The Science of Discworld IV

causing different effects in the cell, to a smell (technically a pheromone) in male mouse urine that affects the oestrous cycle of female mice. All such effects are the results of translating one chemical language – hormones, pheromones – into a different language – physiological effects. So the genetic code is not alone: there are codes everywhere in biology. From this viewpoint, the crucial element in protein formation is not the DNA that prescribes it, or the messenger RNA that transmits the prescription: it’s the ribosome. Which, to complete the analogy, is the pharmacist that makes up the prescription.
    It also seems clear that this very ancient piece of machinery, so central to all living function, pre-dates the bacteria/archaea split, so it probably derives fairly directly from the RNA world. Something back then formed a relationship, a translation, probably from nucleic acid to protein. The ancestor of today’s ribosomes, probably not very different from today’s range of RNA structures, did the trick. So at the beginning of life, we find the translation of one kind of chemistry into another, by a structure that has come down to us almost unchanged.
    Before the ribosome, there was just chemistry. Complicated chemistry, to be sure, but complication alone isn’t quite the point. What matters is complexity, which in this context means ‘organised complication’. Every cook knows that heating sugar with fats, two fairly simple chemical substances, produces caramel. Caramel is enormously complicated on a chemical level. It includes innumerable different molecules, each of which has thousands of atoms. The molecular structure of caramel is far more complicated than most of the molecules you’re using to read this page. But caramel doesn’t do much, aside from tasting good, so mere complication isn’t enough to make interesting new things happen. Similarly, mixing dilute solutions of amino acids, sugars, bases and so on with particular clays generates long, very complicated, polymers. But, like caramel, they’re not very interesting. However, as soon as transactions between thosemolecules came about, via the earliest ribosomes, complexity took over from complication.
    Here, ‘complexity’ refers to
organised
complication. In a complicated system, such as a car, the individual bits – brakes, steering wheel, engine – behave in much the same way outside the system as they do when they’re part of it. Mostly, they just sit there unless they’re pushed, or pulled, or operated, by something else. But you, a fly, or an amoeba, are different. Their components behave differently when they are part of the system compared to what they do on their own. The parts interact more closely, changing their nature in the context of the system.
    A bridge linking an island to the mainland is a complex system in this sense. In order to do its job, it doesn’t much matter what the bridge is made of: it could be rope, steel or concrete. It could even be made of nothing (or air) if it’s a tunnel. The important property is not what it’s made from, but that it links the two ends effectively. That linkage is an
emergent property
of the bridge. That is, it’s not inherent in any of the materials used. It arises because of their relationships to each other and to the local geography. Moreover, once the bridge is in place, the local geographical function is changed. The river that the bridge spans is no longer an obstacle to vehicles, even though they can’t float or travel underwater. Crucially, you won’t understand how that change occurs by studying the materials that made the bridge.
    When the two ends of the bridge are linked, and only then, the local geography changes dramatically. So the real origin of a bridge occurs
when the ends are linked
. For some purposes, this is when the first rope crosses the divide; for other purposes it’s when the first car makes the crossing; for yet other purposes it’s when the Customs Office is set up.
    Similarly, a ribosome in a cell is very different from an isolated one. It has a specific but complex job to do, reading messenger RNA and constructing proteins according to the genetic code. We wonderwhether the chemical transactions made possible by early ribosomes in effect constructed bridges between several different kinds of chemistry, providing energy and materials for the ribosome to replicate itself. It’s mostly RNA, after all.
    Indeed, if we had to point to a