The Science of Discworld Revised Edition

land is the place for building a bit of backbone …’
    There was a clatter from H EX , and the field of vision of the omniscope was pulled back rapidly until the world was just a marble floating in space.
    ‘Oh dear,’ said the Archchancellor, pointing to a trail of gas, ‘Incoming.’
    The wizards watched gloomily as a large part of one hemisphere became a cauldron of steam and fire.
    ‘Is this going to happen
every
time?’ said the Dean, as the smoke died away and spread out across the seas.
    ‘I blame the over-large sun and all those planets,’ said Ridcully. ‘And you fellows should have cleared out the snowballs. Sooner or later, they fall in.’
    ‘It’d just be nice for a species to make a go of things for five minutes without being frozen solid or broiled,’ said the Senior Wrangler.
    ‘That’s life,’ said Ridcully.
    ‘But not for long,’ said the Senior Wrangler.
    There was a whimper from behind them.
    Rincewind hung in the air, the outline of the virtually-there suit shimmering around him.
    ‘What’s up with him?’ said Ridcully.
    ‘Er … I asked him to investigate the crab civilization, sir.’
    ‘The one the comet just landed on?’
    ‘Yes, sir. A billion tons of rock have just evaporated around him, sir.’
    ‘It couldn’t have
hurt
him, though, could it?’
    ‘Probably made him jump, sir.’

THIRTY-TWO
DON’T LOOK UP

    THE WIZARDS HAVE been convinced all along that a planet is not really a good place to put living creatures. A nice, flat disc, with an attendant turtle who can deal with any inbound rocks before they can wreak havoc, makes much more sense.
    It looks increasingly as if they’re right. The more we learn about the history of our planet, and the greater universe in which it resides, the more we have to admit that the wizards have a point. Not about the shape of our world, but about how dangerously exposed it is without a turtle. The universe is riddled with flying rocks and awash with radiation; most of it is either close to absolute zero or hot enough to make a hydrogen bomb seem like a nice, comfortable bonfire. Yet, somehow, life managed to gain a hold on at least one planet, and to retain that hold for four billion years – despite everything that the universe threw at it. (Often literally.) And despite every kind of nasty that the planet itself managed to concoct.
    There are two ways to interpret this.
    One is that life is incredibly fragile, and that Earth is one of the few places where the conditions
necessary
for life managed to hold together long enough for life to develop, diversify, and thrive. At any moment some disaster could undo all that good work and wipe the face of the planet clean of living creatures. The crab civilisation is fictitious, of course, but it’s in our story to make two important points. First, that there has been plenty of time for lifeforms at least as intelligent as us to evolve on Earth; second, that if they had done, they could easily have left no trace of their existence. Oh, and third … that there are plenty of ways in which they could have come to a sticky end. So we’ve been incredibly lucky to avoid going the way of the crab civilisation. On millions of other apparently suitable worlds , life was not so lucky; it either never got started, or something wiped it out. Life is a rarity; Earth may be the only place in the entire universe where that fragile miracle happened.
    The other is that life is incredibly robust, and that the conditions on Earth are
sufficient
for life to arise, but by no means necessary. Just because things worked out in a particular way here, it would be a mistake to conclude that the same events must happen everywhere else. An important implication of evolution is that life automatically adapts itself to whichever environment happens to be available. Boiling water at the bottom of the ocean? Just what extremophile bacteria need. Two miles down in the rocks? Super – it’s nice and warm down there, and there’s plenty sulphur and iron to provide energy. Thank providence there’s none of that poisonous oxygen; terrible stuff, violently reactive, immensely destructive.
Nothing
could survive in an oxygen atmosphere …
    Both points of view have their advocates, and both have a certain amount going for them. Until we get to other worlds and find out what’s there, there will be plenty of room for disagreement and debate. And, perhaps, a synthesis. Already both viewpoints agree that however life