The Science of Discworld Revised Edition

cyanobacteria.
    Cyanobacteria collect together in shallow water to form huge, floating mats, like felt. They secrete a sticky gel as protection against ultraviolet light, and this causes sediment to stick to the mats. When the layer of sediment gets so thick that it blocks out the light, the bacteria form a new layer, and so on. When the layers fossilize they turn into stromatolites, which look rather like big cushions.
    The wizards haven’t been expecting life. Roundworld runs on rules , but life doesn’t – or so they think. The wizards see a sharp discontinuity between life and non-life. This is the problem of expecting
becomings
to have boundaries – of imagining that it ought to be easy to class all objects into either the category ‘alive’ or the category ‘dead’. But that’s not possible, even ignoring the flow of time, in which ‘alive’ can
become
‘dead’ – and vice versa. A ‘dead’ leaf is no longer part of a living tree, but it may well have a few revivable cells.
    Mitochondria, now the part of a cell that generates its chemical energy, once used to be independent organisms. Is a virus alive? Without a bacterial host it can’t reproduce – but neither can DNA copy itself without a cell’s chemical machinery.
    We used to build ‘simple’ chemical models of living processes, in the hope that a sufficiently complex network of chemistry could ‘take off’ – become self-referential, self – copying – by itself. There was the concept of the ‘primal soup’, lots of simple chemicals dissolved in the oceans, bumping into each other at random, and just occasionally forming something more complicated. It turns out that this isn’t quite the way to do it. You don’t have to work hard to make real-world chemistry complex: that’s the default. It’s
easy
to make complicated chemicals. The world is full of them. The problem is to keep that complexity organized.
    What counts as life? Every biologist used to have to learn a list of properties: ability to reproduce, sensitivity to its environment, utilization of energy, and the like. We have moved on. ‘Autopoeisis’ – the ability to make chemicals and structures related to one’s own reproduction – is not a bad definition, except that modern life has evolved away from those early necessities. Today’s biologists prefer to sidestep the issue and define life as a property of the DNA molecule, but this begs the deeper question of life as a general
type
of process. It may be that we’re now defining life in the same way that ‘science fiction’ is defined – it’s what we’re pointing at when we use the term. 1
    The idea that life could somehow be self-starting is still controversial to many people. Nevertheless, it turns out that finding plausible routes to life is easy. There must be at least thirty of them. It ’s hard to decide which, if any, was the actual route taken, because later lifeforms have destroyed nearly all the evidence. This may not matter much: if life hadn’t taken the route that it did, it could easily have taken one of the others, or one of the hundred we haven’t thought of yet.
    One possible route from the inorganic world to life, suggested by Graham Cairns-Smith, is clay. Clay can form complicated microscopic structures, and it often ‘copies’ an existing structure by adding an extra layer to it, which then falls off and becomes the starting point of a new structure. Carbon compounds can stick on to clay surfaces, where they can act as catalysts for the formation of complex molecules of the kind we see in living creatures – proteins, even DNA itself. So today’s organisms may have hitched an evolutionary ride on clay.
    An alternative is Günter Wächtershäuser’s suggestion that pyrite, a compound of iron and sulphur, could have provided an energy source suitable for bacteria. Even today we find bacteria miles underground, and near volcanic vents at the bottom of the oceans, which power themselves by iron/sulphur reactions. These are the source of the ‘upflow of poisonous minerals’ noticed by Rincewind. It’s entirely conceivable that life started in similar environments.
    A potential problem with volcanic vents, though, is that every so often they get blocked, and another one breaks out somewhere else. How could the organisms get themselves safely across the intervening cold water? In 1988 Kevin Speer realized that the Earth’s rotation causes the rising plumes of hot water from