The Science of Discworld Revised Edition

arose here, Earth was no Garden of Eden. Our planet is by no means the ideal habitat for life. In order for living creatures to survive, evolution has had to solve a lot of difficult problems, and adapt to hostile conditions.
    You may not realise just how hostile. But think of the common disasters: fires, hurricanes, tornadoes, earthquakes, volcanoes, tidal waves, floods, droughts … too much rain and we’re up to our necks in water; too little and our crops won’t grow and we starve.
    But those are feeble compared to the
big
disasters.
    We tend to think of the history of Life on Earth as the smooth growth of a single great Evolutionary Tree. But that image is getting very long in the tooth. The history of life is more like a jungle than a tree, and most of the plants in the jungle were strangled, squashed, or suffocated before they took even the first step on the road to maturity. And however that jungle grew, there was nothing smooth about it.
    True, there was a very long time when there were only ‘blobs’ in the seas, and we might think of that period as a fairly featureless trunk of the Tree. As far as the blobs were concerned, life probably was pretty uneventful – but only because they didn’t
notice
what was happening to the planet. They were largely unaffected by a whole succession of events that would have been cosmic catastrophes for later, more complex life.
    There were certainly a few pretty big impacts at the beginning of life on the planet that didn’t put them out of business, such as it was. And Snowball Earth – if in fact it ever happened – can’t have been easy. But despite all these obstacles, or even because of them, life slowly changed, evolved and diversified as the eukaryotes learned to live in an oxygen atmosphere.
    That should have been a disaster. The very composition of the atmosphere changed, and all the biochemical tricks that had evolved to suit the available range of gases became obsolete. Worse, the gas polluting the air was oxygen, an appallingly reactive substance. Think of what would happen today if the atmosphere started to be taken over by fluorine. Some of the nastiest, most explosive substances are fluorine compounds. But oxygen is just as bad, if not worse; think of fires, think of rust, think of decay.
    The eukaryote cell triumphed over oxygen, and subverted it. Oxygen’s negative characteristics were turned into positive ones. So effective was this evolutionary revolution that the deadly, poisonous pollutant became
essential
for (most) life. Deprive a human, a dog, or a fish of its oxygen, and it dies very quickly. Water, food … those it can do without, for a time. But oxygen? You’ll survive for a few minutes at most, maybe half an hour if you’re a whale.
    The oxygen trick was so good that it took over. Eukaryote life radiated – diversified rapidly – in the seas, inventing entire new kinds of ecologies. With that diversification as a springboard, life came out on land. The advantage of moving to the land was that it opened up an entire range of new habitats, new ways of making a living. So many new kinds of living organism could thrive. One disadvantage, though, was that living on land made life much more vulnerable to astronomical insults. Living on land produced many more complicated kinds of plants and animals, able to protect themselves against small local changes, like hot sunshine, or snow. But, ironically, that very complication made them much more vulnerable to big problems – like stones falling from the sky.
    We all know about the meteorite that killed the dinosaurs … and that fits. Dinosaurs were wonderfully effective as long as the environment remained suitable, but they were not at all well-adapted to the sudden changes that the impact created. But bacteria hardly noticed. If anything, it was a good time for them: they got a lot of extra food for a few hundred years, as the corpses decayed, and then went back to the old boring routine.
    We’ll say a bit more about the 200 million year reign of the dinosaurs and their friends soon, and indeed about what killed them off. But we need to give you some context first. Simple forms of life can put up with a lot, and did. And they changed the planet, or at least its outer skin, by putting in feedback loops that made it less liable to change.
    They started Gaia. This is the name that James Lovelock gave in 1982 to the concept of the Earth as a complex living system – metaphorically, an