The Science of Discworld Revised Edition

problem. One key piece of evidence for the Big Freeze is a layer of sedimentary rock that formed just after the glaciers melted and left huge quantities of debris. This layer contains less carbon-13, in proportion to ordinary carbon-12, than normal. Marine photosynthesis converts carbon-12 into carbon dioxide more readily than it does carbon-13, so an excess of carbon-13 is left behind in seawater and in the layers of sediment in it that later turn to rock. So a low ratio of carbon-13 to carbon-12 indicates low biological activity.
    The scientist’s task is to find ways to try to
disprove
things that seem to make sense. In 2001, Martin Kennedy and Nicholas Christie-Blick measured this ratio for sediments that formed
during
the alleged Big Freeze. If the world was miles deep in ice, the ratio ought to be low. But in fact it was high – in Africa, Australia, and North America. This suggests that the global ecosystem was going strong at that time.
    Computer models of the climate system show that the oceans strongly resist freezing over completely, too.
    Like many attractive scientific theories, Snowball Earth is not at all clear-cut, and further research will be needed to find out who is right. Maybe Earth wasn’t a really solid snowball after all. Or maybe , as Schrag responded, there were patches of open water big enough to change the carbon chemistry of the ocean as it absorbed atmospheric carbon dioxide. 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 all 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 triploblasts 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. 1 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