: since we've all watched Close Encounters, we know exactly what an alien must look like ... just as everyone used to know that witches soared through the air on a broomstick. So our visual system knows what shape it should give to whatever it sees when we get that funny feeling that something is haunting us. And flying saucers have come on nicely, too, from being the rivet-studded things that were all the rage in galactic cir­cles in the early Fifties.

  Stories of people seeing ghosts may well have the same explana­tion. You've read the tales, you know what a ghost ought to look like (maybe you watched Ghostbusters or a Stephen King movie), and you're trying to sit up all night in the Haunted House. You're think­ing about ghosts, about headless horsemen and Elizabethan ladies who walk through walls and go transparent, and then you start to doze off because it's 2 am and you've been up all night... The sleep paralysis circuit glitches ... Aaaaagh!

  37. DON'T PLAY GOD

  THE ARCHCHANCELLOR WAS RATHER QUIET OVER TEA.

  Eventually he said, 'Can we stop this project, Ponder?'

  'Er ... are you sure, sir?' 'Well, what is it achieving? I mean, really'? Y'know, I thought, all you had to do is get a world working, and before you could say "creation" there'd be some creature who'd stand up, getting a grip on its surroundings, gaze with a certain amount of intelligence and awe at the infinite sky and say...'

  ‘...that thing's getting bigger, I wonder if it's going to hit us,' said Rincewind.

  'Rincewind, that remark was extremely cynical and accurate.'

  'Sorry, Archchancellor.'

  Bonder's lips were moving quietly as he worked things out.

  'We could start running it down, yes. The thaumic reactor hasn't been putting so much into it in the last week. We've nearly used up the fuel.'

  'Really?'

  'The squash court will have a rather high thaumic index, sir, so whoever goes in to pull the switch will suffer a certain amount of...'

  There was the sound of something spinning. The wizards looked at Rincewind's chair, which finally fell over on to the flagstones. Of its former occupant there was no sign, although there was the dis­tant sound of a slamming door.

  The Dean sniffed.

  'Strange behaviour,' he said.

  'I suggest we give it one more day of our time,' said Ridcully. 'I was hoping we might create a world, gentlemen, but instead it's clear to me that any life in this universe has to get used to living in ... in some kind of huge celestial snow globe. Fire and ice, ice and fire. Gentlemen, round worlds are intrinsically flawed. If there's any hidden gods on ours, they're pretty damn well hidden.'

  'The Omnians say "Don't play God. He always wins",' said the Senior Wrangler.

  'I dare say,' said Ridcully 'So ... one more day, gentlemen? And then we can get on with something sensible.'

  The red sun rose quickly over the parched veldt. The apes stirred in their cave, which was little more than a rocky overhang, and saw the big black rectangle looming over them.

  The Dean tapped it with his pointer

&nb
sp; 'Do try to pay attention today, will you?' He turned and chalked rapidly across the blackboard. 'Here we have R ... O ... C ... K, rock. Can anyone tell me what you do with it? Anyone? Anyone? Look, stop doing that, will you?' He tried to hit an ape with his vir­tual pointer, and then flung it away in disgust. It vanished.

  'Filthy little devils,' he muttered.

  'Not getting anywhere, Dean?' said Ridcully, appearing beside him.

  Wo, Archchancellor. I've tried to explain to them that they've probably got just a few million years, and that's pretty hard to do in sign language, let me tell you. But the only word they know is S-E-X, and they don't waste time spelling it, oh no! For this I skipped breakfast?'

  'Never mind. Let's see how the Senior Wrangler's getting on.'

  'They're just bad copies of humans, if you ask me...'

  The wizards vanished.

  One of the apes knuckled over to the blackboard, and watched it disappear from view as HEX completed the spell.

  He hadn't the faintest idea what had been happening, but he had been impressed by the stick that had been waved about. That seemed to have gone now. That didn't worry the ape, which knew about things vanishing, often, these days, a member of the clan would vanish overnight, with a lot of snarling in the shadows.

