The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,63

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 those molecules 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 wonder whether 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 single innovation that marked biotic from pre-biotic, it would be the ribosome, the translator supreme. Barbieri thinks the ribosome is central to life, and so do we. DNA is simply the rather prosaic, boring text. The ribosome is the orator; the other RNAs are the poetry. Once the ribosome emerged, the future became a living future, and in many ways this step marks the true origin of life.

Most origins also involve more subtle forms of emergence: the beginning of a storm, the acorn’s origin as a bud on the oak, the origin of the Earth. Each of these origins is a quantitative-to-qualitative transformation, an emergent event that localises a real beginning. The first stroke of lightning, the first pair of leaves, the generation of heat that melts the core inside the Earth’s mantle: these are emergent events that can label beginnings of new structures. The ‘becoming’ has divided into two issues, before and after the emergence.

If a phenomenon is emergent, it transcends all that has gone before. It does something that its bits and pieces could not have done on their own, or partially assembled, or assembled with some extra scaffolding that gets in the way. This transition is often the best stab we can make at assigning an origin. An emergent phenomenon does not originate in the bits