Terry Pratchett - The Science of Discworld I

Finally, some drama. About 800 million years ago there was an ice age so severe that it very nearly killed off all of the surface life on Earth. This 'big freeze' lasted for between 10 and 20 million years, the ice reached the equator, and it seems that the seas froze to a depth of half a mile (1 km) or more. According to the 'snowball Earth' theory, ice covered the entire Earth at this time. However, if ice really covered the whole Earth, it should have done more dam­age than the fossil record indicates. So maybe the Earth's axis tilted a lot more than astronomers are willing to concede, and the poles lost their ice while equatorial regions gained it. Or perhaps conti­nental drift was more rapid at that time than we think, and we've mapped out the extent of the ice incorrectly. Whatever the details, though, it was a spectacularly icy world.

Although the big freeze came close to wiping out ail surface life, it may indirectly have created a lot of today's biodiversity. The big shift from single-celled creatures to multi-celled ones also hap­pened 800 million years ago. It is plausible that the big freeze cleared away a lot of the single-celled lifeforms and opened up new possibilities for multi-celled life, culminating in the Cambrian Explosion 540 million years ago. Mass extinctions are typically suc­ceeded by sudden bursts of diversity, in which life reverts from being a 'professional' at the evolutionary game to being an 'ama­teur'. It then takes a while for the less able amateurs to be eliminated, and until they are, all sorts of strange strategies for making a living can temporarily thrive. The succession of icy peri­ods that followed the big freeze could only have assisted this process.

However, it may have been the other way round. The invention of the anus by triplobiasts may have changed the ecology of the seas. Faeces would have dropped to the sea-bed, where bacteria could specialize in breaking them down. Other organisms could then become filter feeders, living on those bacteria, perhaps sending their larvae up into the plankton for dispersal, as modern filter-feeders do. Several new ways of life depended on this primeval composting system. And it's possible that the successful return of phosphorus and nitrogen into the marine cycles led to an explosion of algae, which reduced atmospheric carbon dioxide, cut back on the greenhouse effect, and triggered the big freeze.

Fortunately for us, the big freeze wasn't quite long enough, or cold enough, to kill off everything. (Bacteria in volcanic vents on the ocean floor and in the Earth's crust would have survived no matter what, but evolution would have been set back a long, long way.) So when the Earth warmed, life exploded into a fresh, com­petition-free world. Paradoxically, a major reason why we are here today may be that we very nearly weren't. Our entire evolutionary history is full of these good news-bad news scenarios, where life leaps forward joyously over the bodies of the fallen ...

Rincewind can be forgiven for feeling that Roundworld has it in for him. Life has suffered from many different kinds of natural dis­aster. Here are two more. In the Permian/Triassic extinction of 250 million years ago, 96% of all species died within the space of a few hundred thousand years [40] To the best of our knowledge, based on deduction from the available evi­dence. Certainly it was a big extinction - far bigger than the one that killed off (or helped to kill off) the dinosaurs. We remember the dinosaur one because they've had such good PR people. . William Hobster and Mordeckai Magaritz think this happened because they suffocated. Carbon iso­topes show that a lot of coal and shale oxidized in the run-up to the extinction, probably because of a fall in sea level, which exposed more land. The result was a lot more carbon dioxide and a lot less oxygen, which was reduced to half today's level. Land species were especially badly affected.

Another global extinction, though less severe, occurred 55 mil­lion years ago: the Palaeocene/Eocene boundary. In cores of sediment drilled from the Antarctic, James Kennett and Lowell Stott discovered evidence of the sudden death of a lot of marine species. It seemed that trillions of tons (tonnes) of methane had burst from the ocean, sending temperatures through the roof, methane being a powerful greenhouse gas. Jenny Dickens suggested that the methane was released from deposits of methane hydrates in permafrost and on the seabed. Methane hydrates are a crystal lat­tice of water enclosing methane gas: they are created when bacteria in mud release the gas and it becomes trapped.

Coincidentally, one of the main results of the Palaeocene/Eocene extinction was a burst of evolutionary diversity, leading in particular to the higher primates, and us. Whether something is a disaster depends on your point of view. Rocks may not have a point of view, as Ponder Stibbons pointed out, but we certainly do.

I THINK IT LOOKS MORE LIKE A HOGSWATCHNIGHT ORNAMENT,' said the Senior Wrangler later, as the wizards took a pre-dinner drink and stared into the omniscope at the glittering white world. 'Quite pretty, really.'

'Bang go the blobs,' said Ponder Stibbons.

'Phut,' said the Dean, cheerfully. 'More sherry, Archchancellor?'

'Perhaps some instability in the sun ...' Ponder mused.

'Made by unskilled labour,' said Archchancellor Ridcully. 'Bound to happen sooner or later. And then it's nothing but frozen death, the tea-time of the gods and an eternity of cold.'

'Sniffleheim,' said the Dean, who'd got to the sherry ahead of everyone else.

'According to HEX, the air of the planet has changed,' said Ponder.

'A bit academic now, isn't it?' said the Senior Wrangler.

'Ah, I've got an idea!' said the Dean, beaming. 'We can get HEX to reverse the thaumic flow in the cthonic matrix of the optimized bi-direction octagonate, can't we?'

'Well, that's the opinion of four glasses of sherry,' said the Archchancellor briskly, to break the ensuing silence. 'However, if I may express a preference, something that isn't complete gibberish would be more welcome next time, please. So, Mister Stibbons, is this the end of the world?'

'And if it is,' said the Senior Wrangler, 'are we going to have a lot of heroes turning up?'

'What are you talking about, man?' said Ridcully.

'Well, the Dean seems to think we're like gods, and a great many mythologies suggest that when heroes die they go to feast forever in the halls of the gods,' said the Senior Wrangler. 'I just need to know if I should alert the kitchens, that's all'

'They're only blobs,' said Ridcully. 'What can they do that's heroic?'

'I don't know ... stealing something from the gods is a very clas­sical way,' the Senior Wrangler mused.

'Are you saying we should check our pockets?' said the Archchancellor.

'Well, I haven't seen my penknife lately,' said the Senior Wranger. 'It was just a thought, anyway.'

Ridcully slapped the despondent Stibbons on the back.

'Chin up, lad!' he roared. 'It was a wonderful effort! Admittedly the outcome was a lot of blobs with the intelligence of pea soup, but you shouldn't let utter hopeless failure get you down.'

'We never do,' said the Dean.

It was after breakfast next day when Ponder Stibbons wandered into the High Energy Magic building. A scene of desolation met his eye. There were cups and plates everywhere. Paper littered the floor. Forgotten cigarettes had etched their charred trails on the edge of desks. A half-eaten sardine, cheese and blackcurrant pizza, untouched for days, was inching its way to safety.

Sighing, he picked up a broom, and went over the tray contain­ing HEX's overnight write-out.

It seemed a lot fuller than he would have expected.