  There was probably something you could do with a stick, he thought. Hopefully, it might involve sex.

  He poked around in the debris and found not a stick but a dried-up thighbone, which had a sufficiently stick-like shape.

  He rattled it on the ground a few times. It didn't do anything much. Then he reluctantly decided it would probably be impossible to mate with at the moment, and hurled it high into the air.

  It rose, turning over and over.

  When it fell, it knocked him unconscious.

  The Senior Wrangler was sitting under a virtually-there beach umbrella when the other wizards arrived. He looked as downcast as the Dean.

  A group of apes was playing in the surf.

  'Worse than the lizards,' he said. 'They had some style, at least. When this lot pick up anything, they try to see if they can eat it. What's the point of that?'

  'Well, I suppose they can find out if it's edible,' said Ridcully.

  'But they just mess about,'said the Senior Wrangler. 'Oh, no ... here we go again ...'

  There was a raucous shrieking as the tribe rushed out of the waves and swung up into the nearby mangrove trees. A shadow sped beyond the surf and headed back into the blue water, to an unre­garded chorus of simian catcalls and mangrove seeds.

  'Oh yes, and they like throwing things,' said the Senior Wrangler.

  'Seafood is good for the brain, my granny always said,' said Ridcully.

  'This lot couldn't eat too much of it, then. Yell, throw things, and prod stuff to see what it does, that's the extent of their capabil­ities. Oh, why didn't we discover the lizards earlier? They had class?'

  'Wouldn't have stopped the snowball,' said Ridcully.

  'No. You were right, Archchancellor. It's so pointless.'

  The three wizards stood looking gloomily out to sea. In the mid­dle distance, dolphins stitched their way across the water.

  'Should be coming up to coffee time,' said the Dean, to break the silence.

  'Good thinking, that man.'

  Rincewind was wandering in the next bay, staring at the cliffs. Oh, things were killed off on the Discworld, but... well... sensibly. There were floods, fires and, of course, heroes. There was nothing like a hero for a species whose number was up. But at least some actual thought went into it.

  The cliff was a series of horizontal lines. They represented ancient surfaces, some of which Rincewind had virtually walked on. And in many of them were the bones of ancient creatures, turned into stone by a process Rincewind did not understand and rather distrusted. Life had some how come out of the rocks of this world, and here you could see it going back. There were whole layers of rock made out of life, millions of years of little skeletons. Faced with a natural wonder on that scale, you could only be overawed by the sheer chasms of time or else try to find someone to complain to.

  A few rocks fell out, halfway up the cliff. A couple of small legs waved uncertainly in the strata, and then the Luggage tumbled out, slid down the pile of debris at the foot of the cliff, and landed on its lid.

  Rincewind watched it struggle for a while, sighed, and pushed it the right way up. At least some things didn't change.

  38. ANTHILL INSIDE

  You KNOW WHAT'S GOING TO HAPPEN TO THE APES -they're going to turn into us. But why do we have them playing in the surf? Because it's fun? Yes ... but more significantly, because the seashore is central to one of the two main theories about how our ape ancestors acquired big brains. The other, more orthodox theory places the evolution of the big brain out on the African savannahs, and we know that some of our ancestors lived on the savannahs because we've found fossils. Unfortunately, seashores aren't a good place to leave fossils. You often find them there, but that's because they were deposited when the area wasn't a seashore at all, and the sea has subsequently eroded the rocks to expose the fossils. In the absence of direct evidence of this kind, the surfing apes theory has to take second place ... but it does explain our brains rather neatly, whereas the savannah theory rather sidesteps this issue.

  Our closest living relatives are two species of chimpanzee: the stan­dard boisterous 'zoo' chimp Pan troglodytes and its more slender cousin the bonobo (or pygmy) chimp Pan paniscus. Bonobos live in very inaccessible parts of Zaire, and weren't recognized as a sepa­rate species of chimpanzee until 1929. We can to some extent unravel the past evolutionary history of the great apes by compar­ing their DNA sequences. Human DNA differs from the DNA of either chimpanzee by a mere 1.6%, that is, we have 98.4% of our DNA sequences in common with theirs. (It is interesting to specu­late on what the Victorians would have made of this.) The two species of chimpanzee have DNA that differs by only 0.7%. Gorillas differ from us, and from both chimps, by 2.3%. For orang­utans, the difference from us is 3.6%.

  These differences may seem small, but you can pack an awful lot into a small percentage of an ape genome. A big chunk of what we have in common must surely consist of 'subroutines' that organize basic features of vertebrate and mammalian architecture, tell us how to be an ape, and tell us how to deal with things we've all got -like hair, fingers, internal organs, blood ... The mistake is to imag­ine that everything that makes us human and not a chimpanzee must live in that other 1.6% of 'special' DNA, but DNA doesn't work that way. For example, some of the genes in that 1.6% of the genome may organize the other 98.4% in a completely new way. If you look at the computer code for a wordprocessor and a spread­sheet, you'll find they have an awful lot in common, routines for reading the keyboard, printing to the screen, searching for a given text string, changing fonts to italic, responding to a click on the mouse ... but this doesn't mean that the only distinction between a spreadsheet and a wordprocessor lies in the relatively few routines that are different.

  Since evolution involves changes to DNA, we can use the sizes of those differences to estimate when various ape species diverged from each other. This method was introduced by Charles Sibley and Jon Ahlquist in 1973, and while it needs to be interpreted with caution, it works well here.

  A convenient unit of time for such discussions is the 'Grandfather', which we define to be 50 years. It's a good human length, being about the age difference between the child and the grandparent who says 'When I was young ...' and passes on a sense of history. In these terms, Christ lived 40 Grandfathers ago, and the Babylonians go back about 100 Grandfathers. That's not a lot of grandads, passing down through recorded human history recollec­tions like '... we never had any of this modern cuneiform when I was a lad ...' and'... bronze was good enough for me'. Human time is not very deep. We've just been good at packing a lot into it.

  DNA studies indicate that the two chimp species diverged about 60,000 Grandfathers ago, three million years. Humans a
nd chimps diverged 80,000 Grandfathers earlier, so a chain of only 140,000 grandfathers unites you and your chimplike ancestor. Who was also, we hasten to point out, a modern chimpanzee's manlike ancestor. Humans and gorillas diverged 200,000 Grandfathers ago; humans and orangutans diverged 300,000 Grandfathers ago. So among these animals, we are most closely related to a chimpanzee, and least closely related to the orangutan. This conclusion is borne out by physical appearance and habits, too. Bonobos really like sex.

  If those times seem rather short for all the necessary evolution­ary changes, bear two things in mind. First, that they were estimated by using a realistic rate for DNA mutations; second, that according to Nilsson and Pelger an entire eye can evolve in a mere 8,000 Grandfathers, and lots of different changes can, should, and did evolve in parallel.

  The most striking feature of humans is the size of our brains: bigger, in comparison to body weight, than any other animal. Strikingly bigger. A detailed story of what makes us human must be extraordinarily complicated, but it's clear that big, powerful brains were the main invention that made it all possible. So we now have two obvious questions to think about: 'Why did we evolve big brains?' and 'How did we evolve big brains?'

  The standard theory addresses the 'why'. It maintains that we evolved out on the savannahs, surrounded by lots of big predators, lions, leopards, hyenas, and without much cover. We had to become smart in order to survive. Rincewind would instantly see one flaw in this theory: 'If we were so smart, why did we stay on the savannahs, surrounded by lots of big predators?' But, as we've said, it fits the fossil evidence. The unorthodox theory addresses the 'how'. Big brains need lots of brain cells, and brain cells need lots of chemicals known as 'essential fatty acids'. We have to get these from our food, we can't build them ourselves from anything sim­pler, and they're in short supply out on the savannahs. However, as Michael Crawford and David Marsh pointed out in 1991, they are abundant in seafood.